Table of Contents...Go basic knowledge Go foundation Control statements and functions struct...

TableofContentsIntroduction

InstallationandTools

Installation

Gobasicknowledge

Gofoundation

Controlstatementsandfunctions

struct

Object-oriented

interface

Concurrency

Summary

General

GoProgrammingBasics

WebProgrammingBasics

Implementation

Basicwebapplication

Designingourwebapp

DatabaseHandling

WebappExample

Formhandling

WorkingwithForms

UploadingFiles

Templates

BasicsofTemplates

UserAuthentication

WorkingwithFiles

Routing

Middleware

BuildinganAPI

1.13Contributors

Aboutthebook

ThisbookwaswrittentoteachhowtodevelopwebapplicationsinGofornewbies.

Youwillcreateatodolistapplicationasyougoaheadinthebook.Thebookaimstoteachconceptsbasedonsimpleyetrealexamplesandnotsomedummyexamples.

ThereisacodesectionavailableintheGithubrepo.Ifyouarereadingsomewhereotherthangithub,here'sthelinktotheGithubRepo

ReadOnlineDownloadPDF

CodeThebookcomeswithcorrespondingcode,pleaseuseittounderstandthebookcompletely,thebookisjustplainoldtheoryifyoudonotcheckthecodeout.

ContributingIdon'tprofesstobeaGodofeitherGoorwebdevoranythingingeneral,andIdon'tclaimthatthisisthebestbookforlearninghowtobuildwebappplicationswithGo,butIdobelievethatgoodthingshappenwhenpeoplecollaborate,sopullrequestsarenotonlyappreciated,buttheyarewelcome.

Igotfeedbackfromareddituserthatmaybeitistooearlyformetostartwritingthisbook,decadesago,ayoungstudentfromtheUniversityofHelsinkihadanendlessdebatewithAndrewTannenbaumovercomp.minix,itwasaboutmonolithickernels,hadthestudentlistenedtoAndrewTannenbaum,theworldprobablywouldnothavehadLinux.Thisisthewholepointofopensourceprojects,alittleinitiativefromeveryonegoesalongway.

PhilosophyThroughthisbookwewanttoteachhowtodevelopwebapplicationsinGo.WeexpectthereadertoknowthebasicsofGobutweassumethereaderknowsnothingabouthowtowritewebapplicationsThebookshallcompriseofchapters,ifthetopicishugeanddoesn'tfitintoonechapter,thenwesplitintomultiplechapters,ifpossible.

Introduction

Eachchaptershouldbesplitintologicalpartsorsectionswithameaningfultitlewhich'llteachthereadersomething.EveryconceptshouldbeaccompaniedbytheGocode(ifthereisany),forsneakpeektypesectionswritetheGopseudocode,writingjustthenecessarypartsofthecodeandkeepingeverythingelse.Thecodeshouldn'tbemorethan80characterswidebecauseinthePDFversionsofthebookthecodeisinvisible.Brevityisthesoulofwit,sokeepthedescriptionassmallaspossible.Butthisdoesn'tmeanthatweshouldjustassumethatthereaderknowstheconceptandskipitinsuchcasesdoexplaintheconcept.Inthetodolistmanagerwhichwearecreating,we'llstrivetoimplementasmuchfunctionalityaspossibletogiveatasteofpracticalGoprogrammingtothereader,butweshouldmentionasanotetheotherway,supposeyoure-implementafunctionlikeParseGlobbylistingallhtmlfilesandusingParseFilestoparsethem,weshouldmentionaboutthefunctionParseGlobThemaintitleshouldhaveone#,sectionsshouldhave3#'snoteshouldhave6#'s(noteshouldhaveatitletoo)Multilinecodeshouldhavethreetabsindentation,singlelineofcodecanbeindentedusingtabsorbybackticks.

WrittenwithloveinIndia.

License:BookLicense:CCBY-SA3.0License

Note:1. TheGoProgrammingBasicssectionhasbeenadaptedfrombuild-web-application-with-

golangbyastaxieLinkswereupdatedtoreferthecorrectaspectsofthecurrentbook,titleswereupdatedtofitintothisbook.

2. Thegopherinthecoverpageistakenfromhttps://golang.org/doc/gopher/appenginegophercolor.jpgwithoutmodifications.

LinksNextsection:InstallationandTools

Introduction

InstallationIfyouknowaboutinstallationorhaveinstalledGo,youcanskiptoTools.

Thischapteristakenfrominstallpageverbatim,exceptforthechangestobemadetoadapttothisbook'sstylingformat.

Systemrequirements

Gobinarydistributionsareavailableforthesesupportedoperatingsystemsandarchitectures.Pleaseensureyoursystemmeetstheserequirementsbeforeproceeding.IfyourOSorarchitectureisnotonthelist,youmaybeabletoinstallfromsourceorusegccgoinstead

Operatingsystem Architectures Notes

FreeBSD8-STABLEorlater amd64 DebianGNU/kFreeBSDnotsupported

Linux2.6.23orlaterwithglibc

amd64,386,arm

CentOS/RHEL5.xnotsupported;installfromsourceforARM

MacOSX10.7orlater amd64 usetheclangorgcc†thatcomeswith

Xcode‡

WindowsXPorlater amd64,386 useMinGWgcc†.Noneedforcygwinormsys

†gccisrequiredonlyifyouplantousecgo.

‡YouonlyneedtoinstallthecommandlinetoolsforXcode.IfyouhavealreadyinstalledXcode4.3+,youcaninstallitfromtheComponentstaboftheDownloadspreferencespanel.

InstalltheGotoolsIfyouareupgradingfromanolderversionofGoyoumustfirstremovetheexistingversion.Linux,MacOSX,andFreeBSDtarballs

Downloadthearchiveandextractitinto/usr/local,creatingaGotreein/usr/local/go.Forexample:

tar-C/usr/local-xzfgo$VERSION.$OS-$ARCH.tar.gz

Choosethearchivefileappropriateforyourinstallation.Forinstance,ifyouareinstallingGoversion1.2.1for64-bitx86onLinux,thearchiveyouwantiscalledgo1.2.1.linux-amd64.tar.gz.

(Typicallythesecommandsmustberunasrootorthroughsudo.)

Add/usr/local/go/bintothePATHenvironmentvariable.Youcandothisbyaddingthislinetoyour/etc/profile(forasystem-wideinstallation)or$HOME/.profile:

exportPATH=$PATH:/usr/local/go/bin

Installingtoacustomlocation

TheGobinarydistributionsassumetheywillbeinstalledin/usr/local/go(orc:\GounderWindows),butitispossibletoinstalltheGotoolstoadifferentlocation.InthiscaseyoumustsettheGOROOTenvironmentvariabletopointtothedirectoryinwhichitwasinstalled.

Forexample,ifyouinstalledGotoyourhomedirectoryyoushouldaddthefollowingcommandsto$HOME/.profile:

exportGOROOT=$HOME/go

exportPATH=$PATH:$GOROOT/bin

Note:GOROOTmustbesetonlywheninstallingtoacustomlocation.

MacOSXpackageinstaller

Downloadthepackagefile,openit,andfollowthepromptstoinstalltheGotools.ThepackageinstallstheGodistributionto/usr/local/go.

Thepackageshouldputthe/usr/local/go/bindirectoryinyourPATHenvironmentvariable.YoumayneedtorestartanyopenTerminalsessionsforthechangetotakeeffect.Windows

TheGoprojectprovidestwoinstallationoptionsforWindowsusers(besidesinstallingfromsource):aziparchivethatrequiresyoutosetsomeenvironmentvariablesandanMSIinstallerthatconfiguresyourinstallationautomatically.MSIinstaller

OpentheMSIfileandfollowthepromptstoinstalltheGotools.Bydefault,theinstallerputstheGodistributioninc:\Go.

Theinstallershouldputthec:\Go\bindirectoryinyourPATHenvironmentvariable.Youmayneedtorestartanyopencommandpromptsforthechangetotakeeffect.Ziparchive

Downloadthezipfileandextractitintothedirectoryofyourchoice(wesuggestc:\Go).

Ifyouchoseadirectoryotherthanc:\Go,youmustsettheGOROOTenvironmentvariabletoyourchosenpath.

AddthebinsubdirectoryofyourGoroot(forexample,c:\Go\bin)toyourPATHenvironmentvariable.SettingenvironmentvariablesunderWindows

UnderWindows,youmaysetenvironmentvariablesthroughthe"EnvironmentVariables"buttononthe"Advanced"tabofthe"System"controlpanel.SomeversionsofWindowsprovidethiscontrolpanelthroughthe"AdvancedSystemSettings"optioninsidethe"System"controlpanel.Testyourinstallation

CheckthatGoisinstalledcorrectlybysettingupaworkspaceandbuildingasimpleprogram,asfollows.

Createadirectorytocontainyourworkspace,$HOME/workforexample,andsettheGOPATHenvironmentvariabletopointtothatlocation.

$exportGOPATH=$HOME/work

Youshouldputtheabovecommandinyourshellstartupscript($HOME/.profileforexample)or,ifyouuseWindows,followtheinstructionsabovetosettheGOPATHenvironmentvariableonyoursystem.

Next,makethedirectoriessrc/github.com/user/helloinsideyourworkspace(ifyouuseGitHub,substituteyourusernameforuser),andinsidethehellodirectorycreateafilenamedhello.gowiththefollowingcontents:

packagemain

import"fmt"

funcmain(){

fmt.Printf("hello,world\n")

Thencompileitwiththegotool:

$goinstallgithub.com/user/hello

Theabovecommandwillputanexecutablecommandnamedhello(orhello.exe)insidethebindirectoryofyourworkspace.Executethecommandtoseethegreeting:

$$GOPATH/bin/hello

hello,world

Ifyouseethe"hello,world"messagethenyourGoinstallationisworking.

BeforerushingofftowriteGocodepleasereadtheHowtoWriteGoCodedocument,whichdescribessomeessentialconceptsaboutusingtheGotools.UninstallingGo

ToremoveanexistingGoinstallationfromyoursystemdeletethegodirectory.Thisisusually/usr/local/gounderLinux,MacOSX,andFreeBSDorc:\GounderWindows.

YoushouldalsoremovetheGobindirectoryfromyourPATHenvironmentvariable.UnderLinuxandFreeBSDyoushouldedit/etc/profileor$HOME/.profile.IfyouinstalledGowiththeMacOSXpackagethenyoushouldremovethe/etc/paths.d/gofile.WindowsusersshouldreadthesectionaboutsettingenvironmentvariablesunderWindows.Gettinghelp

Forreal-timehelp,askthehelpfulgophersin#go-nutsontheFreenodeIRCserver.

TheofficialmailinglistfordiscussionoftheGolanguageisGoNuts.

ReportbugsusingtheGoissuetracker.

ToolsForhtml:Brackets,atexteditorforthewebbyAdobe.

ForGo:AnyIDEofyourchoicewhichhasaGolanguageplugin.

Usage:

gofmt<filename>:printstheformattedsourcecodeontheconsole

gofmt-w<filename/foldername>:writestheformattedcodeinsidethefile(s).

gofmtformatsthesourcecodeofaGosourcefile/filesinafolder.ThebasicpointbehindtheGolanguageisthattheyhavestandardizedformatting.Thelanguageauthorswantedtocreatealanguagethatgetsthingsdonequicklyanddidn'twanttheusersofthelanguagetowageintheendlessdebateontrivialissueslikethecodeformatting.MostIDEscanbeconfiguredtorungofmtonsave.Itisrecommendedtorungofmtbeforecommittingtoversioncontrol.

DocumentationinGo,isdoneviacomments,eachexportedfunction/variableissupposedtohaverespectivecommentsonwhatitdoes.Thecommandgodocisthestandarddocumentationgenerationtool.ItextractsdocumentationcommentsonalltheGoprojectspresentin$GOPATH/src.It'sagoodpracticetogiveproperdocumentationwhileprogramming,tomakethecodeeasiertounderstandfornewcomers,forclosedsourceandopensourceprojectsalike.

godoc,bydefaultrunsontheentire$GOPATH,sodependingontheprojectsyouhaveinyour$GOPATH,itmighttakefromfewsecondstofewminutesforgodoctostarttheserver,godocdoesn'tnotifyyouwhentheserverhasstarted,theydohaveaverboseflagwhichprintswhentheserverhasstarted.Itis-v

Therearetwomodesforgodoc,

Webinterface:Usage:godoc-http=:6060-v

Forseeingdocumentationofnet/httpthelinkislocalhost:6060/pkg/net/http

CommandlineinterfaceUsage:godocnet/http

Thiswillprovidethedocumentationofnet/http

gotest

ThisisthetestingtoolchainforGo.Foreachfile.go,thecorrespondingtestcasesshouldbepresentinafilenamedasfile_test.go.Ifmain.goisourGosourcefile,weshouldmakemain_test.gofileinthesamefolderasmain.go.TheGocompilerignoresallthe_test.gofiles.

gobuild

Weusethiscommandtodotobuildourapplication.Itparsesallthe.gofilesexceptthe_test.gofilesintheentirefolderandallsubfoldersalongwithimportedlibrariesifany,andcreatesastaticallylinkedbinary.Thebinarynameisthesameastheprojectfoldername,ifwewantacustomnameweshouldusethe-oflag.

Example:gobuild-otasks

NoteCrosscompilation

WithGo,youcancrosscompileyourapplication.BelowisthecodetocompiletheapplicationforWindowsandMacfromLinux.

envGOOS=darwinGOARCH=386gobuild-otasks.app

envGOOS=windowsGOARCH=386gobuild-otasks.exe

Iftherearenoquirksonthelibraries,shouldgiveyouabinaryfortherespectiveplatforms.

goinstall

Thiscommandcreatesastaticallylinkedbinaryandplacesitin$GOPATH/binfolder.Italsocreatesabinaryversionofallthedependentlibrariesandputsitinthe$GOPATH/pkgfolderintherespectivedirectories.

Whilebuildingyourapplication,firstusegoinstallwhich'llcacheyourdependentlibraries.Thenforsubsequentchangesusegobuild,thiswillsavealotofbuildtime.Thisisbecausegobuilddoesn'tcacheanyresults,itbuildseverything.

Whilerunningyourappthroughthecommandlineyoutypicallhavetodothefollowing:gobuild-oapp./app

goruncombinesthemintoonecommand,itgeneratesandrunsabinaryofyourproject.Thebinaryfile,howeverisn'tretainedaftertherun.

ThisisusedtoinstallpackagesinGo.Itinternallyclonestheversioncontrolrepositoryparameterpassedtoit,canbeanylocal/remotegitrepository.Itthenrunsgoinstallonthelibrary,makingthelibraryavailablein$GOPATH/pkg.

WhatmakesGodifferentfromotherlanguages?TheGoprogramminglanguagewascreatedforwritinglargescalesoftwareeffectively.Thisisthereasonthelanguagehasstrictusageguidelines.InClikelanguagestherearetwofactionsofprogrammers,

thosewhodothis

publicstaticvoidmain(){

andthosewhodothis

publicstaticvoidmain()

ThesamecanbesaidofPythonfourspaces/tabs.

Thismightseemtobeashallowproblematthetop,butwhenthecodebaseandteamsizegrows,thenitisdifficulttomaintainthecode's"beauty"becauseofdifferentuserpreferences.Anyonecanwritecodethesedays,eitherbythemselvesorbycopyingovertheInternet,weshouldstrivetowriteelegantcode,withasmanycommentsaspossible.PythonhasPEP8,whichwascreatedtobringinsomedisciplineforformattingandnamingconventions.

GowasbuiltatGoogle,ascompanyweknowasasynonymforSearchandDistributedComputing,withtheirBorg.Theywantedalanguagethatwasfast,workedwellwithautomatedcodereviewandformattingandallowedalargeteamtowritelargescalesoftwareeffectively.Theydidn'twantusersofGotogetinvolvedintheneverendingtrivialwarsof4spacesvs1tab,hencetherearemanyrestrictionsonthelanguage.Alsoallthemajorlanguagearedecadesold,theywerecreatedinatimewherememorywascostly,thusconcurrencywasn'taproblemfortheircreators.SinceIntelIsraelcameupwiththemulticoreidea,allprocessorchipsaremulticores,Gowasdesignedwithconcurrencyinmind.AsRobPike,inhisamazingtalksaid,Concurrencyisn'tparallelism.

1. Unusedimports/variablesarecompilererrors2. Noneedtoputsemicolon'sbecausethecompileritselfwilladdsemicolonsattheend

Gobasicknowledge

ofline,thisisthereasonyoucan'twritefunctionslikethesecondway,wementionedintheabovesection

3. AllyourGocodeispresentinasinglefolder,$GOPATH,itiscalled.Saygoodbyetoyourcodethrownallaroundinyourmachine

4. gofmtwillformatyourcode,sothereisonestandardwaytowriteGocode.5. Builtinhttp/testingsupport6. Compiledlanguage,thusveryfast.7. Canwritewebappswithoutanyframeworks.

2.1Hello,GoLet'sstartwithasimpleexample,thecustomaryHelloWorld.

Program

packagemain

import"fmt"

funcmain(){

fmt.Printf("Hello,worldor你好，世界orκαλημρακóσμorこんにちは世界\n")

Itprintsfollowinginformation.

Hello,worldor你好，世界orκαλημρακóσμorこんにちは世界

ExplanationGoprogramsusepackages,whicharesameaslibrariesinotherlanguages.MainisaspecialpackageinGo,whenthecompilerstartscompilingthesourcecode,itstartswiththemainpackage.

package<pkgName>(Inthiscaseispackagemain)tellsusthissourcefilebelongstomainpackage,andthekeywordmaintellsusthispackagewillbecompiledtoaprograminsteadofpackagefileswhoseextensionsare.a.

Gobasicknowledge

Perexecutableprogram,therecanbeonlyonemainpackage,andonemainfunctionwithnoargumentspassedorreturned.ThePrintffunction,isimportedfromtheformatpackagecalledfmt,weimportapackageusingimport"fmt".Whencallingafunctionnameorreferringtoavariableinsideanotherpackage,weusepkgName.FunctionNamelikefmt.Println().

Also,Gosupportsmultiplereturnvalues!

Intheexample,weprintednonASCIIcharacters.GosupportsUTF-8bydefault.

Themainpackage

EveryGoprogramshouldbeinapackage,itcanbeeitherMainoranyotherpackage.Eachpackageotherthanmainshouldbepresentasadistinctfolderinto$GOPATH.Thismeansthatyoucandirectlycreateamain.gofilewithpackagemainatit'sstartwithoutcreatingamainfolderandamain.gofileinsideit.

Careneedstobetakenwhilebuilding/runningtheapplication.

Withmainfolder:

[Tasks]$gobuildmain/main.go

[Tasks]$./main/main

Thiswillfunctioncorrectly,becauseweareintheTasksdirectorywhileexecutingourbinary,allthetemplatesandotherfilesarepresentinthisfolder.

Withoutthemainfolder

[Tasks/main]$gobuildmain.go

[Tasks/main]$./main

Here,weareintheTasks/maindirectory,thebinarywillexpectalltheotherfilesintheTasks/maindirectorywhentheyareintheTasksdirectory,

Gobasicknowledge

2.2Gofoundation

DefinevariablesWeusethekeywordvartodefineavariable.NotethatinGothevariabletypecomesafterthevariablename.

//defineavariablewithname“variableName”andtype"type"

varvariableNametype

//definethreevariableswhichtypesare"type"

varvname1,vname2,vname3type

//defineavariablewithname“variableName”,type"type"andvalue"value"

varvariableNametype=value

Definethreevariableswithtype"type",andinitializetheirvalues.

vname1isv1,vname2isv2,vname3isv3

varvname1,vname2,vname3type=v1,v2,v3

Thereisashortcutmethodtodeclarevariables:

Definethreevariableswithouttype"type"andwithoutkeyword"var",andinitializet

heirvalues.

vname1isv1，vname2isv2，vname3isv3

vname1,vname2,vname3:=v1,v2,v3

:=canonlybeusedinsidefunctions,fordefiningglobalvariableswehavetosticktousingvar.

_variableiscalledtheblankvariableanditisusedtoignoreavalue.Thisisauselessexample,butwe'llseeitsexamplesoonenough.

_,b:=34,35//usetheblankoperatortothrowawayavalue

Unusedvariablescausecompilationerrors.Compilethefollowingcodeandseewhathappens.

Gofoundation

packagemain

funcmain(){

variint

ConstantsConstantsarethevaluesthataredeterminedduringcompiletimeandyoucannotchangethemduringruntime.InGo,youcanusenumber,booleanorstringastypesofconstants.

Defineconstantsasfollows.

constconstantName=value

//youcanassigntypeofconstantsifit'snecessary

constPifloat32=3.1415926

Moreexamples.

constPi=3.1415926

consti=10000

constMaxThread=10

constprefix="astaxie_"

Elementarytypes

Boolean

Weusebooltodefineavariableasbooleantype,thevaluecanonlybetrueorfalse,andfalsewillbethedefaultvalue.(Youcannotconvertvariables'typebetweennumberandboolean!)

//samplecode

varisActivebool//globalvariable

varenabled,disabled=true,false//omittypeofvariables

functest(){

varavailablebool//localvariable

valid:=false//briefstatementofvariable

available=true//assignvaluetovariable

Gofoundation

Numericaltypes

Integertypesincludebothsignedandunsignedintegertypes.Gohasintanduintatthesametime,theyhavesamelength,butspecificlengthdependsonyouroperatingsystem.Theyuse32-bitin32-bitoperatingsystems,and64-bitin64-bitoperatingsystems.Goalsohastypesthathavespecificlengthincludingrune,int8,int16,int32,int64,byte,uint8,uint16,uint32,uint64.Notethatruneisaliasofint32andbyteisaliasofuint8.

Oneimportantthingyoushouldknowthatyoucannotassignvaluesbetweenthesetypes,thisoperationwillcausecompileerrors.

varaint8

varbint32

c:=a+b

Althoughint32hasalongerlengththanint8,andhasthesametypeasint,youcannotassignvaluesbetweenthem.(cwillbeassertedastypeinthere)

Floattypeshavethefloat32andfloat64typesandnotypecalledfloat.Thelatteroneisthedefaulttypeifusingbriefstatement.

Gosupportscomplexnumbersaswell.complex128(witha64-bitrealand64-bitimaginarypart)isthedefaulttype,ifyouneedasmallertype,thereisonecalledcomplex64(witha32-bitrealand32-bitimaginarypart).ItsformisRE+IMi,whereREisrealpartandIMisimaginarypart,thelastiistheimaginarynumber.Thereisaexampleofcomplexnumber.

varccomplex64=5+5i

//output:(5+5i)

fmt.Printf("Valueis:%v",c)

String

WejusttalkedabouthowGousestheUTF-8characterset.Stringsarerepresentedbydoublequotes""orbackticks `.

Gofoundation

//samplecode

varfrenchHellostring//basicformtodefinestring

varemptyStringstring=""//defineastringwithemptystring

functest(){

no,yes,maybe:="no","yes","maybe"//briefstatement

japaneseHello:="Ohaiou"

frenchHello="Bonjour"//basicformofassignvalues

It'simpossibletochangestringvaluesbyindex.Youwillgeterrorswhenyoucompilethefollowingcode.

varsstring="hello"

s[0]='c'

WhatifIreallywanttochangejustonecharacterinastring?Trythefollowingcode.

s:="hello"

c:=[]byte(s)//convertstringto[]bytetype

c[0]='c'

s2:=string(c)//convertbacktostringtype

fmt.Printf("%s\n",s2)

Youusethe+operatortocombinetwostrings.

s:="hello,"

m:="world"

a:=s+m

fmt.Printf("%s\n",a)

andalso.

s:="hello"

s="c"+s[1:]//youcannotchangestringvaluesbyindex,butyoucangetvaluesin

stead.

fmt.Printf("%s\n",s)

WhatifIwanttohaveamultiple-linestring?

m:=`hello

world`

willnotescapeanycharactersinastring.

Gofoundation

Errortypes

Gohasoneerrortypeforpurposeofdealingwitherrormessages.Thereisalsoapackagecallederrorstohandleerrors.

err:=errors.New("emitmachodwarf:elfheadercorrupted")

iferr!=nil{

fmt.Print(err)

Underlyingdatastructure

ThefollowingpicturecomesfromanarticleaboutGodatastructureinRussCox'sBlog.Asyoucansee,Goutilizesblocksofmemorytostoredata.

Figure2.1Gounderlyingdatastructure

Someskills

Definebygroup

Ifyouwanttodefinemultipleconstants,variablesorimportpackages,youcanusethegroupform.

Basicform.

import"fmt"

import"os"

consti=100

constpi=3.1415

constprefix="Go_"

variint

varpifloat32

varprefixstring

Groupform.

Gofoundation

import(

const(

pi=3.1415

prefix="Go_"

pifloat32

prefixstring

Unlessyouassignthevalueofconstantisiota,thefirstvalueofconstantinthegroupconst()willbe0.Iffollowingconstantsdon'tassignvaluesexplicitly,theirvalueswillbethesameasthelastone.Ifthevalueoflastconstantisiota,thevaluesoffollowingconstantswhicharenotassignedareiotaalso.

iotaenumerate

Gohasonekeywordcallediota,thiskeywordistomakeenum,itbeginswith0,increasedby1.

const(

x=iota//x==0

y=iota//y==1

z=iota//z==2

w//Ifthereisnoexpressionaftertheconstantsname,itusesthelastexpress

//soit'ssayingw=iotaimplicitly.Thereforew==3,andyandzbothcanomit

"=iota"aswell.

constv=iota//onceiotameetskeyword`const`,itresetsto`0`,sov=0.

const(

e,f,g=iota,iota,iota//e=0,f=0,g=0valuesofiotaaresameinoneline.

Somerules

ThereasonthatGoisconcisebecauseithassomedefaultbehaviors.

Gofoundation

Anyvariablethatbeginswithacapitallettermeansitwillbeexported,privateotherwise.Thesameruleappliesforfunctionsandconstants,nopublicorprivatekeywordexistsinGo.

array,slice,map

arrayisanarrayobviously,wedefineoneasfollows.

vararr[n]type

in[n]type,nisthelengthofthearray,typeisthetypeofitselements.Likeotherlanguages,weuse[]togetorsetelementvalueswithinarrays.

vararr[10]int//anarrayoftype[10]int

arr[0]=42//arrayis0-based

arr[1]=13//assignvaluetoelement

fmt.Printf("Thefirstelementis%d\n",arr[0])

//getelementvalue,itreturns42

fmt.Printf("Thelastelementis%d\n",arr[9])

//itreturnsdefaultvalueof10thelementinthisarray,whichis0inthiscase.

Becauselengthisapartofthearraytype,[3]intand[4]intaredifferenttypes,sowecannotchangethelengthofarrays.Whenyouusearraysasarguments,functionsgettheircopiesinsteadofreferences!Ifyouwanttousereferences,youmaywanttouseslice.We'lltalkaboutlater.

It'spossibletouse:=whenyoudefinearrays.

a:=[3]int{1,2,3}//defineanintarraywith3elements

b:=[10]int{1,2,3}

//defineaintarraywith10elements,ofwhichthefirstthreeareassigned.

//Therestofthemusethedefaultvalue0.

c:=[...]int{4,5,6}//use`…`toreplacethelengthparameterandGowillcalculat

eitforyou.

Youmaywanttousearraysasarrays'elements.Let'sseehowtodothis.

Gofoundation

//defineatwo-dimensionalarraywith2elements,andeachelementhas4elements.

doubleArray:=[2][4]int{[4]int{1,2,3,4},[4]int{5,6,7,8}}

//Thedeclarationcanbewrittenmoreconciselyasfollows.

easyArray:=[2][4]int{{1,2,3,4},{5,6,7,8}}

Arrayunderlyingdatastructure.

Figure2.2Multidimensionalarraymappingrelationship

Inmanysituations,thearraytypeisnotagoodchoice-forinstancewhenwedon'tknowhowlongthearraywillbewhenwedefineit.Thus,weneeda"dynamicarray".ThisiscalledsliceinGo.

sliceisnotreallyadynamicarray.It'sareferencetype.slicepointstoanunderlyingarraywhosedeclarationissimilartoarray,butdoesn'tneedlength.

//justlikedefininganarray,butthistime,weexcludethelength.

varfslice[]int

Thenwedefineaslice,andinitializeitsdata.

slice:=[]byte{'a','b','c','d'}

slicecanredefineexistingslicesorarrays.sliceusesarray[i:j]toslice,whereiisthestartindexandjisendindex,butnoticethatarray[j]willnotbeslicedsincethelengthofthesliceisj-i.

//defineanarraywith10elementswhosetypesarebytes

varar=[10]byte{'a','b','c','d','e','f','g','h','i','j'}

//definetwosliceswithtype[]byte

vara,b[]byte

//'a'pointstoelementsfrom3rdto5thinarrayar.

a=ar[2:5]

//now'a'haselementsar[2],ar[3]andar[4]

//'b'isanothersliceofarrayar

b=ar[3:5]

//now'b'haselementsar[3]andar[4]

Gofoundation

Noticethedifferencesbetweensliceandarraywhenyoudefinethem.Weuse[…]toletGocalculatelengthbutuse[]todefinesliceonly.

Theirunderlyingdatastructure.

Figure2.3Correspondencebetweensliceandarray

slicehassomeconvenientoperations.

sliceis0-based,ar[:n]equalstoar[0:n]Thesecondindexwillbethelengthofsliceifomitted,ar[n:]equalstoar[n:len(ar)].Youcanusear[:]toslicewholearray,reasonsareexplainedinfirsttwostatements.

Moreexamplespertainingtoslice

//defineanarray

vararray=[10]byte{'a','b','c','d','e','f','g','h','i','j'}

//definetwoslices

varaSlice,bSlice[]byte

//someconvenientoperations

aSlice=array[:3]//equalstoaSlice=array[0:3]aSlicehaselementsa,b,c

aSlice=array[5:]//equalstoaSlice=array[5:10]aSlicehaselementsf,g,h,i,j

aSlice=array[:]//equalstoaSlice=array[0:10]aSlicehasallelements

//slicefromslice

aSlice=array[3:7]//aSlicehaselementsd,e,f,g，len=4，cap=7

bSlice=aSlice[1:3]//bSlicecontainsaSlice[1],aSlice[2],soithaselementse,f

bSlice=aSlice[:3]//bSlicecontainsaSlice[0],aSlice[1],aSlice[2],soithasd,e

bSlice=aSlice[0:5]//slicecouldbeexpandedinrangeofcap,nowbSlicecontainsd

,e,f,g,h

bSlice=aSlice[:]//bSlicehassameelementsasaSlicedoes,whichared,e,f,g

sliceisareferencetype,soanychangeswillaffectothervariablespointingtothesamesliceorarray.Forinstance,inthecaseofaSliceandbSliceabove,ifyouchangethevalueofanelementinaSlice,bSlicewillbechangedaswell.

sliceislikeastructbydefinitionanditcontains3parts.

Apointerthatpointstowhereslicestarts.Thelengthofslice.Capacity,thelengthfromstartindextoendindexofslice.

Gofoundation

Array_a:=[10]byte{'a','b','c','d','e','f','g','h','i','j'}

Slice_a:=Array_a[2:5]

Theunderlyingdatastructureofthecodeaboveasfollows.

Figure2.4Arrayinformationofslice

Therearesomebuilt-infunctionsforslice.

lengetsthelengthofslice.capgetsthemaximumlengthofsliceappendappendsoneormoreelementstoslice,andreturnsslice.copycopieselementsfromoneslicetotheother,andreturnsthenumberofelementsthatwerecopied.

Attention:appendwillchangethearraythatslicepointsto,andaffectotherslicesthatpointtothesamearray.Also,ifthereisnotenoughlengthfortheslice((cap-len)==0),appendreturnsanewarrayforthisslice.Whenthishappens,otherslicespointingtotheoldarraywillnotbeaffected.

mapbehaveslikeadictionaryinPython.Usetheformmap[keyType]valueTypetodefineit.

Let'sseesomecode.The'set'and'get'valuesinmaparesimilartoslice,howevertheindexinslicecanonlybeoftype'int'whilemapcanusemuchmorethanthat:forexampleint,string,orwhateveryouwant.Also,theyareallabletouse==and!=tocomparevalues.

//usestringasthekeytype,intasthevaluetype,and`make`initializeit.

varnumbersmap[string]int

//anotherwaytodefinemap

numbers:=make(map[string]int)

numbers["one"]=1//assignvaluebykey

numbers["ten"]=10

numbers["three"]=3

fmt.Println("Thethirdnumberis:",numbers["three"])//getvalues

//Itprints:Thethirdnumberis:3

Somenoteswhenyouusemap.

mapisdisorderly.Everytimeyouprintmapyouwillgetdifferentresults.It'simpossible

Gofoundation

togetvaluesbyindex-youhavetousekey.mapdoesn'thaveafixedlength.It'sareferencetypejustlikeslice.lenworksformapalso.Itreturnshowmanykeysthatmaphas.It'squiteeasytochangethevaluethroughmap.Simplyusenumbers["one"]=11tochangethevalueofkeyoneto11.

Youcanuseformkey:valtoinitializemap'svalues,andmaphasbuilt-inmethodstocheckifthekeyexists.

Usedeletetodeleteanelementinmap.

//Initializeamap

rating:=map[string]float32{"C":5,"Go":4.5,"Python":4.5,"C++":2}

//maphastworeturnvalues.Forthesecondreturnvalue,ifthekeydoesn't

//exist，'ok'returnsfalse.Itreturnstrueotherwise.

csharpRating,ok:=rating["C#"]

fmt.Println("C#isinthemapanditsratingis",csharpRating)

}else{

fmt.Println("WehavenoratingassociatedwithC#inthemap")

delete(rating,"C")//deleteelementwithkey"c"

AsIsaidabove,mapisareferencetype.Iftwomapspointtosameunderlyingdata,anychangewillaffectbothofthem.

m:=make(map[string]string)

m["Hello"]="Bonjour"

m1["Hello"]="Salut"//nowthevalueofm["hello"]isSalut

make,new

makedoesmemoryallocationforbuilt-inmodels,suchasmap,slice,andchannel,whilenewisfortypes'memoryallocation.

new(T)allocateszero-valuetotypeT'smemory,returnsitsmemoryaddress,whichisthevalueoftype*T.ByGo'sdefinition,itreturnsapointerwhichpointstotypeT'szero-value.

newreturnspointers.

Thebuilt-infunctionmake(T,args)hasdifferentpurposesthannew(T).makecanbeusedforslice,map,andchannel,andreturnsatypeTwithaninitialvalue.Thereasonfordoingthisisbecausetheunderlyingdataofthesethreetypesmustbeinitializedbefore

Gofoundation

theypointtothem.Forexample,aslicecontainsapointerthatpointstotheunderlyingarray,lengthandcapacity.Beforethesedataareinitialized,sliceisnil,soforslice,mapandchannel,makeinitializestheirunderlyingdataandassignssomesuitablevalues.

makereturnsnon-zerovalues.

Thefollowingpictureshowshownewandmakearedifferent.

Figure2.5Underlyingmemoryallocationofmakeandnew

Zero-valuedoesnotmeanemptyvalue.It'sthevaluethatvariablesdefaulttoinmostcases.Hereisalistofsomezero-values.

int320

int640

uint0x0

rune0//theactualtypeofruneisint32

byte0x0//theactualtypeofbyteisuint8

float320//lengthis4byte

float640//lengthis8byte

boolfalse

string""

Gofoundation

2.3Controlstatementsandfunctions

Controlstatement

ifdoesn'tneedparenthesesinGo.

ifx>10{

//whenxisgreaterthan10

//programentersthisblock

fmt.Println("xisgreaterthan10")

}else{

//whenxissmallerthan10

//programentersthisblock

fmt.Println("xislessthanorequalto10")

Goallowsustoinitializeandusevariablesiniflikethis:

//initializex,thencheckifxgreaterthan

ifx:=computedValue();x>10{

fmt.Println("xisgreaterthan10")

}else{

fmt.Println("xislessthan10")

//thefollowingcodewillnotcompile

fmt.Println(x)

Formultipleconditionsweusetheelseifblock

ifinteger==3{

fmt.Println("Theintegerisequalto3")

}elseifinteger<3{

fmt.Println("Theintegerislessthan3")

}else{

fmt.Println("Theintegerisgreaterthan3")

Gohasagotokeyword,butbecarefulwhenyouuseit.gotoreroutesthecontrolflowtoapreviouslydefinedlabelwithinthebodyofsamecodeblock.

funcmyFunc(){

Here://labelendswith":"

fmt.Println(i)

gotoHere//jumptolabel"Here"

Thelabelnameiscasesensitive.

Godoesnothavewhile,dowhile.Justafor,whichisthemostpowerfulcontrollogic.Itcanreaddatainloopsanditerativeoperations,justlikewhile.Likeif,fordoesn'tneedparenthesis.

forexpression1;expression2;expression3{

packagemain

import"fmt"

funcmain(){

sum:=0;

forindex:=0;index<10;index++{

sum+=index

fmt.Println("sumisequalto",sum)

//Print：sumisequalto45

Wecanomitoneormoreexpressions.

sum:=1

for;sum<1000;{

sum+=sum

//thisisaninfiniteloop

Usingforlikeawhile

sum:=1

forsum<1000{

sum+=sum

breakandcontinue

1. break:jumpsoutoftheloop.Ifyouhavenestedloops,usebreakalongwithlabels.2. continueskipsthecurrentloopandstartsthenextone

forindex:=10;index>0;index--{

ifindex==5{

break//orcontinue

fmt.Println(index)

}//breakprints10、9、8、7、6//continueprints10、9、8、7、6、4、3、2、1

forcanreaddatafromsliceandmapwhenitisusedtogetherwithrange.

fork,v:=rangemap{

fmt.Println("map'skey:",k)

fmt.Println("map'sval:",v)

BecauseGosupportsmulti-valuereturnsandgivescompileerrorswhenyoudon'tusevaluesthatweredefined,youmaywanttouse_todiscardcertainreturnvalues.

for_,v:=rangemap{

fmt.Println("map'sval:",v)

switch

Sometimesyoumayfindthatyouareusingtoomanyif-elsestatementstoimplementsomelogic,whichmaymakeitdifficulttoreadandmaintaininthefuture.Theswitchstatementsolvesthisproblem.

switchsExpr{

caseexpr1:

someinstructions

caseexpr2:

someotherinstructions

caseexpr3:

someotherinstructions

default:

othercode

ThetypeofsExpr,expr1,expr2,andexpr3mustbethesame.switchisveryflexible.Conditionsdon'thavetobeconstantsanditexecutesfromtoptobottomuntilitmatchesconditions.Ifthereisnostatementafterthekeywordswitch,thenitmatchestrue.

switchi{

case1:

fmt.Println("iisequalto1")

case2,3,4:

fmt.Println("iisequalto2,3or4")

case10:

fmt.Println("iisequalto10")

default:

fmt.Println("AllIknowisthatiisaninteger")

Inthefifthline,weputmanyvaluesinonecase,andwedon'tneedtoaddthebreakkeywordattheendofcase'sbody.Itwilljumpoutoftheswitchbodyonceitmatchedanycase.Ifyouwanttocontinuetomatchingmorecases,youneedtousethefallthroughstatement.

integer:=6

switchinteger{

case4:

fmt.Println("integer<=4")

fallthrough

case5:

fallthrough

case6:

fallthrough

case7:

fallthrough

case8:

fallthrough

default:

fmt.Println("defaultcase")

Thisprogramprintsthefollowinginformation.

integer<=6

integer<=7

integer<=8

defaultcase

FunctionsUsethefunckeywordtodefineafunction.

funcfuncName(input1type1,input2type2)(output1type1,output2type2){

//functionbody

//multi-valuereturn

returnvalue1,value2

Wecanextrapolatethefollowinginformationfromtheexampleabove.

UsekeywordfunctodefineafunctionfuncName.Functionshavezero,oneormorethanonearguments.Theargumenttypecomesaftertheargumentnameandargumentsareseparatedby,.Functionscanreturnmultiplevalues.Therearetworeturnvaluesnamedoutput1andoutput2,youcanomittheirnames

andusetheirtypeonly.Ifthereisonlyonereturnvalueandyouomittedthename,youdon'tneedbracketsforthereturnvalues.Ifthefunctiondoesn'thavereturnvalues,youcanomitthereturnparametersaltogether.Ifthefunctionhasreturnvalues,youhavetousethereturnstatementsomewhereinthebodyofthefunction.

Let'sseeonepracticalexample.(calculatemaximumvalue)

packagemain

import"fmt"

//returngreatervaluebetweenaandb

funcmax(a,bint)int{

ifa>b{

returna

returnb

funcmain(){

max_xy:=max(x,y)//callfunctionmax(x,y)

max_xz:=max(x,z)//callfunctionmax(x,z)

fmt.Printf("max(%d,%d)=%d\n",x,y,max_xy)

fmt.Printf("max(%d,%d)=%d\n",x,z,max_xz)

fmt.Printf("max(%d,%d)=%d\n",y,z,max(y,z))//callfunctionhere

Intheaboveexample,therearetwoargumentsinthefunctionmax,theirtypesarebothintsothefirsttypecanbeomitted.Forinstance,a,bintinsteadofaint,bint.Thesamerulesapplyforadditionalarguments.Noticeherethatmaxonlyhasonereturnvalue,soweonlyneedtowritethetypeofitsreturnvalue-thisistheshortformofwritingit.

Multi-valuereturn

packagemain

import"fmt"

//returnresultsofA+BandA*B

funcSumAndProduct(A,Bint)(int,int){

returnA+B,A*B

funcmain(){

xPLUSy,xTIMESy:=SumAndProduct(x,y)

fmt.Printf("%d+%d=%d\n",x,y,xPLUSy)

fmt.Printf("%d*%d=%d\n",x,y,xTIMESy)

Theaboveexamplereturnstwovalueswithoutnames-youhavetheoptionofnamingthemalso.Ifwenamedthereturnvalues,wewouldjustneedtousereturntoreturnthevaluessincetheyareinitializedinthefunctionautomatically.Noticethatifyourfunctionsaregoingtobeusedoutsideofthepackage,whichmeansyourfunctionnamesstartwithacapitalletter,you'dbetterwritecompletestatementsforreturn;itmakesyourcodemorereadable.

funcSumAndProduct(A,Bint)(addint,multipliedint){

add=A+B

multiplied=A*B

return

Variadicfunctions

Gosupportsfunctionswithavariablenumberofarguments.Thesefunctionsarecalled"variadic",whichmeansthefunctionallowsanuncertainnumbersofarguments.

funcmyfunc(arg...int){}

arg…inttellsGothatthisisafunctionthathasvariablearguments.Noticethattheseargumentsaretypeint.Inthebodyoffunction,theargbecomesasliceofint.

for_,n:=rangearg{

fmt.Printf("Andthenumberis:%d\n",n)

Passbyvalueandpointers

Whenwepassanargumenttothefunctionthatwascalled,thatfunctionactuallygetsthecopyofourvariablessoanychangewillnotaffecttotheoriginalvariable.

Let'sseeoneexampleinordertoprovewhati'msaying.

packagemain

import"fmt"

//simplefunctiontoadd1toa

funcadd1(aint)int{

a=a+1//wechangevalueofa

returna//returnnewvalueofa

funcmain(){

fmt.Println("x=",x)//shouldprint"x=3"

x1:=add1(x)//calladd1(x)

fmt.Println("x+1=",x1)//shouldprint"x+1=4"

Eventhoughwecalledadd1withx,theoriginvalueofxdoesn'tchange.

Thereasonisverysimple:whenwecalledadd1,wegaveacopyofxtoit,notthexitself.

NowyoumayaskhowIcanpasstherealxtothefunction.

Weneedusepointershere.Weknowvariablesarestoredinmemoryandtheyhavesomememoryaddresses.So,ifwewanttochangethevalueofavariable,wemustchangeitsmemoryaddress.Thereforethefunctionadd1hastoknowthememoryaddressofxinordertochangeitsvalue.Herewepass&xtothefunction,andchangetheargument'stypetothepointertype*int.Beawarethatwepassacopyofthepointer,notcopyofvalue.

packagemain

import"fmt"

//simplefunctiontoadd1toa

funcadd1(a*int)int{

*a=*a+1//wechangedvalueofa

return*a//returnnewvalueofa

funcmain(){

x1:=add1(&x)//calladd1(&x)passmemoryaddressofx

fmt.Println("x+1=",x1)//shouldprint"x+1=4"

Nowwecanchangethevalueofxinthefunctions.Whydoweusepointers?Whataretheadvantages?

Allowsustousemorefunctionstooperateononevariable.Lowcostbypassingmemoryaddresses(8bytes),copyisnotanefficientway,bothintermsoftimeandspace,topassvariables.string,sliceandmaparereferencetypes,sotheyusepointerswhenpassingtofunctionsbydefault.(Attention:Ifyouneedtochangethelengthofslice,youhavetopasspointersexplicitly)

Gohasawelldesignedkeywordcalleddefer.Youcanhavemanydeferstatementsinonefunction;theywillexecuteinreverseorderwhentheprogramexecutestotheendoffunctions.Inthecasewheretheprogramopenssomeresourcefiles,thesefileswouldhavetobeclosedbeforethefunctioncanreturnwitherrors.Let'sseesomeexamples.

funcReadWrite()bool{

file.Open("file")

//Dosomework

iffailureX{

file.Close()

returnfalse

iffailureY{

file.Close()

returnfalse

file.Close()

returntrue

Wesawsomecodebeingrepeatedseveraltimes.defersolvesthisproblemverywell.Itdoesn'tonlyhelpyoutowritecleancodebutalsomakesyourcodemorereadable.

funcReadWrite()bool{

file.Open("file")

deferfile.Close()

iffailureX{

returnfalse

iffailureY{

returnfalse

returntrue

Iftherearemorethanonedefers,theywillexecutebyreverseorder.Thefollowingexamplewillprint43210.

fori:=0;i<5;i++{

deferfmt.Printf("%d",i)

Functionsasvaluesandtypes

FunctionsarealsovariablesinGo,wecanusetypetodefinethem.Functionsthathavethesamesignaturecanbeseenasthesametype.

typetypeNamefunc(input1inputType1,input2inputType2[,...])(result1resultType1

[,...])

What'stheadvantageofthisfeature?Theansweristhatitallowsustopassfunctionsasvalues.

packagemain

import"fmt"

typetestIntfunc(int)bool//defineafunctiontypeofvariable

funcisOdd(integerint)bool{

ifinteger%2==0{

returnfalse

returntrue

funcisEven(integerint)bool{

ifinteger%2==0{

returntrue

returnfalse

//passthefunction`f`asanargumenttoanotherfunction

funcfilter(slice[]int,ftestInt)[]int{

varresult[]int

for_,value:=rangeslice{

iff(value){

result=append(result,value)

returnresult

funcmain(){

slice:=[]int{1,2,3,4,5,7}

fmt.Println("slice=",slice)

odd:=filter(slice,isOdd)//usefunctionasvalues

fmt.Println("Oddelementsofsliceare:",odd)

even:=filter(slice,isEven)

fmt.Println("Evenelementsofsliceare:",even)

It'sveryusefulwhenweuseinterfaces.AsyoucanseetestIntisavariablethathasafunctionastypeandthereturnedvaluesandargumentsoffilterarethesameasthoseoftestInt.Therefore,wecanhavecomplexlogicinourprograms,whilemaintaining

flexibilityinourcode.

PanicandRecover

Godoesn'thavetry-catchstructurelikeJavadoes.Insteadofthrowingexceptions,Gousespanicandrecovertodealwitherrors.However,youshouldn'tusepanicverymuch,althoughit'spowerful.

Panicisabuilt-infunctiontobreakthenormalflowofprogramsandgetintopanicstatus.WhenafunctionFcallspanic,Fwillnotcontinueexecutingbutitsdeferfunctionswillcontinuetoexecute.ThenFgoesbacktothebreakpointwhichcausedthepanicstatus.Theprogramwillnotterminateuntilallofthesefunctionsreturnwithpanictothefirstlevelofthatgoroutine.paniccanbeproducedbycallingpanicintheprogram,andsomeerrorsalsocausepaniclikearrayaccessoutofboundserrors.

Recoverisabuilt-infunctiontorecovergoroutinesfrompanicstatus.Callingrecoverindeferfunctionsisusefulbecausenormalfunctionswillnotbeexecutedwhentheprogramisinthepanicstatus.Itcatchespanicvaluesiftheprogramisinthepanicstatus,anditgetsniliftheprogramisnotinpanicstatus.

Thefollowingexampleshowshowtousepanic.

varuser=os.Getenv("USER")

funcinit(){

ifuser==""{

panic("novaluefor$USER")

Thefollowingexampleshowshowtocheckpanic.

functhrowsPanic(ffunc())(bbool){

deferfunc(){

ifx:=recover();x!=nil{

b=true

f()//iffcausespanic,itwillrecover

return

mainfunctionandinitfunction

Gohastworetentionswhicharecalledmainandinit,whereinitcanbeusedinallpackagesandmaincanonlybeusedinthemainpackage.Thesetwofunctionsarenotabletohaveargumentsorreturnvalues.Eventhoughwecanwritemanyinitfunctionsinonepackage,Istronglyrecommendwritingonlyoneinitfunctionforeachpackage.

Goprogramswillcallinit()andmain()automatically,soyoudon'tneedtocallthembyyourself.Foreverypackage,theinitfunctionisoptional,butpackagemainhasoneandonlyonemainfunction.

Programsinitializeandbeginexecutionfromthemainpackage.Ifthemainpackageimportsotherpackages,theywillbeimportedinthecompiletime.Ifonepackageisimportedmanytimes,itwillbeonlycompiledonce.Afterimportingpackages,programswillinitializetheconstantsandvariableswithintheimportedpackages,thenexecutetheinitfunctionifitexists,andsoon.Afteralltheotherpackagesareinitialized,programswillinitializeconstantsandvariablesinthemainpackage,thenexecutetheinitfunctioninsidethepackageifitexists.Thefollowingfigureshowstheprocess.

Figure2.6FlowofprogramsinitializationinGo

import

WeuseimportveryofteninGoprogramsasfollows.

import(

Thenweusefunctionsinthatpackageasfollows.

fmt.Println("helloworld")

fmtisfromGostandardlibrary,itislocatedwithin$GOROOT/pkg.Gosupportsthird-partypackagesintwoways.

1. Relativepathimport"./model"//loadpackageinthesamedirectory,Idon'trecommendthisway.

2. Absolutepathimport"shorturl/model"//loadpackageinpath"$GOPATH/pkg/shorturl/model"

Therearesomespecialoperatorswhenweimportpackages,andbeginnersarealwaysconfusedbytheseoperators.

1. Dotoperator.Sometimeweseepeopleusefollowingwaytoimportpackages.

import(

."fmt"

Thedotoperatormeansyoucanomitthepackagenamewhenyoucallfunctionsinsideofthatpackage.Nowfmt.Printf("Helloworld")becomestoPrintf("Helloworld").

2. Aliasoperation.Itchangesthenameofthepackagethatweimportedwhenwecallfunctionsthatbelongtothatpackage.

import(

f"fmt"

Nowfmt.Printf("Helloworld")becomestof.Printf("Helloworld").

3. _operator.Thisistheoperatorthatisdifficulttounderstandwithoutsomeoneexplainingittoyou.

import(

"database/sql"

_"github.com/ziutek/mymysql/godrv"

The_operatoractuallymeanswejustwanttoimportthatpackageandexecuteitsinitfunction,andwearenotsureifwanttousethefunctionsbelongingtothatpackage.

2.4struct

structWecandefinenewtypesofcontainersofotherpropertiesorfieldsinGojustlikeinotherprogramminglanguages.Forexample,wecancreateatypecalledpersontorepresentaperson,withfieldsnameandage.Wecallthiskindoftypeastruct.

typepersonstruct{

namestring

ageint

Therearetwofields.

nameisastringusedtostoreaperson'sname.ageisaintusedtostoreaperson'sage.

Let'sseehowtouseit.

typepersonstruct{

namestring

ageint

varPperson//pispersontype

P.name="Astaxie"//assign"Astaxie"tothefield'name'ofp

P.age=25//assign25tofield'age'ofp

fmt.Printf("Theperson'snameis%s\n",P.name)//accessfield'name'ofp

Therearethreemorewaystodefineastruct.

Assigninitialvaluesbyorder

P:=person{"Tom",25}

Usetheformatfield:valuetoinitializethestructwithoutorder

P:=person{age:24,name:"Bob"}

struct

Defineananonymousstruct,theninitializeit

P:=struct{namestring;ageint}{"Amy",18}

Let'sseeacompleteexample.

packagemain

import"fmt"

//defineanewtype

typepersonstruct{

namestring

ageint

//comparetheageoftwopeople,thenreturnthe

//olderpersonanddifferencesofage

//structispassedbyvalue

funcOlder(p1,p2person)(person,int){

ifp1.age>p2.age{

returnp1,p1.age-p2.age

returnp2,p2.age-p1.age

funcmain(){

vartomperson

//initialization

tom.name,tom.age="Tom",18

//initializetwovaluesbyformat"field:value"

bob:=person{age:25,name:"Bob"}

//initializetwovalueswithorder

paul:=person{"Paul",43}

tb_Older,tb_diff:=Older(tom,bob)

tp_Older,tp_diff:=Older(tom,paul)

bp_Older,bp_diff:=Older(bob,paul)

fmt.Printf("Of%sand%s,%sisolderby%dyears\n",

tom.name,bob.name,tb_Older.name,tb_diff)

tom.name,paul.name,tp_Older.name,tp_diff)

bob.name,paul.name,bp_Older.name,bp_diff)

struct

embeddedfieldsinstruct

I'vejustintroducedtoyouhowtodefineastructwithfieldnamesandtype.Infact,Gosupportsfieldswithoutnames,butwithtypes.Wecalltheseembeddedfields.

Whentheembeddedfieldisastruct,allthefieldsinthatstructwillimplicitlybethefieldsinthestructinwhichithasbeenembdedded.

Let'sseeoneexample.

packagemain

import"fmt"

typeHumanstruct{

namestring

ageint

weightint

typeStudentstruct{

Human//embeddedfield,itmeansStudentstruct

//includesallfieldsthatHumanhas.

specialtystring

funcmain(){

//initializeastudent

mark:=Student{Human{"Mark",25,120},"ComputerScience"}

//accessfields

fmt.Println("Hisnameis",mark.name)

fmt.Println("Hisageis",mark.age)

fmt.Println("Hisweightis",mark.weight)

fmt.Println("Hisspecialtyis",mark.specialty)

//modifynotes

mark.specialty="AI"

fmt.Println("Markchangedhisspecialty")

fmt.Println("Hisspecialtyis",mark.specialty)

//modifyage

fmt.Println("Markbecomeold")

mark.age=46

fmt.Println("Hisageis",mark.age)

//modifyweight

fmt.Println("Markisnotanathletanymore")

mark.weight+=60

fmt.Println("Hisweightis",mark.weight)

struct

Figure2.7EmbeddinginStudentandHuman

WeseethatwecanaccesstheageandnamefieldsinStudentjustlikewecaninHuman.Thisishowembeddedfieldswork.It'sverycool,isn'tit?Holdon,there'ssomethingcooler!YoucanevenuseStudenttoaccessHumaninthisembeddedfield!

mark.Human=Human{"Marcus",55,220}

mark.Human.age-=1

AllthetypesinGocanbeusedasembeddedfields.

packagemain

import"fmt"

typeSkills[]string

typeHumanstruct{

namestring

ageint

weightint

typeStudentstruct{

Human//structasembeddedfield

Skills//stringsliceasembeddedfield

int//built-intypeasembeddedfield

specialtystring

funcmain(){

//initializeStudentJane

jane:=Student{Human:Human{"Jane",35,100},specialty:"Biology"}

//accessfields

fmt.Println("Hernameis",jane.name)

fmt.Println("Herageis",jane.age)

fmt.Println("Herweightis",jane.weight)

fmt.Println("Herspecialtyis",jane.specialty)

//modifyvalueofskillfield

jane.Skills=[]string{"anatomy"}

fmt.Println("Herskillsare",jane.Skills)

fmt.Println("Sheacquiredtwonewones")

jane.Skills=append(jane.Skills,"physics","golang")

fmt.Println("Herskillsnoware",jane.Skills)

//modifyembeddedfield

jane.int=3

fmt.Println("Herpreferrednumberis",jane.int)

struct

Intheaboveexample,wecanseethatalltypescanbeembeddedfieldsandwecanusefunctionstooperateonthem.

Thereisonemoreproblemhowever.IfHumanhasafieldcalledphoneandStudenthasafieldwithsamename,whatshouldwedo?

Gouseaverysimplewaytosolveit.Theouterfieldsgetupperaccesslevels,whichmeanswhenyouaccessstudent.phone,wewillgetthefieldcalledphoneinstudent,nottheoneintheHumanstruct.Thisfeaturecanbesimplyseenasfieldoverloading.

packagemain

import"fmt"

typeHumanstruct{

namestring

ageint

phonestring//Humanhasphonefield

typeEmployeestruct{

Human//embeddedfieldHuman

specialtystring

phonestring//phoneinemployee

funcmain(){

Bob:=Employee{Human{"Bob",34,"777-444-XXXX"},

"Designer","333-222"}

fmt.Println("Bob'sworkphoneis:",Bob.phone)

//accessphonefieldinHuman

fmt.Println("Bob'spersonalphoneis:",Bob.Human.phone)

struct

Object-orientedGodoesn'tallowustohavefunctionsasapartofstructs,butitdoesallowustobindfunctionstostructs,thesefunctionsarecalledmethods.Thesemethodscanonlybecalledbyaninstanceofthestruct.

methodSupposeyoudefinea"rectangle"structandyouwanttocalculateitsarea.We'dtypicallyusethefollowingcodetoachievethisgoal.

packagemain

import"fmt"

typeRectanglestruct{

width,heightfloat64

funcarea(rRectangle)float64{

returnr.width*r.height

funcmain(){

r1:=Rectangle{12,2}

r2:=Rectangle{9,4}

fmt.Println("Areaofr1is:",area(r1))

fmt.Println("Areaofr2is:",area(r2))

Theaboveexamplecancalculatearectangle'sarea.Weusethefunctioncalledarea,butit'snotamethodoftherectanglestruct(likeclassmethodsinclassicobject-orientedlanguages).Thefunctionandstructaretwoindependentthingsasyoumaynotice.

It'snotaproblemsofar.However,ifyoualsohavetocalculatetheareaofacircle,square,pentagon,oranyotherkindofshape,youaregoingtoneedtoaddadditionalfunctionswithverysimilarnames.

Figure2.8Relationshipbetweenfunctionandstruct

Obviouslythat'snotcool.Also,theareashouldreallybethepropertyofacircleorrectangle.

Object-oriented

Forthosereasons,wehavethemethodconcept.methodisaffiliatedwithtype.Ithasthesamesyntaxasfunctionsdoexceptforanadditionalparameterafterthefunckeywordcalledthereceiver,whichisthemainbodyofthatmethod.

Usingthesameexample,Rectangle.area()belongsdirectlytorectangle,insteadofasaperipheralfunction.Morespecifically,length,widthandarea()allbelongtorectangle.

AsRobPikesaid.

"Amethodisafunctionwithanimplicitfirstargument,calledareceiver."

Syntaxofmethod.

func(rReceiverType)funcName(parameters)(results)

Let'schangeourexampleusingmethodinstead.

Object-oriented

packagemain

import(

"math"

typeRectanglestruct{

width,heightfloat64

typeCirclestruct{

radiusfloat64

func(rRectangle)area()float64{

returnr.width*r.height

func(cCircle)area()float64{

returnc.radius*c.radius*math.Pi

funcmain(){

r1:=Rectangle{12,2}

r2:=Rectangle{9,4}

c1:=Circle{10}

c2:=Circle{25}

fmt.Println("Areaofr1is:",r1.area())

fmt.Println("Areaofr2is:",r2.area())

fmt.Println("Areaofc1is:",c1.area())

fmt.Println("Areaofc2is:",c2.area())

Notesforusingmethods.

Ifthenameofmethodsarethesamebuttheydon'tsharethesamereceivers,theyarenotthesame.Methodsareabletoaccessfieldswithinreceivers.Use.tocallamethodinthestruct,thesamewayfieldsarecalled.

Figure2.9Methodsaredifferentindifferentstructs

Intheexampleabove,thearea()methodsbelongtobothRectangleandCirclerespectively,sothereceiversareRectangleandCircle.

Object-oriented

Onethingthat'sworthnotingisthatthemethodwithadottedlinemeansthereceiverispassedbyvalue,notbyreference.Thedifferencebetweenthemisthatamethodcanchangeitsreceiver'svalueswhenthereceiverispassedbyreference,anditgetsacopyofthereceiverwhenthereceiverispassedbyvalue.

Canthereceiveronlybeastruct?Ofcoursenot.Anytypecanbethereceiverofamethod.Youmaybeconfusedaboutcustomizedtypes.Structisaspecialkindofcustomizedtype-therearemorecustomizedtypes.

Usethefollowingformattodefineacustomizedtype.

typetypeNametypeLiteral

Examplesofcustomizedtypes:

typeagesint

typemoneyfloat32

typemonthsmap[string]int

m:=months{

"January":31,

"February":28,

"December":31,

Ihopethatyouknowhowtousecustomizedtypesnow.SimilartotypedefinC,weuseagestosubstituteintintheaboveexample.

Let'sgetbacktotalkingaboutmethod.

Youcanuseasmanymethodsincustomtypesasyouwant.

packagemain

import"fmt"

const(

WHITE=iota

YELLOW

typeColorbyte

Object-oriented

typeBoxstruct{

width,height,depthfloat64

colorColor

typeBoxList[]Box//asliceofboxes

func(bBox)Volume()float64{

returnb.width*b.height*b.depth

func(b*Box)SetColor(cColor){

b.color=c

func(blBoxList)BiggestsColor()Color{

v:=0.00

k:=Color(WHITE)

for_,b:=rangebl{

ifb.Volume()>v{

v=b.Volume()

k=b.color

returnk

func(blBoxList)PaintItBlack(){

fori,_:=rangebl{

bl[i].SetColor(BLACK)

func(cColor)String()string{

strings:=[]string{"WHITE","BLACK","BLUE","RED","YELLOW"}

returnstrings[c]

funcmain(){

boxes:=BoxList{

Box{4,4,4,RED},

Box{10,10,1,YELLOW},

Box{1,1,20,BLACK},

Box{10,10,1,BLUE},

Box{10,30,1,WHITE},

Box{20,20,20,YELLOW},

fmt.Printf("Wehave%dboxesinourset\n",len(boxes))

fmt.Println("Thevolumeofthefirstoneis",boxes[0].Volume(),"cm³")

fmt.Println("Thecolorofthelastoneis",boxes[len(boxes)-1].color.String())

fmt.Println("Thebiggestoneis",boxes.BiggestsColor().String())

Object-oriented

fmt.Println("Let'spaintthemallblack")

boxes.PaintItBlack()

fmt.Println("Thecolorofthesecondoneis",boxes[1].color.String())

fmt.Println("Obviously,now,thebiggestoneis",boxes.BiggestsColor().String())

Wedefinesomeconstantsandcustomizedtypes.

UseColorasaliasofbyte.DefineastructBoxwhichhasfieldsheight,width,lengthandcolor.DefineastructBoxListwhichhasBoxasitsfield.

Thenwedefinedsomemethodsforourcustomizedtypes.

Volume()usesBoxasitsreceiverandreturnsthevolumeofBox.SetColor(cColor)changesBox'scolor.BiggestsColor()returnsthecolorwhichhasthebiggestvolume.PaintItBlack()setscolorforallBoxinBoxListtoblack.String()useColorasitsreceiver,returnsthestringformatofcolorname.

Isitmuchclearerwhenweusewordstodescribeourrequirements?Weoftenwriteourrequirementsbeforewestartcoding.

Usepointerasreceiver

Let'stakealookatSetColormethod.ItsreceiverisapointerofBox.Yes,youcanuse*Boxasareceiver.Whydoweuseapointerhere?BecausewewanttochangeBox'scolorinthismethod.Thus,ifwedon'tuseapointer,itwillonlychangethevalueinsideacopyofBox.

Ifweseethatareceiveristhefirstargumentofamethod,it'snothardtounderstandhowitworks.

Youmightbeaskingwhywearen'tusing(*b).Color=cinsteadofb.Color=cintheSetColor()method.EitheroneisOKherebecauseGoknowshowtointerprettheassignment.DoyouthinkGoismorefascinatingnow?

Youmayalsobeaskingwhetherweshoulduse(&bl[i]).SetColor(BLACK)inPaintItBlackbecausewepassapointertoSetColor.Again,eitheroneisOKbecauseGoknowshowtointerpretit!

Inheritanceofmethod

Object-oriented

Welearnedaboutinheritanceoffieldsinthelastsection.Similarly,wealsohavemethodinheritanceinGo.Ifananonymousfieldhasmethods,thenthestructthatcontainsthefieldwillhaveallthemethodsfromitaswell.

packagemain

import"fmt"

typeHumanstruct{

namestring

ageint

phonestring

typeStudentstruct{

Human//anonymousfield

schoolstring

typeEmployeestruct{

companystring

//defineamethodinHuman

func(h*Human)SayHi(){

fmt.Printf("Hi,Iam%syoucancallmeon%s\n",h.name,h.phone)

funcmain(){

mark:=Student{Human{"Mark",25,"222-222-YYYY"},"MIT"}

sam:=Employee{Human{"Sam",45,"111-888-XXXX"},"GolangInc"}

mark.SayHi()

sam.SayHi()

Methodoverload

IfwewantEmployeetohaveitsownmethodSayHi,wecandefineamethodthathasthesamenameinEmployee,anditwillhideSayHiinHumanwhenwecallit.

Object-oriented

packagemain

import"fmt"

typeHumanstruct{

namestring

ageint

phonestring

typeStudentstruct{

schoolstring

typeEmployeestruct{

companystring

func(e*Employee)SayHi(){

fmt.Printf("Hi,Iam%s,Iworkat%s.Callmeon%s\n",e.name,

e.company,e.phone)//Yesyoucansplitinto2lineshere.

funcmain(){

mark:=Student{Human{"Mark",25,"222-222-YYYY"},"MIT"}

sam:=Employee{Human{"Sam",45,"111-888-XXXX"},"GolangInc"}

mark.SayHi()

sam.SayHi()

YouareabletowriteanObject-orientedprogramnow,andmethodsuseruleofcapitallettertodecidewhetherpublicorprivateaswell.

Object-oriented

2.6Interface

InterfaceOneofthesubtlestdesignfeaturesinGoareinterfaces.Afterreadingthissection,youwilllikelybeimpressedbytheirimplementation.

Whatisaninterface

Inshort,aninterfaceisasetofmethodsthatweusetodefineasetofactions.

Liketheexamplesinprevioussections,bothStudentandEmployeecanSayHi(),buttheydon'tdothesamething.

Let'sdosomemorework.We'lladdonemoremethodSing()tothem,alongwiththeBorrowMoney()methodtoStudentandtheSpendSalary()methodtoEmployee.

Now,StudenthasthreemethodscalledSayHi(),Sing()andBorrowMoney(),andEmployeehasSayHi(),Sing()andSpendSalary().

ThiscombinationofmethodsiscalledaninterfaceandisimplementedbybothStudentandEmployee.So,StudentandEmployeeimplementtheinterface:SayHi()andSing().Atthesametime,Employeedoesn'timplementtheinterface:SayHi(),Sing(),BorrowMoney(),andStudentdoesn'timplementtheinterface:SayHi(),Sing(),SpendSalary().ThisisbecauseEmployeedoesn'thavethemethodBorrowMoney()andStudentdoesn'thavethemethodSpendSalary().

TypeofInterface

Aninterfacedefinesasetofmethods,soifatypeimplementsallthemethodswesaythatitimplementstheinterface.

typeHumanstruct{

namestring

ageint

phonestring

typeStudentstruct{

schoolstring

loanfloat32

interface

typeEmployeestruct{

companystring

moneyfloat32

func(h*Human)Sing(lyricsstring){

fmt.Println("Lala,lalala,lalalalala...",lyrics)

func(h*Human)Guzzle(beerSteinstring){

fmt.Println("GuzzleGuzzleGuzzle...",beerStein)

//EmployeeoverloadsSayhi

func(e*Employee)SayHi(){

func(s*Student)BorrowMoney(amountfloat32){

s.loan+=amount//(againandagainand...)

func(e*Employee)SpendSalary(amountfloat32){

e.money-=amount//Morevodkaplease!!!Getmethroughtheday!

//defineinterface

typeMeninterface{

SayHi()

Sing(lyricsstring)

Guzzle(beerSteinstring)

typeYoungChapinterface{

SayHi()

Sing(songstring)

BorrowMoney(amountfloat32)

typeElderlyGentinterface{

SayHi()

Sing(songstring)

SpendSalary(amountfloat32)

interface

Weknowthataninterfacecanbeimplementedbyanytype,andonetypecanimplementmanyinterfacessimultaneously.

Notethatanytypeimplementstheemptyinterfaceinterface{}becauseitdoesn'thaveanymethodsandalltypeshavezeromethodsbydefault.

Valueofinterface

Sowhatkindofvaluescanbeputintheinterface?Ifwedefineavariableasatypeinterface,anytypethatimplementstheinterfacecanassignedtothisvariable.

Liketheaboveexample,ifwedefineavariable"m"asinterfaceMen,thenanyoneofStudent,HumanorEmployeecanbeassignedto"m".SowecouldhaveasliceofMen,andanytypethatimplementsinterfaceMencanassigntothisslice.Beawarehoweverthatthesliceofinterfacedoesn'thavethesamebehaviorasasliceofothertypes.

packagemain

import"fmt"

typeHumanstruct{

namestring

ageint

phonestring

typeStudentstruct{

schoolstring

loanfloat32

typeEmployeestruct{

companystring

moneyfloat32

func(hHuman)SayHi(){

func(hHuman)Sing(lyricsstring){

fmt.Println("Lalalala...",lyrics)

func(eEmployee)SayHi(){

interface

//InterfaceMenimplementedbyHuman,StudentandEmployee

typeMeninterface{

SayHi()

Sing(lyricsstring)

funcmain(){

mike:=Student{Human{"Mike",25,"222-222-XXX"},"MIT",0.00}

paul:=Student{Human{"Paul",26,"111-222-XXX"},"Harvard",100}

sam:=Employee{Human{"Sam",36,"444-222-XXX"},"GolangInc.",1000}

tom:=Employee{Human{"Sam",36,"444-222-XXX"},"ThingsLtd.",5000}

//defineinterfacei

variMen

//icanstoreStudent

i=mike

fmt.Println("ThisisMike,aStudent:")

i.SayHi()

i.Sing("Novemberrain")

//icanstoreEmployee

fmt.Println("ThisisTom,anEmployee:")

i.SayHi()

i.Sing("Borntobewild")

//sliceofMen

fmt.Println("Let'suseasliceofMenandseewhathappens")

x:=make([]Men,3)

//thesethreeelementsaredifferenttypesbuttheyallimplementedinterfaceMen

x[0],x[1],x[2]=paul,sam,mike

for_,value:=rangex{

value.SayHi()

Aninterfaceisasetofabstractmethods,andcanbeimplementedbynon-interfacetypes.Itcannotthereforeimplementitself.

Emptyinterface

Anemptyinterfaceisaninterfacethatdoesn'tcontainanymethods,soalltypesimplementanemptyinterface.Thisfactisveryusefulwhenwewanttostorealltypesatsomepoint,andissimilartovoid*inC.

interface

//defineaasemptyinterface

varainterface{}

variint=5

s:="Helloworld"

//acanstorevalueofanytype

Ifafunctionusesanemptyinterfaceasitsargumenttype,itcanacceptanytype;ifafunctionusesemptyinterfaceasitsreturnvaluetype,itcanreturnanytype.

Methodargumentsofaninterface

Anyvariablecanbeusedinaninterface.Sohowcanweusethisfeaturetopassanytypeofvariabletoafunction?

Forexampleweusefmt.Printlnalot,buthaveyouevernoticedthatitcanacceptanytypeofargument?Lookingattheopensourcecodeoffmt,weseethefollowingdefinition.

typeStringerinterface{

String()string

ThismeansanytypethatimplementsinterfaceStringercanbepassedtofmt.Printlnasanargument.Let'sproveit.

interface

packagemain

import(

"strconv"

typeHumanstruct{

namestring

ageint

phonestring

//Humanimplementedfmt.Stringer

func(hHuman)String()string{

return"Name:"+h.name+",Age:"+strconv.Itoa(h.age)+"years,Contact:"+h.

funcmain(){

Bob:=Human{"Bob",39,"000-7777-XXX"}

fmt.Println("ThisHumanis:",Bob)

LookingbacktotheexampleofBox,youwillfindthatColorimplementsinterfaceStringeraswell,soweareabletocustomizetheprintformat.Ifwedon'timplementthisinterface,fmt.Printlnprintsthetypewithitsdefaultformat.

fmt.Println("Thebiggestoneis",boxes.BiggestsColor().String())

fmt.Println("Thebiggestoneis",boxes.BiggestsColor())

Attention:Ifthetypeimplementedtheinterfaceerror,fmtwillcallerror(),soyoudon'thavetoimplementStringeratthispoint.

Typeofvariableinaninterface

Ifavariableisthetypethatimplementsaninterface,weknowthatanyothertypethatimplementsthesameinterfacecanbeassignedtothisvariable.Thequestionishowcanweknowthespecifictypestoredintheinterface.TherearetwowayswhichIwillshowyou.

AssertionofComma-okpattern

Gohasthesyntaxvalue,ok:=element.(T).Thischeckstoseeifthevariableisthetypethatweexpect,where"value"isthevalueofthevariable,"ok"isavariableofbooleantype,"element"istheinterfacevariableandtheTisthetypeofassertion.

interface

Iftheelementisthetypethatweexpect,okwillbetrue,falseotherwise.

Let'suseanexampletoseemoreclearly.

packagemain

import(

"strconv"

typeElementinterface{}

typeList[]Element

typePersonstruct{

namestring

ageint

func(pPerson)String()string{

return"(name:"+p.name+"-age:"+strconv.Itoa(p.age)+"years)"

funcmain(){

list:=make(List,3)

list[0]=1//anint

list[1]="Hello"//astring

list[2]=Person{"Dennis",70}

forindex,element:=rangelist{

ifvalue,ok:=element.(int);ok{

fmt.Printf("list[%d]isanintanditsvalueis%d\n",index,value)

}elseifvalue,ok:=element.(string);ok{

fmt.Printf("list[%d]isastringanditsvalueis%s\n",index,value)

}elseifvalue,ok:=element.(Person);ok{

fmt.Printf("list[%d]isaPersonanditsvalueis%s\n",index,value)

}else{

fmt.Printf("list[%d]isofadifferenttype\n",index)

It'squiteeasytousethispattern,butifwehavemanytypestotest,we'dbetteruseswitch.

switchtest

Let'suseswitchtorewritetheaboveexample.

interface

packagemain

import(

"strconv"

typeElementinterface{}

typeList[]Element

typePersonstruct{

namestring

ageint

func(pPerson)String()string{

return"(name:"+p.name+"-age:"+strconv.Itoa(p.age)+"years)"

funcmain(){

list:=make(List,3)

list[0]=1//anint

list[1]="Hello"//astring

list[2]=Person{"Dennis",70}

forindex,element:=rangelist{

switchvalue:=element.(type){

caseint:

fmt.Printf("list[%d]isanintanditsvalueis%d\n",index,value)

casestring:

fmt.Printf("list[%d]isastringanditsvalueis%s\n",index,value)

casePerson:

fmt.Printf("list[%d]isaPersonanditsvalueis%s\n",index,value)

default:

fmt.Println("list[%d]isofadifferenttype",index)

Onethingyoushouldrememberisthatelement.(type)cannotbeusedoutsideoftheswitchbody,whichmeansinthatcaseyouhavetousethecomma-okpattern.

Embeddedinterfaces

ThemostbeautifulthingisthatGohasalotofbuilt-inlogicsyntax,suchasanonymousfieldsinstruct.Notsuprisingly,wecanuseinterfacesasanonymousfieldsaswell,butwecallthemEmbeddedinterfaces.Here,wefollowthesamerulesasanonymousfields.More

interface

specifically,ifaninterfacehasanotherinterfaceembeddedwithinit,itwillbehaveasifithasallthemethodsthattheembeddedinterfacehas.

Wecanseethatthesourcefileincontainer/heaphasthefollowingdefinition:

typeInterfaceinterface{

sort.Interface//embeddedsort.Interface

Push(xinterface{})//aPushmethodtopushelementsintotheheap

Pop()interface{}//aPopmethodthatpopselementsfromtheheap

Weseethatsort.Interfaceisanembeddedinterface,sotheaboveInterfacehasthethreemethodscontainedwithinthesort.Interfaceimplicitly.

typeInterfaceinterface{

//Lenisthenumberofelementsinthecollection.

Len()int

//Lessreturnswhethertheelementwithindexishouldsort

//beforetheelementwithindexj.

Less(i,jint)bool

//Swapswapstheelementswithindexesiandj.

Swap(i,jint)

Anotherexampleistheio.ReadWriterinpackageio.

//io.ReadWriter

typeReadWriterinterface{

Reader

Writer

Reflection

ReflectioninGoisusedfordetermininginformationatruntime.Weusethereflectpackage,andthisofficialarticleexplainshowreflectworksinGo.

Therearethreestepsinvolvedwhenusingreflect.First,weneedtoconvertaninterfacetoreflecttypes(reflect.Typeorreflect.Value,thisdependsonthesituation).

t:=reflect.TypeOf(i)//getmeta-dataintypei,andusettogetallelements

v:=reflect.ValueOf(i)//getactualvalueintypei,andusevtochangeitsvalue

Afterthat,wecanconvertthereflectedtypestogetthevaluesthatweneed.

interface

varxfloat64=3.4

v:=reflect.ValueOf(x)

fmt.Println("type:",v.Type())

fmt.Println("kindisfloat64:",v.Kind()==reflect.Float64)

fmt.Println("value:",v.Float())

Finally,ifwewanttochangethevaluesofthereflectedtypes,weneedtomakeitmodifiable.Asdiscussedearlier,thereisadifferencebetweenpassbyvalueandpassbyreference.Thefollowingcodewillnotcompile.

varxfloat64=3.4

v:=reflect.ValueOf(x)

v.SetFloat(7.1)

Instead,wemustusethefollowingcodetochangethevaluesfromreflecttypes.

varxfloat64=3.4

p:=reflect.ValueOf(&x)

v:=p.Elem()

v.SetFloat(7.1)

Wehavejustdiscussedthebasicsofreflection,howeveryoumustpracticemoreinordertounderstandmore.

interface

ConcurrencyItissaidthatGoistheClanguageofthe21stcentury.Ithinktherearetworeasons:first,Goisasimplelanguage;second,concurrencyisahottopicintoday'sworld,andGosupportsthisfeatureatthelanguagelevel.

goroutinegoroutinesandconcurrencyarebuiltintothecoredesignofGo.They'resimilartothreadsbutworkdifferently.Morethanadozengoroutinesmaybeonlyhave5or6underlyingthreads.Goalsogivesyoufullsupporttosharingmemoryinyourgoroutines.Onegoroutineusuallyuses4~5KBofstackmemory.Therefore,it'snothardtorunthousandsofgoroutinesonasinglecomputer.Agoroutineismorelightweight,moreefficientandmoreconvenientthansystemthreads.

goroutinesrunonthethreadmanageratruntimeinGo.Weusethegokeywordtocreateanewgoroutine,whichisafunctionattheunderlyinglevel(main()isagoroutine).

gohello(a,b,c)

Let'sseeanexample.

packagemain

import(

"runtime"

funcsay(sstring){

fori:=0;i<5;i++{

runtime.Gosched()

fmt.Println(s)

funcmain(){

gosay("world")//createanewgoroutine

say("hello")//currentgoroutine

Output：

Concurrency

Weseethatit'sveryeasytouseconcurrencyinGobyusingthekeywordgo.Intheaboveexample,thesetwogoroutinessharesomememory,butwewouldbetterofffollowingthedesignrecipe:Don'tuseshareddatatocommunicate,usecommunicationtosharedata.

runtime.Gosched()meanslettheCPUexecuteothergoroutines,andcomebackatsomepoint.

Thescheduleronlyusesonethreadtorunallgoroutines,whichmeansitonlyimplementsconcurrency.IfyouwanttousemoreCPUcoresinordertotakeadvantageofparallelprocessing,youhavetocallruntime.GOMAXPROCS(n)tosetthenumberofcoresyouwanttouse.Ifn<1,itchangesnothing.Thisfunctionmayberemovedinthefuture,seemoredetailsaboutparallelprocessingandconcurrencyinthisarticle.

channelsgoroutinesruninthesamememoryaddressspace,soyouhavetomaintainsynchronizationwhenyouwanttoaccesssharedmemory.Howdoyoucommunicatebetweendifferentgoroutines?Gousesaverygoodcommunicationmechanismcalledchannel.channelislikeatwo-waypipelineinUnixshells:usechanneltosendorreceivedata.Theonlydatatypethatcanbeusedinchannelsisthetypechannelandthekeywordchan.Beawarethatyouhavetousemaketocreateanewchannel.

ci:=make(chanint)

cs:=make(chanstring)

cf:=make(chaninterface{})

channelusestheoperator<-tosendorreceivedata.

ch<-v//sendvtochannelch.

v:=<-ch//receivedatafromch,andassigntov

Let'sseemoreexamples.

Concurrency

packagemain

import"fmt"

funcsum(a[]int,cchanint){

total:=0

for_,v:=rangea{

total+=v

c<-total//sendtotaltoc

funcmain(){

a:=[]int{7,2,8,-9,4,0}

c:=make(chanint)

gosum(a[:len(a)/2],c)

gosum(a[len(a)/2:],c)

x,y:=<-c,<-c//receivefromc

fmt.Println(x,y,x+y)

Sendingandreceivingdatainchannelsblocksbydefault,soit'smucheasiertousesynchronousgoroutines.WhatImeanbyblockisthatagoroutinewillnotcontinuewhenreceivingdatafromanemptychannel,i.e(value:=<-ch),untilothergoroutinessenddatatothischannel.Ontheotherhand,thegoroutinewillnotcontinueuntilthedataitsendstoachannel,i.e(ch<-5),isreceived.

BufferedchannelsIintroducednon-bufferedchannelsabove.Goalsohasbufferedchannelsthatcanstoremorethanasingleelement.Forexample,ch:=make(chanbool,4),herewecreateachannelthatcanstore4booleanelements.Sointhischannel,weareabletosend4elementsintoitwithoutblocking,butthegoroutinewillbeblockedwhenyoutrytosendafifthelementandnogoroutinereceivesit.

ch:=make(chantype,n)

n==0!non-buffer（block）

n>0!buffer（non-blockuntilnelementsinthechannel）

Youcantrythefollowingcodeonyourcomputerandchangesomevalues.

Concurrency

packagemain

import"fmt"

funcmain(){

c:=make(chanint,2)//change2to1willhaveruntimeerror,but3isfine

fmt.Println(<-c)

RangeandCloseWecanuserangetooperateonbufferchannelsasinsliceandmap.

packagemain

import(

funcfibonacci(nint,cchanint){

x,y:=1,1

fori:=0;i<n;i++{

x,y=y,x+y

close(c)

funcmain(){

c:=make(chanint,10)

gofibonacci(cap(c),c)

fori:=rangec{

fmt.Println(i)

fori:=rangecwillnotstopreadingdatafromchanneluntilthechannelisclosed.Weusethekeywordclosetoclosethechannelinaboveexample.It'simpossibletosendorreceivedataonaclosedchannel;youcanusev,ok:=<-chtotestifachannelisclosed.Ifokreturnsfalse,itmeansthethereisnodatainthatchannelanditwasclosed.

Remembertoalwaysclosechannelsinproducersandnotinconsumers,orit'sveryeasytogetintopanicstatus.

Concurrency

Anotherthingyouneedtorememberisthatchannelsarenotlikefiles.Youdon'thavetoclosethemfrequentlyunlessyouaresurethechanneliscompletelyuseless,oryouwanttoexitrangeloops.

SelectIntheaboveexamples,weonlyuseonechannel,buthowcanwedealwithmorethanonechannel?Gohasakeywordcalledselecttolistentomanychannels.

selectisblockingbydefaultanditcontinuestoexecuteonlywhenoneofchannelshasdatatosendorreceive.Ifseveralchannelsarereadytouseatthesametime,selectchooseswhichtoexecuterandomly.

packagemain

import"fmt"

funcfibonacci(c,quitchanint){

x,y:=1,1

select{

casec<-x:

x,y=y,x+y

case<-quit:

fmt.Println("quit")

return

funcmain(){

c:=make(chanint)

quit:=make(chanint)

gofunc(){

fori:=0;i<10;i++{

fmt.Println(<-c)

quit<-0

fibonacci(c,quit)

selecthasadefaultcaseaswell,justlikeswitch.Whenallthechannelsarenotreadyforuse,itexecutesthedefaultcase(itdoesn'twaitforthechannelanymore).

Concurrency

select{

casei:=<-c:

//usei

default:

//executesherewhencisblocked

TimeoutSometimesagoroutinebecomesblocked.Howcanweavoidthistopreventthewholeprogramfromblocking?It'ssimple,wecansetatimeoutintheselect.

funcmain(){

c:=make(chanint)

o:=make(chanbool)

gofunc(){

select{

casev:=<-c:

println(v)

case<-time.After(5*time.Second):

println("timeout")

o<-true

RuntimegoroutineThepackageruntimehassomefunctionsfordealingwithgoroutines.

runtime.Goexit()

Exitsthecurrentgoroutine,butdeferedfunctionswillbeexecutedasusual.

runtime.Gosched()

Letstheschedulerexecuteothergoroutinesandcomesbackatsomepoint.

runtime.NumCPU()int

ReturnsthenumberofCPUcores

Concurrency

runtime.NumGoroutine()int

Returnsthenumberofgoroutines

runtime.GOMAXPROCS(nint)int

SetshowmanyCPUcoresyouwanttouse

Concurrency

2.8SummaryInthischapter,wemainlyintroducedthe25Gokeywords.Let'sreviewwhattheyareandwhattheydo.

breakdefaultfuncinterfaceselect

casedefergomapstruct

chanelsegotopackageswitch

constfallthroughifrangetype

continueforimportreturnvar

varandconstareusedtodefinevariablesandconstants.packageandimportareforpackageuse.funcisusedtodefinefunctionsandmethods.returnisusedtoreturnvaluesinfunctionsormethods.deferisusedtodefinedeferfunctions.goisusedtostartanewgoroutine.selectisusedtoswitchovermultiplechannelsforcommunication.interfaceisusedtodefineinterfaces.structisusedtodefinespecialcustomizedtypes.break,case,continue,for,fallthrough,else,if,switch,gotoanddefaultwereintroducedinsection2.3.chanisthetypeofchannelforcommunicationamonggoroutines.typeisusedtodefinecustomizedtypes.mapisusedtodefinemapwhichissimilartohashtablesinotherlanguages.rangeisusedforreadingdatafromslice,mapandchannel.

Ifyouunderstandhowtousethese25keywords,you'velearnedalotofGoalready.

Summary

GoProgrammingBasics

Workspace

$GOPATHand$GOROOT$GOROOT:ThisisanenvironmentvariablewhichstoresthepathwhereyourGoinstallationispresentifyouhavecustomizedit.$GOPATH:TheGouniverseforyourmachine.TheideaisthatallyourGocodeshouldresideinadirectorytreesothecodeisn'tlyingaroundinrandomplaces.

InmostunixsystemsitisdonewithexportGOPATH=/usr/home/suraj/Go.

My$GOPATHis/usr/home/suraj/Go,ithasthefollowingstructure

pkg:Thelibrarieswhich'llbeimportedinourcode,thereisa.afilecreatedattherespectivepath.bin:ThebinaryfileofourprojectwillbeinstalledhereoncallingGoinstallintheprojectfoldersrc:Willholdtheactualcode,ifyouhaveagithubaccount,thenyoucancreateafoldertreeinsidelike

srcgithub.com

thewhitetulipwsharepicsort

golang.netsourcegraph.com

bin(binaries)wsharepicsort

GotakesthisuniqueapproachsothatalltheGocodeiswellorganizedandnotthrowninahaphazardmanner.Thismakesiteasytolocatecodeforhumansandsoftwarealike.

Packages

General

Packages,inGo,arejustoneormorefile(s)whichcontainsGosourcecode.Itcanbenamedanything,butitneedstobeainafolderandeachfileinthepackageshouldhavethelinepackage<name>atthetop.Thefoldernamehastobeexactlysameasthe<name>wespecifiyineachpackagesourcefile.

Therecanbeanynumberoffilesinapackagedirectory,butonlyonefilewiththemainpackage.Whilebuildingthecode,thecompilerstartswiththemainpackage.

MVCPatterns

WhenIstartedlearningbuildingwebappsinDjango,IfirstreadabouttheMVCpattern,themodels,theviewsandwhatnot.ButthatknowledgewastobeassumedbymeandIhadnoideawhythatdecisionwasmade,alongwiththatDjangoworkslikemagic,onehastoadapttoit'squirks.HencewhileprogramminginGo,it'llbeimmenselybenefitialtogrowuptotheMVCpatternbystartingoutsmall.Thefirstappshouldentirelyresideinthemain.gofile,thenslowly,astheappgrows,itneedstobestructuredwell,sotheallthehandlersgointoaviewspackage,templatesgoinatemplatesfolder,databaserelatedentitiesgoinanotherpackage.

Learningbydoingandexperimentingisthebestway.Challengethestatusquo.

PackagenamingTheactualpackagenameisjust"views"or"views",thepackageisfoundoutbythecompilerfromit'sabsolutepathfromthe$GOPATH/srcdirectory.FortheTasksapp,itresidesin$GOPATH/src/github/thewhitetulip/Tasks,thus,mypackageswillliewithintheTasksfolder.Thismakesiteasyfordistributingthelibraries.Theoretically,onecancreateapackagedirectlyinthe$GOPATH/srcfoldertoskipoutthelongnamingconventions,butdoingsobreaksthecodedistributionviaversioncontrol.

Whenpackagesareresolved,bytheGocompiler,itfirstlooksinthestandardlibraryandthenstartslookingforthefullnameinthe$GOPATH/srcfolder.

Internaldeployment

We'llfollowthestandardpracticeandputourcodein$GOPATH/src/github.com/thewhitetulip/Tasksfolder.Whiletestingourapp,weneedadeploymentversion.IuseedTaskswhielbuildingTasks,Ihavemadeafolder~/TasksandhereIkeepthedeploymentversionofTasks,whichcontainsthebinaryversionandthestaticfiles.Everynewbuildinthesrcgetspushedinthisfolder.

General

Runningaserver

WedeploywebappsonIPaddressesonspecificports.Typicallywechoosealargeportnumberlike8000soitdoesn'taffectsomeotherapplications.Certainportsarerestrictedandneedsudoaccess,like80.

TheIPaddresswebindtheservertocanbeapublicorprivate.Publicmeansusing":8080"or"0.0.0.0:8080".Privatemeans"127.0.0.1:8080"

Publicmeansanycomputerinthesamenetworkcanaccessthewebapp.Privatemeansonlyyourcomputerwillbeabletoaccessit.

//Public

log.Fatal(http.ListenAndServe(":8080",nil))

//Private

log.Fatal(http.ListenAndServe("127.0.0.1:8080",nil))

Note:127.0.0.1

127.0.0.1,calledlocalhost,isaspecialIPaddresswhichisgiventoeverymachinetorefertoitself.itdoesn'tmatterifyouareconnectedtothenetworkornot,yourmachinewillalwayshavethisIPaddress,soifyouarewantprivacyuse127.0.0.1.

Ifyoudowantyourwebapplicationtobeaccessiblethenuse0.0.0.0.0:8080orjust:8080,whenyougivejusttheportnumber,theGolanguageassumestheIPaddressthemachineisonbutnot127.0.0.1.

General

WebProgrammingBasicsWebserversareprogramswhichgetanHTTPrequestandsendbackanHTTPresponsetorequests,theformatoftherequestistheHTTPprotocol.

TheGolanguagehashttpsupportinit'sstandardlibraryasnet/http.

Initially,HTTPwasbuiltfortransferringplaintext,lateritallowedmultimediacontenttoo.

Whenwetypewww.github.comonourbrowser'saddressbar

1. Browseraddseitherhttps://orhttp://andatrailingforwardslash2. OurrequestbecomesHTTPGET/github.com.3. WesendouttheHTTPGETrequest4. Github'sserverswillprocesstherequestandsendbackaresponse.5. Ourbrowserwillrenderthepage.

BeforeAJAXwasinvented,youhadtorefreshthepageeachtime.WithAJAX,wecanmodifythepage'scontentswithoutrefresingthepage.

HTTPMethods

HTTPhasvariousmethodslikeGET,POST,DELETE,PUT.

GET:usedtoretrievetheURL,GET/willgetthehomepage.POST:usedtocreatedatastoredontheURL.PUT:usedtoupdatedataontheURL.DELETE:usedtodeletedataintheURL.

//Createanewcategory.

//POST/categories

//Updateanexistingcategory.

//PUT/categories/12

//Viewthedetailsofacategory.

//GET/categories/12

//Deleteanexistingcategory.

//DELETE/categories/12

Thinkofcategoriesasadocument,POSTtocreateit,GETtofetchit,PUTtoupdateit,andDELETEtodeleteit.Fordeletingatask,ratherthansendingGET/delete/1234,weshouldsendaDELETE/tasks/1234.Thus,ratherthanoverusingtheGETmethodforeverything,

wecanreallyusecontextbasedonthemethodoftherequest.

GETvsPOST

Apartfromtheirfunctionaldifferences,GETandPOSTdifferinsecurityperspectives.

GETtransfersdataviatheURL.POSTsendsdataintherequest'sbodyorpayload,butthatisn'thiddenorencryptedbydefault,butitisn'tvisibleontheURL,itiseasilyaccessibletoanyonewhoknowshowtoreadaHTTPrequest.

Securityissomethingyoubuildyourapplicationaround.Inshortthereisn'tmuchdifferencebetweenGETandPOSTwhenweconsidersecurity,bothtransferdatainplaintextGETisjustrelativelyalittlelesssecuresinceURLsareloggedbyaproxyserver/firewall/browserhistoryandthatGETrequestscanbedonebythebrowseronbehalfoftheuserwithoutconfirmation.Botsarecommonovertheinternet,botscanvisitrandomlytoeverylinkpresentinyourapplication,buttheydon'tsendrandomdatatoanyFormyouhave,orifso,veryfewbotscandothat.

Forprotectingdataofthewebapp,onehastosticktousingHTTPSandsanitizeanydatathatcomesfromtheuser.

Example

Ablogconsistsofacollectionofposts,aposthastags,iswrittenbysomeauthor,atsometimeandhassomeprimarykeytouniquelyidentifyitinourdatabaseandithasaslugwhichmeanstheURL.

Thisistheeraofsemanticweb,thusthenewbeautifulURLslike,surajblog.com/posts/welcome-the-new-year,theslugisthewelcome-the-new-year.

WhentheservergetsaHTTPGETrequestof/posts/welcome-the-new-year,it'llsearchforURLhandlersstartingwiththelistofURLhandlerswehavegiven,thenit'llfindtheclosestmatch,inourcaseit'llbe/post/,thenit'llcallthehandlerofthisURL.

Our/rootURLshouldbeattheverybottomofourlist.Becausewhileexecuting,checksaredonefromtoptobottom.

//samplehandlerdefinition

http.HandleFunc("/post/",ShowPostBySlug)

http.HandleFunc("/",ShowAllPosts)

Handlertalktothedatabase,fetchthedataandrendertemplateswhichshowupasHTMLpagesinourbrowser.

Whatisatemplate?

Templatesareawaytopresentdatatotheuser.TheserverpopulatedthetemplatesandsendstheHTMLpagebacktothebrowser.Forablog,itdoesn'tmakesensetogeneratehtmlpageforeachpost.Thisiswhythereisaposttemplateandtheserverwillgetallthedetailslikecontent,title,datepublishedandpopulatetheposttemplateandreturnitbacktothebrowser.

AwebapplicationisbasicallyawayofrepresentingdatastoredinthedatabasetotheenduserusingHTTP.

Writingawebapplication:

1. Fixthedatabasestructure.2. UnderstandhowdataflowsanddecidetheURLs3. Writetemplatestocorrespondingto(almost)eachURL4. WritefunctionsinGotohandleeachURLpattern,calledhandlers.5. Handlersfetchdatafromthedatabaseandpopulatedatainthetemplates.

Notabusingtemplates

Thelogicbehindcreatingtemplateswastonottorepeatourhtmlpage.Templatesupportvariableswhichourhandlersaregoingtopopulate.Thestandardwayistohandlethebusinesslogininsidethehandlerandusetemplatesjustforrenderingthedata.Templatesaretobeusedonlyforthepresentationlogicnotthebusinesslogic.Itbecomesdifficulttomaintainapplicationswhichhavebusinesslogicinsidethetemplate.Itshouldneverbedone.

Example:Wearegoingtobuildatodolistmanagerinthisbook.Itsupportmultipleusers.

Wrongway:Fetchalltasksinthetemplateandonlyshowthoseofthecurrentuser.i.e.filterthetasksinthetemplateCorrectway:Fetchonlythetasksbelongingtothecurrentuser.i.e.filterthetasksinthehandler.

FunctionalityofourEditTaskURLwhichis/edit/<id>.

fileviews/addViews.go

//EditTaskFuncisourhandlerwhichwillhandlethe/edit/<id>URL

funcEditTaskFunc(whttp.ResponseWriter,r*http.Request){

//Code

task:=db.GetTaskByID(id)

editTemplate.Execute(w,task)

//Code

filedb/tasks.go

funcGetTaskByID(idint)types.Context{

//Codetofetchtasksofthecurrentuser

context:=types.Context{Tasks:tasks}

returncontext

TheEditTaskFunctalkstothedatabasewiththeGetTaskByIDfunctionandfetchesthetasksforthecurrentuserandpopulatestheeditTemplate.

Thuswecansplitanapplicationintoviews,database,templatesandcontroller(mainpackage).

StaticFiles

Staticfilesarerequiredfortemplates,theyarealltheCSS/JS/Imageswhichweloadintoourhtmltemplates.

TheURLwillbe/static/.

Execution:

1. Wegetarequestlike/static/<filepath>2. Wegotothepublicdirectoryofourapplicationandlookfor3. Ifwegetafileofthatpaththenweservethefile,othewisesenda404error.

Thepublicfoldercontainsallyourstaticfiles.Wewillhaveatemplatesfolderonthesamefolderwherethepublicispresent.

Thereasontemplatesisaseparatefolderisthatitisaseparateentityandshouldn'tbepubliclyavailableusingthe/static/URL.

public

||--static

|||--css

|||`--styles.css

..andmore

||`--js

|||--bootstrap.min.js

||....andmore

templates

||--completed.html

||...andmore

NoteOutput

Theaboveoutputisofthetreeprogram.

BasicwebapplicationMakeafolderinyour$GOPATH/src/github.com/<yourname>/Taskssubstituteyourusernameinlieuof<yourname>.

Createafilemain.go

filemain.go

packagemain

import(

"net/http"

funcmain(){

PORT:="127.0.0.1:8080"

log.Fatal(http.ListenAndServe(PORT,nil))

Weimportthehttppackageinourapplicationwithimportnet/http.

Nowgotoyourterminalandtype

[Tasks]$gorunmain.go

Youwillnoticethattheprogramdoesn'tprintanythingbecausewetolditonlytolistenontheport.Ifwewanttousertoknowthatwearerunningaserveronthatport,weshouldprintamessagesayingso,asshowninthebelowexample.

codeexamplefile:3.1basicServer.go

Implementation

packagemain

import(

"net/http"

funcmain(){

PORT:=":8080"

log.Print("Runningserveron"+PORT)

Wheneverwerunthis,we'llgetthebelowoutput

2016/01/0122:00:36Runningserveron:8080

Openlocalhost:8080inabrowserandyou'llgetthemessage"404pagenotfound"

Thisisbecauseasofnowwehavejuststartedaservertolistenontheport8080,butwehaven'thandledtheURL.TheHTTP404errorwillbedisplayedeachtimetheserverwon'tbeabletofigureouthowtoservetherequest.

HandlingURLs

http.HandleFunc("/complete/",ShowCompleteTasksFunc)

//ShowCompleteTasksFuncisusedtopopulatethe"/completed/"URL

funcShowCompleteTasksFunc(whttp.ResponseWriter,r*http.Request){

WeHandleFuncinthenet/httppackagetohandleURLs.ThefirstargumentistheURLtobehandledandthesecondparameteristhefunction.Wecandefinefunctionsinthesecondparameter,butyouareadvisedagainstit.

Thehandlerfunctionrequirestwoarguments,aResponseWriterobjectandaRequestobject.WearegoingtowritetotheResponseWriterdependingonwhatwegetintheRequestobject.

HandlerExample

WewanttowritetheURLthattheuserisvisitingonourwebapp.TheURLcanbefoundintherequestobject,r.URL.Path.

Implementation

packagemain

import(

"net/http"

funcmain(){

PORT:=":8080"

log.Print("Runningserveron"+PORT)

http.HandleFunc("/",CompleteTaskFunc)

funcCompleteTaskFunc(whttp.ResponseWriter,r*http.Request){

w.Write([]byte(r.URL.Path))

Parameterizedrouting

WhenwegettheURL/tasks/124,wewanttogetthetasknumber124,wedothefollowing

//GetTaskFuncisusedtodeleteatask,

funcGetTaskFunc(whttp.ResponseWriter,r*http.Request){

ifr.Method=="GET"{

id:=r.URL.Path[len("/tasks/"):]

w.Write([]byte("Getthetask"+id))

WetakeasubstringoftheURLandremovethe/delete/partandwehavetheIDofthetask.

Thisexamplemakesuseoftheslicingconcept.Itissimple,ifPathisourstringvariablethenPath[1:]isthesubstringwhichincludeseverythingfromthefirstcharacter,indexbeingzero.

Note:Parameterizedrouting

IdeallyweshouldcheckingtheURLpathinsideourview,wearesupposedtousearouter.Forrealprojects,youshouldusearouter.Herewearelearning,sowewon'tbeusingarouter.

Servingstaticfiles

http.Handle("/static/",http.FileServer(http.Dir("public")))

Implementation

Forservingstaticfiles,weusetheFileServermethodofthehttppackage.Ittakesafolderasanargument.Makesureyougiveonlythepublicfolderpathintheargument.

Homework

Readthedocumentationofnet/http&logandgettoknowofthemethods/functionsinthepackages[1]FindouthowmanyalternativesaretheretoListenAndServe.Writeahandlertoservestaticfiles,iffileisavailableonthatpaththenitshouldservethefile,otherwiseitmustreturnanHTTP404error.Writeacommandlineapplicationtosharefiles/folderoverHTTP,thesyntaxshouldbelikethis./wshare-ffile.pdffileonlink=192.168.2.1:8080.

Footnotes

[1]:Learnhowtoreaddocumentation,itsavesalotoftime.

Implementation

DesigningourwebappThedesignphaseisthemostimportantinanysoftwareprojectasonesmallmistakeindesigningcoststheprojectserverly.Itbecomesverycostlytofixthatlater.

Ourwebappisgoingtobeatodolistmanagerforasingleuser,soit'llhavethefollowingfeatures:

1.Abilitytoadd/delete/modifytask.

2.Abilitytomarktasksascomplete/incomplete.

3.Displaythetasksasaonecolumnlist.

4.Abilitytoswitchmodesbetweenpending/completed/deletedtasks

5.Displaynotificationslikenoteadded/deleted/archived

6.Searchfortextandhighlightthetextinthesearchresultspage.

TheDesign

TranslatingourdesigntoAPI,wegetthefollowing.

/add/POST=addnewtask

/GET=showpendingtasks

/complete/GET=showcompletedtasks

/deleted/GET=showdeletedtasks

/edit/<id>POST=editpost

/edit/<id>GET=showtheeditpage

/trash/<id>POST=trashposttorecyclebin

/delete/<id>POST=permanentlydeletepost

/complete/<id>POST=markpostascomplete

/login/POST=dothelogin

/login/GET=showloginpage

/logout/POST=logtheuserout

/restore/<id>POST=restorethattask

/update/<id>POST=updatetask

/change/GET=willallowchangingpassword

/register/GET=showtheregisterpage

/register/POST=willaddentriesintodatabase

file~/main/main.go

Designingourwebapp

packagemain

import(

"net/http"

funcmain(){

http.HandleFunc("/complete/",CompleteTaskFunc)

http.HandleFunc("/delete/",DeleteTaskFunc)

http.HandleFunc("/deleted/",ShowTrashTaskFunc)

http.HandleFunc("/trash/",TrashTaskFunc)

http.HandleFunc("/edit/",EditTaskFunc)

http.HandleFunc("/completed/",ShowCompleteTasksFunc)

http.HandleFunc("/restore/",RestoreTaskFunc)

http.HandleFunc("/add/",AddTaskFunc)

http.HandleFunc("/update/",UpdateTaskFunc)

http.HandleFunc("/search/",SearchTaskFunc)

http.HandleFunc("/login",GetLogin)

http.HandleFunc("/register",PostRegister)

http.HandleFunc("/admin",HandleAdmin)

http.HandleFunc("/add_user",PostAddUser)

http.HandleFunc("/change",PostChange)

http.HandleFunc("/logout",HandleLogout)

http.HandleFunc("/",ShowAllTasksFunc)

http.Handle("/static/",http.FileServer(http.Dir("public")))

log.Print("runningonport8080")

Createallfunctionswementionedaboveandmakethenecessarychangesasperonedefinitionthatweshowbelowinthisfile.

funcShowAllTasksFunc(whttp.ResponseWriter,r*http.Request){

ifr.Method=="GET"{

message:="allpendingtasksGET"

}else{

message:="allpendingtasksPOST"

w.Write([]byte(message))

Afteryoucreatethesefunctionsruntheserverasbelow,

[Tasks]$gobuild

[Tasks]$./Tasks

Designingourwebapp

Inyourbrowsertypelocalhost:8080andtypealltheseURLsandseewhatmessageyouget.

Homework

Checkthedocumentationforhttp.ResponseWriterandhttp.Requestobjectsandgettoknowallthevariables/functions/constantsforhttppackageandthesetwowhichwementioned

Designingourwebapp

UsingdatabasesinGoGodoesn'tprovideoutoftheboxsupportforanydatabase,butitprovidesaninterface,whichcanbeusedbydatabaselibrarycreatorstokeepallthedatabaselibrariescompatiblewitheachother.

Wewillusesqliteforthisbook.

Creatingandconfiguringdatabase

UsethisDDLtocreateatableinourdatabase

[Tasks]$sqlite3tasks.db

SQLiteversion3.8.22013-12-0614:53:30

Enter".help"forinstructions

EnterSQLstatementsterminatedwitha";"

sqlite>CREATETABLEtask(

idintegerprimarykeyautoincrement,

titlevarchar(100),

contenttext,

is_deletedchar(1)default'N',

created_datetimestamp,

last_modified_attimestamp,

finish_datetimestamp

Usethefollowinginsertstatementstoenterdatainourtable,sowe'llbeginreadingdatainourShowAllTasksfunctionwhichwewroteinthepreviouschapter

DatabaseHandling

INSERTINTO"task"VALUES(1,'Publishongithub',

'Publishthesourceoftasksandpicsortongithub',

'N','2015-11-1215:30:59','2015-11-2114:19:22',

'2015-11-1717:02:18');

INSERTINTO"task"VALUES(4,'gofmtall',

'Theideaistorungofmt-wfile.goonevery

gofileinthelisting,*Editturnsoutthisisisdifficult

todoingolang**Editbarely3linebashscript:P'

,'N','2015-11-1216:58:31',

'2015-11-1410:42:14','2015-11-1313:16:48');

INSERTINTO"task"VALUES(7,'modificationstotask',

'1.addsearchfunction#Done

2.apriorityfeaturetotask

3.commentsontasks

4.Duedate

5.removedependencyonhttprouter#Done',

'N','2015-11-1304:23:27','2015-11-1304:23:27',NULL);

Installingthesqlitedriverforgo

We'llusethego-sqlite3drivercreatedbymattn.Thereasonbeingitimplementsthedatabase/sqlinterface.Typethisinyourterminal:

goget"github.com/mattn/go-sqlite3"

Accessingdatabaseingo

Weimportthelibraryasimport_"github.com/mattn/go-sqlite3"

Thepackageisimportedanonymously,whenweimportnet/httpwehavetousehttp.toaccessthepackage.Tobeabletoswapunderlyingdatabases,wewon'tdothat.Thustheanonymouspackageimport.

Underthehood,thedriverregistersitselftothedatabase/sqlpackage.

NoteThe'_'operator

The_operatorhasauniqueroleingo,itisusedtodenoteanemptyvariable,usedwhenwewanttoignoredata.numsisanarray,whileweloopthrougharraysusingrangeitreturnsa(key,value)pair.hereweignorethekeyandprintthevalue.

DatabaseHandling

for_,value:=rangenums{

fmt.Println(value)

Alsonotethatwecangiveapackagealiasinsteadoftheunderscorecharactershouldwewantanaliasforourpackage.

Everydatabasehasaconnectionmechanism,fileforsqliteandIPaddressforMySQL/Postgres.

Weencapsulateourdbobjectinsideastruct.Wealsoencapsulatethedatabaseactionsasshownbelow

vardatabaseDatabase

//Databaseencapsulatesdatabase

typeDatabasestruct{

db*sql.DB

func(dbDatabase)begin()(tx*sql.Tx){

tx,err:=db.db.Begin()

iferr!=nil{

log.Println(err)

returnnil

returntx

func(dbDatabase)prepare(qstring)(stmt*sql.Stmt){

stmt,err:=db.db.Prepare(q)

iferr!=nil{

log.Println(err)

returnnil

returnstmt

func(dbDatabase)query(qstring,

args...interface{})(rows*sql.Rows){

rows,err:=db.db.Query(q,args...)

iferr!=nil{

log.Println(err)

returnnil

returnrows

funcinit(){

database.db,err=

DatabaseHandling

sql.Open("sqlite3","./newtask.db")

iferr!=nil{

log.Fatal(err)

//Closedatabaseconnection

funcClose(){

database.db.Close()

//taskQueryencapsulatesExec()

functaskQuery(sqlstring,args...interface{})error{

SQL:=database.prepare(sql)

tx:=database.begin()

_,err=tx.Stmt(SQL).Exec(args...)

iferr!=nil{

log.Println("taskQuery:",err)

tx.Rollback()

}else{

tx.Commit()

returnerr

Noteinit()

Theinitfunctionisthefirstfunctiontorunwhenthepackageisimportedorexecuted.Thisiswhywedotheinitializationinit.

NoteDBfromgodoc

typeDBstruct{

//containsfilteredorunexportedfields

DBisadatabasehandlerepresentingapoolofzeroormoreunderlyingconnections.It'ssafeforconcurrentusebymultiplegoroutines.

Thesqlpackagecreatesandfreesconnectionsautomatically;italsomaintainsafreepoolofidleconnections.Ifthedatabasehasaconceptofper-connectionstate,suchstatecanonlybereliablyobservedwithinatransaction.Wedonotwantthisvariabletobeaccessiblefromoutsidethepackagesoitisunexported.Openmayjustvalidateitsargumentswithoutcreatingaconnectiontothedatabase.Toverifythatthedatasourcenameisvalid,callPing().ThereturnedDBissafeforconcurrentusebymultiplegoroutinesandmaintainsitsownpoolofidleconnections.Thus,theOpenfunctionshouldbecalledjustonce.ItisrarelynecessarytocloseaDB.

DatabaseHandling

err=db.Ping()

iferr!=nil{

//dosomethingaboutit

Thesql.DBobjectshoudn'tbeopenedandclosedfrequently,itshouldbeclosedonlywhenwenolongerneedtoaccessthedatabase.

Pleaserefertothedocumentationofdatabase/sqlformoredetails.

Noteexportedandunexportedvariables

Exported:variablestartswithacapitalletterandisaccessibleforthosewhoimportthispackageUnexported:variableisprivateforthepackage,notaccessibleoutsideit'sdeclaration.

Queryingthedatabase

Statementsthatdon’treturnrowsshouldnotuseQueryfunctions;theyshoulduseExec().

Fromthegodocumentation

func(*DB)Exec

func(db*DB)Exec(querystring,args...interface{})(Result,error)

Execexecutesaquerywithoutreturninganyrows.Theargsareforany

placeholderparametersinthequery.

WeusetheQuerymethodtoquerythedatabasewhenweexpectsomeresultfromthedatabase.

DatabaseHandling

getTaskSQL="selectid,title,content,created_datefromtask

wherefinish_dateisnullandis_deleted='N'orderbycreated_dateasc"

rows,err:=database.Query(getTaskSQL)

iferr!=nil{

log.Println(err)

deferrows.Close()

forrows.Next(){

err:=rows.Scan(&TaskID,&TaskTitle,&TaskContent,&TaskCreated)

TaskContent=strings.Replace(TaskContent,"\n","<br>",-1)

iferr!=nil{

log.Println(err)

fmt.Println(TaskID,TaskTitle,TaskContent,TaskCreated)

taskSQL:="deletefromtask"

tx:=database.begin()

_,err=tx.Stmt(SQL).Exec(args...)

iferr!=nil{

tx.Rollback()

}else{

tx.Commit()

Notedeferkeyword

Weusedeferinsideafunctioncall.

packagemain

import(

"io/ioutil"

funcmain(){

file,err:=os.Open('file.dat')

iferr!=nil{

fmt.Println("Filedoesn'texistoryoudon'thave

readpermission")

deferfile.Close()

inputReader:=bufio.NewReader(file)

//dosomethingaboutinputReader

DatabaseHandling

Thedeferstatementputsthefunctioncallatthebottomofthecallstack,sowheneverthefunctionreturns,deferistriggered.Onehastobecarefulwithusingdefer,itcancausedifficulttofindbugs.

file~/main/main.go

Findandfixthebug:

packagemain

import(

_"github.com/mattn/go-sqlite3"

vardatabase*sql.DB

funcinit(){

deferdatabase.Close()

database,err=sql.Open("sqlite3","./tasks.db")

iferr!=nil{

fmt.Println(err)

//intentionalbugexists,fixit

funcmain(){

getTaskSQL="selectid,title,content,created_datefromtask

wherefinish_dateisnullandis_deleted='N'orderbycreated_dateasc"

rows,err:=database.Query(getTaskSQL)

iferr!=nil{

fmt.Println(err)

deferrows.Close()

forrows.Next(){

TaskContent=strings.Replace(TaskContent,"\n","<br>",-1)

iferr!=nil{

fmt.Println(err)

fmt.Println(TaskID,TaskTitle,TaskContent,TaskCreated)

err=rows.Err()

iferr!=nil{

log.Fatal(err)

Alwaysdeferrows.Close(),tofreethedatabaseconnectioninthepool.Solongasrowscontainstheresultset,thedatabaseconnectionisinuseandnotavailableintheconnectionpool.

DatabaseHandling

Whentherows.Next()functionreturnsEOF(EndofFile),whichmeansthatithasreachedtheendofrecords,it'llcallrows.Close()foryou,Close()canbecalledmultipletimeswithoutsideeffects.

Single-RowQueries

Ifaqueryreturnsatmostonerow,youcanuseashortcut:

vartaskDescriptionstring

query:="selecttaskDescriptionfromtaskwhereid=?"

err=db.QueryRow(query,1).Scan(&taskDescription)

iferr!=nil{

log.Fatal(err)

fmt.Println(taskDescription)

ErrorsfromthequeryaredeferreduntilScan()iscalled,andthenarereturnedfromthat.YoucanalsocallQueryRow()onapreparedstatement:

query:="selecttaskDescriptionfromtaskwhereid=?"

stmt,err:=db.Prepare(query,1).Scan(&taskDescription)

iferr!=nil{

log.Fatal(err)

vartaskDescriptionstring

err=stmt.QueryRow(1).Scan(&taskDescription)

iferr!=nil{

log.Fatal(err)

fmt.Println(taskDescription)

Writingdataintothedatabase

Abovefunctionsfetcheddatafromthedatabase,transactionsaretobeusedtowritedatainthedatabase.

Belowliesanexampleofusingtransaction

filedb/db.go

DatabaseHandling

//RestoreTaskisusedtorestoretasksfromtheTrash

funcRestoreTask(idint)error{

query:="updatetasksetis_deleted='N',last_modified_at=datetime()whereid=?"

restoreSQL,err:=database.Prepare(query)

iferr!=nil{

fmt.Println(err)

tx,err:=database.Begin()

iferr!=nil{

fmt.Println(err)

_,err=tx.Stmt(restoreSQL).Exec(id)

iferr!=nil{

fmt.Println("doingrollback")

tx.Rollback()

}else{

tx.Commit()

returnerr

Prepare

Preparecreatesapreparedstatementforlaterqueriesorexecutions.Multiplequeriesorexecutionsmayberunconcurrentlyfromthereturnedstatement.Thecallermustcallthestatement'sClosemethodwhenthestatementisnolongerneeded.

NoteErrorhandling

BecauseofGo'suniquemethodofhandlingerrors,wecanbestuckintheiferr!=nilland.But,ontheotherhanditsimplifieserrorhandlingaswejusthavetoreturntheerrormessage.

AnexcellentresourceaboutGoanddatabasescanbefoundathttp://go-database-sql.org

Thefaultinourcode:

Fixingtheintentionalbugintheabovecode:

funcinit(){

deferdatabase.Close()

iferr!=nil{

fmt.Println(err)

DatabaseHandling

Homework

Seethe/code/chapter-4/4.5databaseinourcoderepositoryandmodifythefiletoinsertdatafromthe4.3formsuploadfolder.Wehavetwoworkingcodeset,oneofprintingformvaluesontheconsoleandoneoffetchingdbvaluesandrenderingatemplate.Whatyouhavetodoisbasedonthischapter,writemethodstoinsertvaluesfromtheformtothedatabase.

DatabaseHandling

AnExampleExpectedoutput:

openyourtask.dbfileinsqlite3likethis

[Tasks]$sqlite3task.db

sqlite>selecttitlefromtasklimit1;

Publishongithub

Nowthisoutputshouldmatchwiththeoneweseeatlocalhost:8080

Afterrunningthefile,gotolocalhost:8080andlocalhost:8080/add

file~/main/main.go

packagemain

import(

"database/sql"

_"github.com/mattn/go-sqlite3"

"net/http"

"time"

vardatabase*sql.DB

varerrerror

//Taskisthestructusedtoidentifytasks

typeTaskstruct{

Titlestring

Contentstring

Createdstring

//Contextisthestructpassedtotemplates

typeContextstruct{

Tasks[]Task

Navigationstring

Searchstring

Messagestring

funcinit(){

WebappExample

iferr!=nil{

fmt.Println(err)

funcmain(){

http.HandleFunc("/",ShowAllTasksFunc)

http.HandleFunc("/add/",AddTaskFunc)

fmt.Println("runningon8080")

//ShowAllTasksFuncisusedtohandlethe"/"URLwhichisthedefaultone

ifr.Method=="GET"{

context:=GetTasks()//truewhenyouwantnondeletednotes

w.Write([]byte(context.Tasks[0].Title))

}else{

http.Redirect(w,r,"/",http.StatusFound)

funcGetTasks()Context{

vartask[]Task

varcontextContext

varTaskIDint

varTaskTitlestring

varTaskContentstring

varTaskCreatedtime.Time

vargetTasksqlstring

getTasksql="selectid,title,content,created_datefromtask;"

rows,err:=database.Query(getTasksql)

iferr!=nil{

fmt.Println(err)

deferrows.Close()

forrows.Next(){

iferr!=nil{

fmt.Println(err)

TaskCreated=TaskCreated.Local()

a:=Task{Id:TaskID,Title:TaskTitle,Content:TaskContent,

Created:TaskCreated.Format(time.UnixDate)[0:20]}

task=append(task,a)

context=Context{Tasks:task}

returncontext

WebappExample

//AddTaskFuncisusedtohandletheadditionofnewtask,"/add"URL

funcAddTaskFunc(whttp.ResponseWriter,r*http.Request){

title:="randomtitle"

content:="randomcontent"

truth:=AddTask(title,content)

iftruth!=nil{

log.Fatal("Erroraddingtask")

w.Write([]byte("Addedtask"))

//AddTaskisusedtoaddthetaskinthedatabase

funcAddTask(title,contentstring)error{

query:="insertintotask(title,content,created_date,last_modified_at)\

values(?,?,datetime(),datetime())"

restoreSQL,err:=database.Prepare(query)

iferr!=nil{

fmt.Println(err)

_,err=tx.Stmt(restoreSQL).Exec(title,content)

iferr!=nil{

fmt.Println(err)

tx.Rollback()

}else{

log.Print("insertsuccessful")

tx.Commit()

returnerr

HomeworkThehomeworkistosplitthecodeintopackagesandgetittowork,thetypedefinitiongoesintothetypes/types.gofile,thehandlerdefinitiongoesintotheviews/views.go,thedatabasereadandwritemethodsgointothedb/db.go.Makesurethatafteryourefactorthecode,thatthecoderuns.

WebappExample

WorkingwithFormsHTMLformsareusedtogetdatafromtheuser.FormscanuseboththeGETandPOSTmethodsfortransferringdatatotheserver,butitisrecommendedtouseHTTPPOSTmethodjustbecauseitdoesn'thighlightdataintheURLandbecauseofthemanythingswediscussedinthechapterWebProgrammingBasics.

Formsarerenderedbytemplating,whichwe'llseeinalaterchapter.Asofnowwewanttounderstandhowtogetdatafromtheenduserbyusingforms.

Therearetwopartsofworkingwithforms,theHTMLpartandtheGopart.TheHTMLpagegetsthedataandsendsittotheserverasaPOST/GETandtheGopartwillparsetheformtodosometasklikelettingtheusertologinorinsertingdatainthedatabase.

Eachformelementhasanamewhichisreferencedintheserversidepartoftheform,belowwehavethefileuploadandadropdownlist.Bothofwhichhaveauniquename.

<formaction="/add/"method="POST">

<divclass="form-group">

<inputtype="text"name="title"class="form-control"id="add-note-title"place

holder="Title"

style="border:none;border-bottom:1pxsolidgray;box-shadow:none;">

</div>

<divclass="form-group">

<textareaclass="form-control"name="content"id="add-note-content"placeholde

r="Content"

rows="10"style="border:none;border-bottom:1pxsolidgray;box-shadow:none;"><

/textarea>

File:<inputtype="file"name="uploadfile"/><br>

Priority:<selectname="priority">

<optionvalue="3">High</option>

<optionvalue="2">Medium</option>

<optionvalue="1">Low</option>

</select>

</div>

<divclass="modal-footer">

<buttontype="button"class="btnbtn-default"data-dismiss="modal">Close</butt

<inputtype="submit"text="submit"class="btnbtn-default"/>

</div>

</form>

Formhandling

WhenwearepopulatingthisHTMLpage,wecanalsopopulateitdynamically,withoutgettingboggeddownbysyntax,ignoretemplatingforthewhileWehaveavariablecalledNavigationandonecalledCategories,weloopthroughCategoriesandiftheNavigationisequaltothatcategorythenthecheckedvalueistrue.

Category:

<selectname="category"class="dropdown">

<optionvalue="{{$cat.Name}}"{{ifeq$cat.Name$navigation}}checked="checke

d"{{end}}>{{$cat.Name}}</option>

</select>

Herewehaveusedadropdownbox,youcanuseradiobuttonslikebelow

<inputtype="radio"name="gender"value="1">Female

<inputtype="radio"name="gender"value="2">Male

<inputtype="radio"name="gender"value="3">Other

Orcheckboxes

<inputtype="checkbox"name="gender"value="female">Female

<inputtype="checkbox"name="gender"value="male">Male

<inputtype="checkbox"name="gender"value="other">Other

Wegetthevalueofadropdownbox/radiobutton/checkboxontheserversidebyusingthenamefieldlikebelow:

value:=r.Form.Get("gender")

value:=r.FormValue("gender")

Aswesawearlier,ourwebserversbasicallytakeaHTTPRequestobjectandreturnanHTTPResponseObject,belowisanexampleofasampleHTTPRequestobject.

ThehostistheIPaddresssendingthereq,UserAgent:fingerprintofthemachine,theAccept-fieldsdefinevariouspartslikethelanguage,encodingRefereriswhichIPmadethecall,CookieisthevalueofthecookiestoredonthesystemandConnectionisthetypeofconnection.

Inthisrequestsnapshot,wealsohaveafilewhichweupload,forfileupload,thecontenttypeismultipart/form-data

Formhandling

RequestHeader

Host:127.0.0.1:8081

User-Agent:...

Accept:text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8

Accept-Language:en-US,en;q=0.5

Accept-Encoding:gzip,deflate

Referer:http://127.0.0.1:8081/

Cookie:csrftoken=abcd

Connection:keep-alive

RequestBody

Content-Type:multipart/form-data;

boundary=---------------------------6299264802312704731507948053

Content-Length:15031

-----------------------------6299264802312704731507948053

Content-Disposition:form-data;name="title"

workingwithforms

-----------------------------6299264802312704731507948053

Content-Disposition:form-data;name="CSRFToken"

-----------------------------6299264802312704731507948053

Content-Disposition:form-data;name="content"

finishthechapterworkingwithforms

-----------------------------6299264802312704731507948053

Content-Disposition:form-data;name="uploadfile";filename="2.4workingwithform.md"

Content-Type:text/x-markdown

--filecontent--

-----------------------------6299264802312704731507948053

Content-Disposition:form-data;name="priority"

-----------------------------6299264802312704731507948053--

IfyouhadwonderedhowGoogle'shomepageshowsapopupwhenyouvisitgoogle.comonIEofFirefox,itchecksyourUser-Agent.ThethingwithHTTPrequestisthattheycanbemodifiedtoanyextent,theChromedevelopertoolsgivesyouquitesophisticatedtoolstomodifyyourrequests,evenUserAgentspoofingisadefaultfeatureavaiable,thisfeatureisavailablesowecantestourwebappsinonewindowsimulatingmanyinternetbrowsersatonego.

Formhandling

Thebasicpartofworkingwithformsistoidentifywhichuserthatparticularformbelongsto,therearewaystoattainthat,wecaneitherhaveastatefulorastatelesswebserver.

Astatelessserverdoesn'tstoresessions,itrequiresanauthenticationkeyforeachrequestwhileastatefulserverstoressessions.Forstoringsessionsacookieisused,whichisafilewhichisstoredintheprivatememoryofthewebbrowserwhichweuse.Onlythewebsitewhichcreatedthecookiecanaccessthecookie,nothirdpartywebsitescanaccessthecookies,buttheOSusercanread/edit/deletecookiesusingthewebbrowser.

CSRFstandsforCrossRequestSiteForgery.AnywebsitecansendaPOSTrequesttoyourwebserver,whosenttherequestcanbefoundintheRefererfieldofyourHTTPresponse.Itisaformofconfuseddeputyattackinwhichthedeputyisyourwebbrowser.Amalicioususerdoesn'thavedirectaccesstoyourwebsite,soitmakesuseofyourbrowsertosendamaliciousrequest.Typicallycookiesenableyourbrowsertoauthenticateitselftoawebserver,sowhatthesemaliciouswebsitesdois,theysendinaHTTPrequestonbehalfofyourbrowser.

Wecanthwartthisattackbyrestrictingthereferertoyourowndomain,butitisquiteeasytomanipulatethemisspeltrefererfieldofaHTTPrequest.

Anotherwayistousetokens.Whilerenderingourform,wesendinahiddenfieldwithcryptogeneratedstringof256characters,sowhenweprocessthePOSTrequest,wefirstcheckifthetokenisvalidornotandthendecideifthedatacamefromagenuinesourceorfromamalicioussource.Itdoesn'thavetobemaliciousactually,evenifalegitimateusertriedtotrickyourwebserverintoacceptingdata,weshouldn'tentertainit.

Tocheckthecsrftoken,weservethetokentotheformandstoreitinacookie,whenwegetthePOSTrequest,wecheckifbothareequalornot.Thisisbecauseamaliciouspersonmighttrickausertoclickonaformbuttheycan'tsetcookiesforyourwebapplication.

Apointtonotehereisthatnevertrustuserdata.Alwaysclean/sanitizedatawhichyougetfromtheenduser.

NoteJavascript

Ifyouareseriousaboutwebdevelopment,yououghttolearnJavascriptindetail.Whilebuildingawebapp,therewillbetimeswhenyouwouldwanttoimprovetheUIofyourapplication,whichwouldmeanachangeinthehtmlpage.UsingJSisinevitablewhilebuildingbeautifulwebapps,whileaddingsomenewhtmlfeature,openthe"webinspector"presentinthedevelopertoolsanddynamicallyaddthehtmlcode.ThewebinspectorallowsustomanipulatetheCSSandHTMLpart.Nowopenthejavascriptconsole,thatenables

Formhandling

youtotesttheJSfeaturewhichyouarewillingtoadd.Forinstance,inthetasksapplication,therewasnoprovisiontoexpand/contractthesizeofthetask,soIaddedabuttoninthewebinspector,

<buttonclass="toggle"></button>

IntheJSconsole,totogglethevisibilityofmy.noteContentfield,Ididthis:

$('.toggle').next().toggle()

Thisprovedthatitworks,sonowgotoyourtemplateandactuallyaddthecode.Makesurethehtmliscorrectbecausewhilerunning,thehtmlfilesareparsedonce,soforanyhtmlchange,youhavetorunthewebappforeachHTMLchange.Sooncethehtmlissetup,ifyouchangetheJS/CSSthenyoujusthavetorefreshthepage,becausethehtmlpagegetstheJS/CSSeachtimethepageisloaded.

Aswesawintheaboveparagraph,forpreventingCSRF,weneedtogenerateatokenandsendasahiddenfieldintheformandstoreitinacookie,whenwegetthePOSTrequestfromthe

FormsinGo

Inthebelowfunctionwesetthecookie,wefirstgenerateaCSRFtoken,which'llbeuniqueforeachHTTPrequestwhichwegetandstoreitinacookie.

//ShowAllTasksFuncisusedtohandlethe"/"URLwhichisthedefaultons

ifr.Method=="GET"{

context:=db.GetTasks("pending")//truewhenyouwantnondeletednotes

ifmessage!=""{

context.Message=message

context.CSRFToken="abcd"

message=""

expiration:=time.Now().Add(365*24*time.Hour)

cookie:=http.Cookie{Name:"csrftoken",Value:"abcd",Expires:expiration}

http.SetCookie(w,&cookie)

homeTemplate.Execute(w,context)

}else{

message="Methodnotallowed"

Formhandling

ThebelowhandlerhandlesthePOSTrequestsentbyourform,itfetchesthevalueofthecsrftokencookieandgetsthevalueofthehiddenCSRFTokenfieldoftheaddtaskform.Ifthevalueofthecookieisequaltothevaluefetchedbytheform,thenweallowittogotothedatabase.

ThecalltoParseFormwillparsethecontentsoftheformintoGettablefieldswhichwecanfetchusingtheGetfunction.Thiscalliscompulsory.

//AddTaskFuncisusedtohandletheadditionofnewtask,"/add"URL

funcAddTaskFunc(whttp.ResponseWriter,r*http.Request){

ifr.Method=="POST"{

r.ParseForm()

file,handler,err:=r.FormFile("uploadfile")

iferr!=nil{

log.Println(err)

taskPriority,priorityErr:=strconv.Atoi(r.FormValue("priority"))

ifpriorityErr!=nil{

log.Print("unabletoconvertprioritytointeger")

priorityList:=[]int{1,2,3}

for_,priority:=rangepriorityList{

iftaskPriority!=priority{

log.Println("incorrectprioritysent")

//mightwanttologassecurityincident

taskPriority=1//thisdefaultstheprioritytolow

title:=template.HTMLEscapeString(r.Form.Get("title"))

content:=template.HTMLEscapeString(r.Form.Get("content"))

formToken:=template.HTMLEscapeString(r.Form.Get("CSRFToken"))

cookie,_:=r.Cookie("csrftoken")

ifformToken==cookie.Value{

ifhandler!=nil{

r.ParseMultipartForm(32<<20)//definedmaximumsizeoffile

deferfile.Close()

f,err:=os.OpenFile("./files/"+handler.Filename,os.O_WRONLY|os.O_CR

EATE,0666)

iferr!=nil{

log.Println(err)

return

deferf.Close()

io.Copy(f,file)

filelink:=

"<br><ahref=/files/"+handler.Filename+">"+handler.Filename+"

content=content+filelink

Formhandling

truth:=db.AddTask(title,content,taskPriority)

iftruth!=nil{

message="Erroraddingtask"

log.Println("erroraddingtasktodb")

}else{

message="Taskadded"

log.Println("addedtasktodb")

}else{

log.Fatal("CSRFmismatch")

}else{

NoteCookies

Cookieisawaytostoredataonthebrowser,HTTPisastatelessprotocol,itwasn'tbuiltforsessions,basicallytheInternetitselfwasn'tbuiltconsideringsecurityinmindsinceinitiallyitwasjustawaytosharedocumentsonline,henceHTTPisstateless,whenthewebserverreceivesrequests,itcan'tdistinguishbetweentwoconsequitiverequests,hencetheconceptofcookieswereadded,thuswhilestartingasession,wegenerateasessionIDandstoreitonthedatabaseinmemoryoronthedatabaseandwestorethesameIDonacookieonthewebbrowserandwevalidatebothofthemtoauthenticatethem.

Wehavetonotethat,ifwesetanexpirydateforacookie,thenitisstoredonthefilesystemotherwiseitisstoredinmemoryinthebrowser.Intheincognitomode,thisisthecase,allthecookiesarestoredinmemoryandnotinthefilesystem.

HTTPRequest

Host:localhost:8080

User-Agent:.....

Accept:text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8

Accept-Language:en-US,en;q=0.5

Accept-Encoding:gzip,deflate

Referer:http://localhost:8080/

Cookie:csrftoken=abcd

Connection:keep-alive

Cache-Control:max-age=0

Formhandling

Thebrowser,whilesendingaresponseappendsallthecookiedatastoredinitsmemoryorfilealongwiththeotheraspectsoftheHTTPrequestsowecanaccessthecookieasr.Cookie,it'llcontaineverycookieforthatparticulardomain,we'dthenloopthroughittofetchthedatawhichwewant.Thisisimportantforsecurityreasons,supposeIsetacookieonmyimaginedomain.comandifitcontainsacsrftokenandifthatcookieisaccessibletosomeotherwebappthenitishorriblesincetheycanmasqueradeasanylegitmateuser.Butthisain'tpossible,sinceawebsitecanonlyaccessthecookiestoredforitsowndomain,pluswehavethefeaturetosetHTTPonlyvalueofacookieastrue,whichsaysthatevenjavascriptcan'taccessthecookies.

HTTPResponse

Content-Type:text/html;charset=utf-8

Date:Tue,12Jan201616:43:53GMT

Set-Cookie:csrftoken=abcd;Expires=Wed,11Jan201716:43:53GMT

Transfer-Encoding:chunked

Whenwesetcookies,wewritethentotheHTTPresponsewhichwesendtotheclientandthebrowserreadstheCookieinformationandstorestheminthememoryorthefilesystemdependingontheExpiresfieldoftheresponse.

Fromthegodocumentation:

typeCookiestruct{

Namestring

Valuestring

Pathstring//optional

Domainstring//optional

Expirestime.Time//optional

RawExpiresstring//forreadingcookiesonly

//MaxAge=0meansno'Max-Age'attributespecified.

//MaxAge<0meansdeletecookienow,equivalently'Max-Age:0'

//MaxAge>0meansMax-Ageattributepresentandgiveninseconds

MaxAgeint

Securebool

HttpOnlybool

Rawstring

Unparsed[]string//Rawtextofunparsedattribute-valuepairs

InputValidation

Formhandling

Thebasicaspectofwebapplicationsisthatnodatacanbetrusted,eveniftheuserisn'tmaliciousherself,therearemanywaystotrickthebrowserintosendingHTTPrequestsandfoolingthewebservertorespondtowhatseemslikealegitimaterequest.Hencewehavetoverifyeverythingthatcomesfromtheuser.

Onemightargueherethatwedoallsortsofvalidationusingjavascript,buttherearewaystoevadeJSvalidations,thesimplestwayistodisableJSandmoresophisticatedwaysaretomanipulatetheHTTPrequestbeforethebrowsersendsit,itisliterallytrivialwhenweusejustthewebdevelopertoolsthatthesedaysbrowsersprovide.

ifformToken==cookie.Valueandtitle!=nilandcontent!=nil

Thetitleandcontentofthetaskismandatory,henceweaddedthenotnilpart.

Wedoinputvalidationwhenwedon'twantjunkdatatogointoourdatabase,supposetheuserhitthesubmitbuttontwice,orsomeotherscenario.Butwealsohavetoconsiderthecasewheretheuserwantstorunsomescriptonourwebsite,whichisdangerous,soweusethetemplate.HTMLEscapeStringmethodtoescapewhatmightruninmemoryofthecurrentbrowsersession.Eventhedatawhichcomesfromyourdropdownlistshouldbevalidated.

EverythinginawebformissentviaaRequestaswesawintheaboveexample,weuseadropdownlistwhenwehaveareexpectingaparticularsetofinputsbutforsomeonewhoknowswhatfirefoxdevtoolsare,caneasilymodifyanythingintherequest,forexamplewehavethepriorityasdropdownlistwemightthinkthatsincewehaveonlythreeentriesinthedropdownlist,shouldwekeepacheckinourviewhandlertonotacceptanythingbutthethreevalueswhicharepresentinourtemplate.Weoughttokeepachecklikebelow:

priorityList:=[]int{1,2,3}

for_,priority:=rangepriorityList{

iftaskPriority!=priority{

log.Println("incorrectprioritysent")

//mightwanttologassecurityincident

Thisisthepriorityfieldofourrequest

Content-Disposition:form-data;name="priority"

Formhandling

Allamalicioususerhastodoischangethisvalueandresendtherequest,theycanliterallyinsertanyvaluehere,justthinkwhatiftheysendrm-fr*.*andaccidentallyenoughthiscommandisexecutedonourserver.Thisalsobringsinanotheraspectofsecurity,neverrunyourmachineinrootmode,alwayskeeptherootmodeforadmintasksanduseanonrootmode.Evenifthatisn'tthecase,alessdangerousexamplewillbesendingahugenumberwiththerequest,assumingthatwehaveusedintegerasourvariable,theprogrammightcrashifithastohandleanumberbeyonditsstoragecapacity.Thismightbetermedasadenialofserviceattack.

Formhandling

UploadingfilesUploadingfilesisthenextstepinformprocessing,incaseoffiles,wesendtheentirefiledataintheHTTPheader,sowehavetosettheformencodingtoenctype="multipart/form-data".Thiswillinformourserverthatwearegoingtogetafilefromtheformalongwiththerestofthefields,ifany.

Thismeanswecangeteithereitherfile(s)anddataorjustfile(s)orjustdataandnofile(s).

AtthefirstlineofourHTTPhandler,wehavetowritethisline,ifthislineisnotpresentinthefirstlinethenitgivesunexpectedresults

file,handler,err:=r.FormFile("uploadfile")

ifhandler!=nil{

deferfile.Close()

f,err:=os.OpenFile("./files/"+handler.Filename,os.O_WRONLY|os.O_CR

EATE,0666)

iferr!=nil{

log.Println(err)

return

deferf.Close()

io.Copy(f,file)

filelink:="<br><ahref=./files/"+handler.Filename+">"+handler.File

name+"</a>"

Wefirstprovidethemaximumsizeofthefilewhichis32^20,whichisgiganticforourwebapp,noteveryonehastheInternetinfrastructuretouploadthatbigafile,butwewanttobeflexible.

Webasicallygetafilefromtheformrequest,intheformhandlerweopenanotherfilewiththesame/differentnameandthenreadthefileformtherequestandwriteitontheserver.Weneedtohandlethescenewherewenamethefiledifferentlysowe'dneedtostoretheoldfilename->newfilenamerelationsomewhere,itcanbeadatabasetable.

Thefilenameshouldbescrubbed,sincetheusercangiveanymaliciousnametodamageourapplication.

Wenowwanttorandomizethefilenameofthefileswhichtheusersupload.InyourAddTaskFuncaddthefollowinglines

UploadingFiles

ifhandler!=nil{

deferfile.Close()

randomFileName:=md5.New()

io.WriteString(randomFileName,strconv.FormatInt(time.Now().Unix(),10))

io.WriteString(randomFileName,handler.Filename)

token:=fmt.Sprintf("%x",randomFileName.Sum(nil))

f,err:=os.OpenFile("./files/"+token,os.O_WRONLY|os.O_CREATE,0666)

iferr!=nil{

log.Println(err)

return

deferf.Close()

io.Copy(f,file)

filelink:="<br><ahref=/files/"+token+">"+handler.Filename+"</a

fileTruth:=db.AddFile(handler.Filename,token)

iffileTruth!=nil{

message="Erroraddingfilenameindb"

log.Println("erroraddingtasktodb")

file~/Tasks/db/db.go

//AddFileisusedtoaddthemd5ofafilenamewhichisuploadedtoourapplicat

//thiswillenableustorandomizetheURLwithoutworryingaboutthefilenames

funcAddFile(fileName,tokenstring)error{

SQL,err:=database.Prepare("insertintofilesvalues(?,?)")

iferr!=nil{

log.Println(err)

iferr!=nil{

log.Println(err)

_,err=tx.Stmt(SQL).Exec(fileName,token)

iferr!=nil{

log.Println(err)

tx.Rollback()

}else{

log.Println(tx.Commit())

returnerr

UploadingFiles

tablestructure

CREATETABLEfiles(namevarchar(1000)notnull,autoNamevarchar(255)notnull);

Theseblockofcodedothefollowingthings:

1. Createaversionofnameforeachuploadedfile2. Insertitindatabase3. Referencethefileas/files/<randomName>4. Fileisnowreferencedbyournameratherthantheusersuppliedname

Thenextparttodoisregisteringthe/files/handler.

file:~/Tasks/views/views.go

//UploadedFileHandlerisusedtohandletheuploadedfilerelatedrequests

funcUploadedFileHandler(whttp.ResponseWriter,r*http.Request){

ifr.Method=="GET"{

log.Println("intothehandler")

token:=r.URL.Path[len("/files/"):]

//file,err:=db.GetFileName(token)

//iferr!=nil{

log.Println("servingfile./files/"+token)

http.ServeFile(w,r,"./files/"+token)

UploadingFiles

Templatespackage:text/template

Inthefirstchapterwehadacursoryglanceovertheconceptoftemplates.Thischapterisdedicatedentirelytotemplates.Aswesaidpreviously,awebapplicationrespondstocertainURLsandgivesanhtmlpagetothebrowserwhichthebrowsertheninterpretsandshowstotheenduser.Thishtmlpagewhichissenttothebrowseriswhatiscalledtemplatesinthebackendforwehaveatemplatewhichstoressomevariables,andinrealtimedataisprovidedintothetemplatewhichmakesitacompletehtmlpage.

Let'stakeapracticalexample.Supposewearebuildingamicrobloggingsite.Wewouldstartwithcreatingthefrontendinhtml.OurmicrobloggingsitewillshowHiUserontherightcorner.

Inourstatichtmlwewritethis<p>HiUser</p>

Butifweservethispageonourwebserverit'llnotchangeanything,it'llshowHiUser,thenameoftheuserwon'tcomemagically,wehavetoputitintothepagesomehow,hereweuseavariablesoinGowe'dapproachthisbyusingavariablenamed{{.Name}}soourhtmlnowwillbe<p>Hi{{.Name}}</p>.The.ismandatory.

Now,thisisthelogicweapplytoallourhtmlpages,keepingsuch{{}}variableexpansionparametersandservingthevalueoftheparameterwhileexecutingthetemplate.Ifyouarewonderinghowwe'ddothat,thisisthesyntax

//Contextisthestructpassedtotemplates

typeContextstruct{

Tasks[]Task

Namestring

Searchstring

Messagestring

Thesearethethreepartsofusingtemplates,firstyouneedtocreatetypeslikewehavecreatedtheContexttype,thenweneedtoreadthetemplate,thenweneedtousethecomponentsofthattypeinourtemplatefile.Sowhatremainsnowispassinganobjectofthattypeduringtemplateexecution.

Everytemplaterequiresthecontextobjectbecausethatiswhatdefinesthedatatobepopulatedinthetemplate.

Templates

Readingtemplate:

templates,err=template.Must(template.ParseFiles(allFiles...))

Note:template.Must

Mustisahelperthatwrapsacalltoafunctionreturning(*Template,error)andpanicsiftheerrorisnon-nilWhereallFilesispopulatedasbelow:

varallFiles[]string

templatesDir:="./public/templates/"

files,err:=ioutil.ReadDir(templatesDir)

iferr!=nil{

fmt.Println("Errorreadingtemplatedir")

for_,file:=rangefiles{

filename:=file.Name()

ifstrings.HasSuffix(filename,".html"){

allFiles=append(allFiles,templatesDir+filename)

ForthesakeofdemonstrationofhowtoparsemultiplefileswehaveusedtheParseFilesmethodtoparseallthe.htmlfiles,youcanusetheParseGlobmethodwhichisavailableinthestandardlibrary.

template.Must(template.ParseGlob(templatesDir+"*.html"))

ThedefinitionofParseGlobis:funcParseGlob(patternstring)(*Template,error)

WehavetospecifythePatternfortheParseGlobfunction,butwehavepassedthepathandthepatternbecausejustpassingthepatternisuseless,weneedthepathwherethepatternwillbeappliedtofindthelistofallfilesmeetingthecriteria.

1. ...operator:allFilesisastringsliceandallFiles...passesthefunctionaparameterasastring.

2. ParseFilesperformance:

Thereisonepointtonotehereaboutperformanceinparsingfiles,typicallyatemplatefilewon'tchangeuntilthereissomemajorchangetothecodebasesoweshouldonlyparsethefilesonce,ratherthankeepthiscodeineachviewhandleranddoinga

Templates

template.ParseFiles("home.html")

template.Execute(w,context)

Thisblockofcodewillunnecessarilyreadthehtmlpageeachtimewhileservingtheresponseofarequest,whichmeansiftenpeopleareusingourbloggingsitethenforeachpagetheyvisitthecodewillreadthehtmlpage,butthereisnoneedtodothingsthisway,wecanreadthehtmlfilesonceatthestartandthenusetheLookupmethodofthetemplateclass,

homeTemplate=templates.Lookup("home.html")

SubtemplatingWelearnthowtopassdatatotemplatesanddisplayitinthehtmlpage,itsohappensthatalotofcodeisusedinalltemplatessupposethenavigationbarortheheader,thenweneednotwritethesamechunkeverywhere,wecancreateatemplatetostorethat,andusesubtemplating{{template"_head.html".}}

Here,wehaveidentifiedachunkofcodewhichwewanttoreplicateandputitin_head.html.Thenweputtheabovestatementineachtemplatefilewherewewishtohaveourheader.Thiswaytemplatesbecomealotsmalleranddon'tcontainreplicatedcodeeverywhere.Donotethatthe.beforethefirst}isintentionalanditmeansthatallthevariableswhichwerepassedtothecurrenttemplatearepassedtothesubtemplate.

Thesubtemplateswhichwecreatedependsonourrequirement,butthebasicpointbehinditisthatifwearegoingtorepeatablockofHTMLcodethenweshouldformitasatemplate.

NoteUsingSubTemplates

Themainpointtonoteoverhereisthatwhenwearegoingtouseourtemplatesorsubtemplates,allthosehtmlfilesneedtobeparsed.Thebasicpointintemplatingisthatwehaveavariablewhichstoresalltemplatesevenifwearen'tgoingtorefertothatdirectlyinourcodeusingtheLookupmethodonthetemplatevariable.Thelookupmethodtakesthenameofthetemplate.Whenthe{{template_head.html.}}isevaluated,itgoestoourtemplatevariableandtriestofindoutthetemplateparsedwiththeexactname,ifitisnotpresentthenitdoesn'tcomplainbydefault,weshoulduseMustmethodifwewantittocomplain.

Templates

Examplefileviews/views.go

packageviews

import(

"io/ioutil"

"net/http"

"strconv"

"strings"

"text/template"

homeTemplate*template.Template

deletedTemplate*template.Template

completedTemplate*template.Template

loginTemplate*template.Template

editTemplate*template.Template

searchTemplate*template.Template

templates*template.Template

messagestring

//messagewillstorethemessagetobeshownasnotification

errerror

//PopulateTemplatesisusedtoparsealltemplatespresentin

//thetemplatesfolder

funcPopulateTemplates(){

varallFiles[]string

templatesDir:="./public/templates/"

files,err:=ioutil.ReadDir(templatesDir)

iferr!=nil{

fmt.Println("Errorreadingtemplatedir")

for_,file:=rangefiles{

filename:=file.Name()

ifstrings.HasSuffix(filename,".html"){

allFiles=append(allFiles,templatesDir+filename)

iferr!=nil{

fmt.Println(err)

os.Exit(1)

templates,err=template.Must(template.ParseFiles(allFiles...))

//templates,err:=template.Must(template.ParseGlob(templatesDir+".html"

Templates

iferr!=nil{

fmt.Println(err)

os.Exit(1)

homeTemplate=templates.Lookup("home.html")

deletedTemplate=templates.Lookup("deleted.html")

editTemplate=templates.Lookup("edit.html")

searchTemplate=templates.Lookup("search.html")

completedTemplate=templates.Lookup("completed.html")

loginTemplate=templates.Lookup("login.html")

//ShowAllTasksFuncisusedtohandlethe"/"URL

//TODOaddhttp404error

ifr.Method=="GET"{

context:=db.GetTasks("pending")

//truewhenyouwantnondeletednotes

ifmessage!=""{

context.Message=message

message=""

}else{

Loopingthrougharrays

<divclass="timeline">

<divclass="note">

<pclass="noteHeading">{{.Title}}</p>

<pclass="noteContent">{{.Content}}</p>

</div>

<divclass="note">

<pclass="noteHeading">NoTaskshere</p>

<pclass="notefooter">Createnewtask<button>here</button></p>

</div>

Templates

The{{if.Tasks}}blockchecksifthearrayisemptyornot,ifitisnotthenit'llgotothe{{.range.Tasks}}whichwillloopthroughthearray,thenthe{{.Title}}willaccessthetitleand{{.Content}}willaccesstheContentofthatparticularTaskinstanceandwe'llseeallthetasksasalist.

Templatevariables

Wehavethisscenario,wehavearangeofcategoriesinthenavigationdrawerandifwevisitthe/category/study,thenourcategorynameshouldbehighlightedinthenavigationdrawer.Wedothisbystoringthevalueofthe.Navigationfield-whichtellsifitisaEditpage/Trashpage/Categorypage

<liclass="sidebar-item">

<ahref="/category/{{$cat.Name}}"{{ifeq$cat.Name$nav}}class="active"{{

end}}>

<spanclass="nav-item">{{$cat.Name}}</span><spanclass="badgepull-right

">{{$cat.Count}}</span></a>

Creatingvariables

{{$url:=""}}willcreateablankvariable,onehastonotthatallvariablesarepracticallystringsoncetheyarerendered.{{$url:=.Navigation}}willcreateanewvariableandinitializeitwiththevalueof.Navigation

Forunderstandingwhytemplatevariablesarerequired,weneedtogointotheaboveblockofcode,whenweareusingtherangeoperator,weareparsingthearrayandtherangeblockgetstheelementsoftheblockbydefault.

MyContexttypeis

typeContextstruct{

Tasks[]Task

Navigationstring

Searchstring

Messagestring

CSRFTokenstring

Categories[]CategoryCount

Refererstring

Templates

Hencewhenwedoa{{range.Categories}}wewillbeaccessingeachelementas{{.}}perloop.Ifweuseanyothervalidvariablehere,likethe.Navigation,thentheblocktriestofindthe.Navigationinside.Categories,whichobviouslyisn'tpresent.

Nowweneedtomakethepageawareofwhichcategoryitisshowing,shouldtheusergotothe/categories/page.

ThelogicbehindmakingthatpagecategoryawareisthatwecreateaCSSclasstomarkthatparticularcategoryasactive,butforthat,we'dneedtoaccesstheCategorynameandNavigationwithintherange.Categoriesblock,thuswecreatetwovariables,onetostorethecategorynamefromthe.Navigationvariableandusetheifstatementlike

{{ifeq$cat$nav}}class="active"{{end}}

Thiswillmarkonlythatparticularcategoryasactiveandnotallofthem.

Intemplatinglogictheoperatorisfirstandthenthetwooperands.

eq:equal,le:lessthanequal,ge:greaterthanequal

Youcanalsouseifelseclauselikebelow:

<divclass="navbar-header">

{{if.Search}}<aclass="navbar-brand">Resultsfor:{{.Search}}</a>{{else}}{{i

feq.Navigation"pending"}}

{{$url:=""}}{{elseifeq.Navigation"completed"}}{{$url:=""}}{{elseife

q.Navigation"deleted"}}

{{$url:=""}}{{elseifeq.Navigation"edit"}}{{$url:=""}}{{else}}{{$url:=

"/category"}}{{end}}

<pclass="navbar-brand"href="{{$url}}/{{.Navigation}}">

{{ifeq.Navigation"pending"}}Pending{{elseifeq.Navigation"completed"}

}Completed{{elseifeq.Navigation"deleted"}}Deleted

{{elseifeq.Navigation"edit"}}Edit{{else}}{{.Navigation}}{{end}}{{en

Herewehadsomecomplicatedstuff,ifourpageisasearchone,wehadtoshowResultsfor:<query>,pending,deleted,edit,completedforrespectiveand/category/ifweareinthecategory.SowedefinedanemptyURLandassignedtheURLvaluesaccordingtothecomplicatedifelsestructure.

Homework

Templates

1. Takethehtmlpagesfromhttp://github.com/thewhitetulip/omninoteswebandmodifythemtosuitourpurposesWewouldneedtocreateonetemplateeachfortheoneswementionedintheabovevariabledeclaration,usetemplatingasfaraspossibleandlatercheckyourresultswithhttp://github.com/thewhitetulip/Tasks,pleasedotheexerciseonyourownfirstandthenonlychecktheTasksrepository.

2. Implementasearchinterface.Takeaqueryasinput,searchtasksforthatqueryandreturnanhtmlpagewiththequeryhighlightedintheresultingpage.

Templates

AuthenticationAuthenticationisusedtoverifyiftheusershaveaccesstothatparticularpartofyourwebapplication.Forunderstandinghowtoimplementauthenticationweneedtounderstandwhathappensbehindthescenesofabrowser.Supposewerunabankwebapplication.Wewantonlyourlegitimateuserstoaccessourwebapplication.Wesetupaloginpageandprovideouruserswiththeirusernameandpasswordwhichtheycanusetovalidatetheirclaimtoourwebapp.

Whenwesubmittheloginform,thebrowsertakesourusername,passwordandsendsaPOSTrequesttothewebserver,whichagainrespondswithaHTTPredirectresponseandwearereturnedtoourbankdashboard.

TheHTTPprotocolisstateless,whichmeanseveryrequestisunique.Thereisnowayforidentifyingautomaticallyifarequestisrelatedtoanotherrequest.Thisbringsabouttheproblemofauthentication,howthencanwevalidateiftheusershaveaccesstoourwebapp?

WecansendtheusernamealongwitheachHTTPrequest,eitherintheURLviaaGETrequestorinthePOSTrequest.Butthisisinefficientsinceforeachrequest,thewebserverwouldneedtohitthedatabasetovalidatetheusername,alsothiswouldmeanweaksecuritysinceifIknowyourusername,Icanimpersonateyouprettyeasilyandthewebserverishelplesstoidentifythisimpersonation.

TosolvethisproblemsSessionswereinvented,sessionsneedtousecookiesonthebrowsertofunction.ThebasicideaisthattheservergeneratesasessionIDandstoresitinacookie.Withsubsequentrequests,thebrowserwillsendthesessionIDalongwiththerequest,thewebserverwillthencometoknowfromthatsessionIDiftherequestisafakeoneornot.Alsowegettoknowwhotheuserisfromthat.

Cookies

Cookies,aswesawinapreviouschaptercanbeusedtostoreakey,valuepair.WeusedacookietostoretheCSRFtoken,thecookiehadthenameasCSRFandvalueasthetoken.

Pleasedon'tconfusesessionswithcookies,becausesessionsaren'takey,valuepair.Sessionsareawayofworkingwithcookiesontheserverside.ThereisagapoftheentireInternetbetweensessionsandcookies.

UserAuthentication

Cookiesarestoredinourbrowsers,forsecurityreasonsweneedtoenablethe"isHTTPOnly"fieldofourcookies,soonlyourwebapplicationcanreadthecookie.Otherwiseanyonejavascriptapplicationcaneasilyreadourcookiedefeatingitspurpose,wemightaswellnotkeepanauthenticationmechanismforourwebapp.

FromthegodocumentationtypeCookiestruct{NamestringValuestring

Pathstring//optional

Domainstring//optional

Expirestime.Time//optional

RawExpiresstring//forreadingcookiesonly

//MaxAge=0meansno'Max-Age'attributespecified.

//MaxAge<0meansdeletecookienow,equivalently'Max-Age:0'

//MaxAge>0meansMax-Ageattributepresentandgiveninseconds

MaxAgeint

Securebool

HttpOnlybool

Rawstring

Unparsed[]string//Rawtextofunparsedattribute-valuepairs

Thedomainofourcookieenablesarestrictedaccesstoourcookie.Avisitorgoestoourfictionalbankwebsite,sbank.comandentersausernameandpassword,acookieisstoredinthebrowserwhichonlythesbank.comdomaincanaccesssincewearesecurityawareandwehavesettheHttpOnlyfieldtotruewhilesettingthecookie.Thismeanssomemaliciouswebsitewhichissetupbyanattackertointentionallytargetourbankisn'tabletoaccessthecookieusingjavascript.

Onehastorememberthatcookieisnothingbutafilestoredinauser'sbrowser,ifcanbeaccessedoverHTTPbyourwebserver,aclientbrowserdoesallowaccesstoitthroughbrowsersettingsorcustomjavascript.Thebrowser,afterallisaplatform,andwehaveAPIstothatplatform.

Sessions

Asessionisaseriesofactionsperformedbetweenyouandthewebappyouareacting,enabledbythebrowserandtheInternetyouareusing.Whilegeneratinganewsession,weneedtocheckifasessionsisalreadyactive,ifsowereturnthesamesessionIDratherthancreateanewone,ifnot,wegenerateanewsessionID.SessionIDsneedtobesufficientlyrandom.Ofcoursewecan'tgeneratesomethingtotallyrandom,butwehavetoensuretogeneratesomethingthatnobodyelsecanreplicate,unlesstheyhaveaccesstoourprivatekeywhichweusetogenerateourrandomnumber.

UserAuthentication

Sessionhandlingusinggorilla/sessions

Tillnowweneverusedanythirdpartylibraryoraframeworkinthisbook,thisisforthefirsttimethatwearedoingso,asperthearrogantpeopleonHNwebetteruselibrariesforhandlingsessions,sincesecurityisthe#1aspectofanywebapplication,untilthetimeweareaspecialistinwebdevelopmentandcanwriteourownsessionmoduleitiswiserandsafertousingpre-builtpackages

Path:~/Tasks/sessions/sessions.go

packagesessions

import(

"net/http"

"github.com/gorilla/sessions"

//Storethecookiestorewhichisgoingtostoresessiondatainthecookie

varStore=sessions.NewCookieStore([]byte("secret-password"))

//IsLoggedInwillcheckiftheuserhasanactivesessionandreturnTrue

funcIsLoggedIn(r*http.Request)bool{

session,_:=Store.Get(r,"session")

ifsession.Values["loggedin"]=="true"{

returntrue

returnfalse

Thisisthesessionspackagewhichwewilluseinourapplication.

WecreateaCookieStorewhichstoresthesessionsinformationunderthe"sessions"inthebrowser.WegetthesessionIDstoredunderthesessioncookieandstoreitthesessionvariable.Whenitcomestousingthisfunction,wehavetheAddCommentFuncviewbelowwhichisgoingtohandlethecommentingfeatureofourapplication,it'llfirstcheckiftheuserhasanactivesessionandifso,it'llhandlethePOSTrequesttoaddacomment,ifnot,it'llredirecttheusertotheloginpage.

Path:~/Tasks/Views/addViews.go

UserAuthentication

//AddCommentFuncwillbeused

funcAddCommentFunc(whttp.ResponseWriter,r*http.Request){

ifsessions.IsLoggedIn(r){

ifr.Method=="POST"{

r.ParseForm()

text:=r.Form.Get("commentText")

id:=r.Form.Get("taskID")

idInt,err:=strconv.Atoi(id)

if(err!=nil)||(text==""){

log.Println("unabletoconvertintointeger")

message="Erroraddingcomment"

}else{

err=db.AddComments(idInt,text)

iferr!=nil{

log.Println("unabletoinsertintodb")

message="Commentnotadded"

}else{

message="Commentadded"

}else{

http.Redirect(w,r,"/login",302)

Thebelowfilecontainsthecodetologinandlogoutofourapplication,webasicallyaregoingtosetthe"loggedin"propertyofourcookiestore.

Path:~/Tasks/Views/sessionViews.go

UserAuthentication

packageviews

import(

"net/http"

"github.com/thewhitetulip/Tasks/sessions"

//LogoutFuncImplementsthelogoutfunctionality.

//WIlldeletethesessioninformationfromthecookiestore

funcLogoutFunc(whttp.ResponseWriter,r*http.Request){

session,err:=sessions.Store.Get(r,"session")

iferr==nil{//Ifthereisnoerror,thenremovesession

ifsession.Values["loggedin"]!="false"{

session.Values["loggedin"]="false"

session.Save(r,w)

http.Redirect(w,r,"/login",302)

//redirecttologinirrespectiveoferrorornot

//LoginFuncimplementstheloginfunctionality,will

//addacookietothecookiestoreformanagingauthentication

funcLoginFunc(whttp.ResponseWriter,r*http.Request){

iferr!=nil{

loginTemplate.Execute(w,nil)

//incaseoferrorduring

//fetchingsessioninfo,executelogintemplate

}else{

isLoggedIn:=session.Values["loggedin"]

ifisLoggedIn!="true"{

ifr.Method=="POST"{

ifr.FormValue("password")=="secret"

&&r.FormValue("username")=="user"{

session.Values["loggedin"]="true"

session.Save(r,w)

http.Redirect(w,r,"/",302)

return

}elseifr.Method=="GET"{

}else{

UserAuthentication

Thereisabetterwayofhandlingsessionsusingmiddleware,we'llintroducethatconceptinthenextchapter.

Signingusersup

Theaboveexamplejusthardcodestheusernameandpassword,ofcoursewe'dwantpeopletosignuptoourservice.Wecreateausertable.

CREATETABLEuser(

idintegerprimarykeyautoincrement,

usernamevarchar(100),

passwordvarchar(1000),

emailvarchar(100)

Forthesakeofsimplicity,it'llonlycontainID,username,passwordandemail.

Therearetwopartshere,firstonewhereweallowuserstosignup,andanotherpartwherewereplacethehardcodedusernameandpasswordinourloginlogic.

file:~/Tasks/main.go

http.HandleFunc("/signup/",views.SignUpFunc)

file:~/Tasks/views/sessionViews.go

UserAuthentication

//SignUpFuncwillenablenewuserstosignuptoourservice

funcSignUpFunc(whttp.ResponseWriter,r*http.Request){

ifr.Method=="POST"{

r.ParseForm()

username:=r.Form.Get("username")

password:=r.Form.Get("password")

email:=r.Form.Get("email")

log.Println(username,password,email)

err:=db.CreateUser(username,password,email)

iferr!=nil{

http.Error(w,"Unabletosignuserup",http.StatusInternalServerError)

}else{

http.Redirect(w,r,"/login/",302)

file:~/Tasks/db/user.go

//CreateUserwillcreateanewuser,takeasinputtheparametersand

//insertitintodatabase

funcCreateUser(username,password,emailstring)error{

err:=taskQuery("insertintouser(username,password,email)values(?,?,?)",user

name,password,email)

returnerr

WesawTaskQueryinourchapteronDB,itisasimplewrapperarounddb.Exec().

Note:Inarealwebapp,you'dwanttoencryptthepasswordandnotstoreitinplaintext,thisisadummyappwhich'llneverseethelightofthedaysoIamkeepingitplaintext.

file~/Tasks/views/sessionViews.go

UserAuthentication

//LoginFuncimplementstheloginfunctionality,willaddacookietothecookiestore

formanagingauthentication

funcLoginFunc(whttp.ResponseWriter,r*http.Request){

iferr!=nil{

log.Println("erroridentifyingsession")

return

switchr.Method{

case"GET":

case"POST":

log.Print("InsidePOST")

r.ParseForm()

if(username!=""&&password!="")&&db.ValidUser(username,password){

session.Values["loggedin"]="true"

session.Values["username"]=username

session.Save(r,w)

log.Print("user",username,"isauthenticated")

return

log.Print("Invaliduser"+username)

file~/Tasks/db/user.go

UserAuthentication

//ValidUserwillcheckiftheuserexistsindbandifexistsiftheusernamepassword

//combinationisvalid

funcValidUser(username,passwordstring)bool{

varpasswordFromDBstring

userSQL:="selectpasswordfromuserwhereusername=?"

log.Print("validatinguser",username)

rows:=database.query(userSQL,username)

deferrows.Close()

ifrows.Next(){

err:=rows.Scan(&passwordFromDB)

iferr!=nil{

returnfalse

//Ifthepasswordmatches,returntrue

ifpassword==passwordFromDB{

returntrue

//bydefaultreturnfalse

returnfalse

Sinceweareusinggorilla/sessions,wejusthavetopluginthefunctionalitythatthepackageprovidesanddon'thavetobotherabouttheactualimplementationofsessionshandling,ifyouareinterestedthenbyallmeansgoaheadandcheckoutthesourcecodeofgorilla/sessions!

Wekeepresettingthepasswordasanexercise,formulateamechanismtoresettingthepassword,requiresustocreateausertableandstoresomeprofileinformation,eitheremail-IDfromwherewe'llsendasecuritycodeorbydoingsomethingelse!Brainstorm!

UserAuthentication

FilesJSONandXMLaretwoofthemostcommonwaystotransmitdatabetweenwebapplications.We'lluseJSONforourconfigurationfile.

Forourwebapplicationwehaveasetofconfigurationvaluesliketheserverportwhereourapplicationwillrun.Supposeyouaredevelopingyourapplicationin$GOPATHandalsousingitsomewhereelse,thenyoucan'truntheminparallelbecausebothsourcesusethesameportnumber.Naturallywewantawaytoparameterizethatportnumber.Theparameterorconfigurationvaluelistmaycontainmorethingslikedatabaseconnectioninformation.Asofnowwewilluseaconfig.jsonfileandreadtheserverPortvariableandbindourserveronthatport.

Ourconfigurationfileusesafixedstructure,henceitissimpleenoughtoUnMarshalittoastructtype,wecanusesomeadvanceconceptstoaccomodateunstructuredJSONfiles,becausethatisthewholepointofJSON,wecanhavedatainanunstructuredformat.

NoSQLhasbeenfamouslately,theyarebasicallyJSONdocumentstores.Wehaveprojectslikeboltdbwhichstoredatainakeyvaluepair,ultimatelyinflatfilesorinmemory.

file:$GOPATH/src/github.com/thewhitetulip/Tasks/config/config.go

WorkingwithFiles

packageconfig

import(

"encoding/json"

"io/ioutil"

//Storesthemainconfigurationfortheapplication

//defineastructobjectcontaining

typeConfigurationstruct{

ServerPortstring

varerrerror

varconfigConfiguration

//ReadConfigwillreadtheconfig.jsonfiletoreadtheparameters

//whichwillbepassedintheconfigobject

funcReadConfig(fileNamestring)Configuration{

configFile,err:=ioutil.ReadFile(fileName)

iferr!=nil{

log.Fatal("Unabletoreadlogfile")

//log.Print(configFile)

err=json.Unmarshal(configFile,&config)

iferr!=nil{

log.Print(err)

returnconfig

file:$GOPATH/src/github.com/thewhitetulip/Tasks/config.json

"ServerPort":":8081"

file:$GOPATH/src/github.com/thewhitetulip/Tasks/main.go

values:=config.ReadConfig("config.json")

//valuesistheobjectnow,wecanusethe

//belowstatementtoaccesstheportname

values.ServerPort

WorkingwithFiles

Weusethejson.UnmarshaltoreadtheJSONfileintoourstructureobject.ThisisaverysimpleandbasicexampleofparsingJSONfiles,youcanhavenestedstructuresofmanylevelsinsidethemainconfigobject,butthatisthefeaturesofGo,solongasitcanberepresentedasaJSONdocumentyoucanusetheUnmarshalmethodtotranslatethefileintoanobjectwhichyoucanuseinyourprogram.

Homework

Altertheconfig.jsonfiletotakethenameofthesqlitedatabaseasaconfigurationparameter.ReadabouttheJSONlibraryingodoc

WorkingwithFiles

RoutingTillnowweusedroutingdirectlyinsideofourhandlers.Foralargeapplicationthough,it'dbebettertohavearouterinplace.Wecaneitheruseathirdpartyonewithcountlessfeaturesorthestandardmux.

Asourapplicationmatures,routingplaysabigroleintoit.Weintentionallyavoidedroutingtillnowbecauseasawebdeveloper,wemustunderstandwhathappensinthebackgroundofourapplication.Webframeworksallowustobuildapplicationsquickly,butthedownsideofthemisthattheyprovideuswithaframeworkasthenamesuggests,whichmeansyouaretotallyrestrictedbytheAPIwhichtheframeworkwillprovide.Thusweneedtoknowhowtoimplementbarebonestuff,soinfuturewemightwanttomodifytheframeworkweneed,orrathercreateourownone.

Firstofallwe'dneedtoinstallthehttprouter,doagoget-ugithub.com/julienschmidt/httprouter

Fromthedocumentation

PackagehttprouterisatriebasedhighperformanceHTTPrequestrouter.

Routing

packagemain

import(

"github.com/julienschmidt/httprouter"

"net/http"

funcIndex(whttp.ResponseWriter,r*http.Request,_httprouter.Params){

fmt.Fprint(w,"Welcome!\n")

funcHello(whttp.ResponseWriter,r*http.Request,pshttprouter.Params){

fmt.Fprintf(w,"hello,%s!\n",ps.ByName("name"))

funcmain(){

router:=httprouter.New()//createsanewrouter

router.GET("/",Index)//willdirecttheGET/requesttotheIndexfunction

router.GET("/hello/:name",Hello)//willredirecttheGET/nametoHello,stores

thenameoftheparameter

//intheavariableofhttprouter.Params

log.Fatal(http.ListenAndServe(":8080",router))

httprouterusesacustomHttp.HandleFuncmethodtoaccomodateparameterizedrouting.

Here,wecanrouteourrequestsdependingontheHTTPmethodthroughwhichitisused,andwecanhandleparameterizedroutingforfree.

Tohandlethatscenariowewereinitiallyusingther.URL.Pathvariableandthenextractingtheparameters,itisjustthesolutionoffindingthevariableparameteroftheURL,thereisawideareawherehttprouterisawesome,thereisnoneedtohandlethe/foreachrequests.

Inwebapplicationssometimeshavingaforwardslashisverycritical,theURLindexistotallydifferentfrom/indexandismarginallydifferentfrom/index/httproutertakescareofthetrailingslashes.TheGodefaultMUXrequiresyoutohandletheroutesinthedescendingorder,meaningthemostgenericURLthe"/"shouldbeatthebottomandtheleastgenericshouldbeatthetop,asitgoessequentiallyupanddownmatchingtheURLs,httprouterprovidesawidevarietyofadvantageswhenitcomestorouting.

Thisisbecauseourapplicationshouldseparateroutingfromthehandlerlogic,weweremixingitjusttogetafeelforhowtoprograminGo,buteventuallywhentheappgrows,itistiresomeandnonmaintainablefordoingtheifr.Method==POSTcheckineachhandler,

Routing

ratherthanthatwecanhave3differenthandlersoneforwhenrequestcomesviaAJAX,onefornormalGETandonefornormalPOST.Thiswaywedonothaveonefunctionhandlercheckingthetypeofrequestandthenwritingthreeseparatelogicinsideonefunction.

Butthatdoesn'tmeanwehavetousehttprouter,ifinanapptherearen'tmuchsophisticatedroutingrequiredthenwecanusethedefaultMuxsinceitisgoodenough,butifwehavecomplicatedroutingthenhttprouteristhebest.

Homework

Readthehttprouterdocumentationandsourcecodetogetadeeperunderstandingofrouters,sinceroutersareanintegralpartofanywebapplication.Thenrewriteourapplicationusingthehttprouter.

Routing

MiddlewaresMiddlewareisreallyanythingthatextendsyourwebappinamodularway.Mostcommonexamplesareprobablyparsingtherequestparameters/bodyandstoringtheminaneasily-accessibleformatsoyoudon'thavetodoitineverysinglehandler,orsessionhandlingaswementionedinthepreviouschapter.OtherexamplescouldbethrottlingorIPfiltering,whichwouldalsohappenbeforeyoustartbuildingyourresponse,orcompression,whichwouldhappenafteryou'vebuiltyourresponse.

//RequiresLoginisamiddlewarewhichwillbeusedforeach

//httpHandlertocheckifthereisanyactivesession

funcRequiresLogin(handlerfunc(whttp.ResponseWriter,r*http.Request))

func(whttp.ResponseWriter,r*http.Request){

returnfunc(whttp.ResponseWriter,r*http.Request){

if!sessions.IsLoggedIn(r){

return

handler(w,r)

Theabovefunctioncountsasmiddleware-itdoesn'tknowanythingaboutyourappexcepthowyouhandlesessions.Ifyou'renotloggedin,itredirects,otherwiseitdoesn'tdoanythingandpassesalongtothenexthandler.Thatnexthandlermightbewhereyouactuallybuildyourresponse,oritcouldbeanothermiddlewarecomponentthatdoessomethingelsefirst.

Toknowsomeone'sloggedin,yes,youwanttocreateasessionidentifierandstorethatsomewhereontheserverside(inmemoryoradatabase)andalsosetitintheuser'scookies.YoursessionIDsshouldbesufficientlyrandomandlongthattheycouldn'tbeeasilyguessed.IthinkacommonwayofsatisfyingthatiscreatingaUUIDandthenbase64encodethat.Or,youcouldjustgenerateabunchofrandombytes.

Toknowwhichuserisloggedin,thesessionIDshouldbethekeythatmapstoauserID.So,you'dmakeamapofSessionID=>UserID,orsomethingsimilarinyourdatabase.

Then,beforeeveryrequest,you'd

1. Checkuser'scookiesforSessionID.Ifnone,userisnotloggedin.2. Checkyourstoreforuser'sSessionID.Ifit'snotfound,thenit'sinvalid-userisnot

loggedin.3. Ifyoufoundit,useittolookuptheuser'sID.Userisnowloggedin.

Middleware

4. Nowyou'vegottheuserIDandcanuseitasafilterwhenqueryingyourDBifyouonlywanttoshowthatuser'stasks.

ExampleWithoutmiddleware:

//IsLoggedInwillcheckiftheuserhasanactivesessionandreturnTrue

funcIsLoggedIn(r*http.Request)bool{

session,_:=Store.Get(r,"session")

ifsession.Values["loggedin"]=="true"{

returntrue

returnfalse

//SearchTaskFuncisusedtohandlethe/search/url,handlesthesearchfunction

funcSearchTaskFunc(whttp.ResponseWriter,r*http.Request){

ifsessions.IsLoggedin(r){

ifr.Method=="POST"{

r.ParseForm()

query:=r.Form.Get("query")

context:=db.SearchTask(query)

categories:=db.GetCategories()

context.Categories=categories

searchTemplate.Execute(w,context)

}else{

WithMiddleware:

http.HandleFunc("/",views.RequiresLogin(views.ShowAllTasksFunc))

//SearchTaskFuncisusedtohandlethe/search/url,

//handlesthesearchfunction

funcSearchTaskFunc(whttp.ResponseWriter,r*http.Request){

ifr.Method=="POST"{

r.ParseForm()

query:=r.Form.Get("query")

context:=db.SearchTask(query)

categories:=db.GetCategories()

context.Categories=categories

searchTemplate.Execute(w,context)

Middleware

Thisway,wedonothavetorepeattheifsessions.IsLoggedin()blockineachofourviewwhichrequiresauthentication.Inthisexamplewehaveuseditforsessionhandling,butitcanbeusedforanypurposewhichrequiressomekindofprehandlingofanyview.

Middleware

BuildinganAPIAPIstandsforApplicationProgrammingInterface,itisjustaninterfacetothewebapp.Whenweuseabrowsertoaccessawebapplication,weinteractinHTTPandgetbackHTMLpages,whichthebrowserwillrenderforus.Let'ssaywewanttointeractwithourwebapptogetsomedataoutofitusingahostprogramminglanguagelikeGoorPython.We'dhavetomaintaincookiesinPython,orwriteapythonmoduleasanextensionofabrowsertohandlethisscenario.

Thereisasimplewayoutofthis,weequipourwebapplicationitselftointeractwithanyhostlanguagewhichcantalkinHTTP.Thiswaydeveloperscaneasilygetaccesstooursystem,usingvalidcredentials,ofcourse.

Browser:

1. Wesendtheusername,passwordandgetacookiestoredonourmachine.2. WeusethetokeninthecookieuntilitisvalidtosendHTTPrequests.3. ThebrowserisresponsibleforrenderingtheHTMLpagessentbytheserver.

1. Wesendtheusername,passwordandgetatoken.2. Wesendthistokenineachofourrequesttotheserver

TypicallywesendthetokeninacustomHTTPheadercalledtoken.

Whenweuseabrowser,theserverstoresourinformationasasession,whenwesenditarequest,itisawareofoursession.AwebapptypicallyusescookiestostorethesessionID,whichisusedtoidentifytheuser.Suchaserveriscalledastatefulserver.

WhenwewriteAPIs,theyarestatelessservers,theydonotstoresessionsinformationanywhereontheserver.Toit,eachrequestisunique.Whichiswhy,weneedtopassalongtheauthenticationtokenineachrequest.

Note:Don'tmessaroundwithtokens

Thereareappswhere"singlesignin"featureisavailable,theuserhastologinonlyonceandtheyareloggedinforever,thisisverydangerous.Becauseifamaliciouspersongetstheirhandsonthesecuritytoken,theycansendmaliciousrequestsfordatawhichlookgenuineandareimpossibletoclasifyasmalicious.Don'tdothis,alwayshavesomeexpirationtimeforsecuritytokens,dependsonyourapplicationreally,twohours,sixhours,butneverinfinitehours.

BuildinganAPI

JWTJavascriptWebTokensisastandardforgeneratingtokens.Wewillusethejwt-golibrary.

Letsstartbydefiningourroutes

http.HandleFunc("/api/get-task/",views.GetTasksFuncAPI)

http.HandleFunc("/api/get-deleted-task/",views.GetDeletedTaskFuncAPI)

http.HandleFunc("/api/add-task/",views.AddTaskFuncAPI)

http.HandleFunc("/api/update-task/",views.UpdateTaskFuncAPI)

http.HandleFunc("/api/delete-task/",views.DeleteTaskFuncAPI)

http.HandleFunc("/api/get-token/",views.GetTokenHandler)

http.HandleFunc("/api/get-category/",views.GetCategoryFuncAPI)

http.HandleFunc("/api/add-category/",views.AddCategoryFuncAPI)

http.HandleFunc("/api/update-category/",views.UpdateCategoryFuncAPI)

http.HandleFunc("/api/delete-category/",views.DeleteCategoryFuncAPI)

file:main.go

WewillhavethesameURLsfortheAPI,butit'llstartwith/api/

OurlogicisthatwewillsendtheusernameandpasswordinaPOSTrequestto/api/get-token/thatwillreturnthetokenforus.

file:views/api.go

BuildinganAPI

import"github.com/dgrijalva/jwt-go"

varmySigningKey=[]byte("secret")

//GetTokenHandlerwillgetatokenfortheusernameandpassword

funcGetTokenHandler(whttp.ResponseWriter,r*http.Request){

ifr.Method=="POST"{

//specifythealgorithmtogeneratetoken

token:=jwt.New(jwt.SigningMethodHS256)

r.ParseForm()

ifdb.ValidUser(username,password){

/*Settokenclaimsliketheusernamd

andtheexpirationtime*/

token.Claims["username"]=username

token.Claims["exp"]=time.Now().Add(time.Hour*2).Unix()

/*Signthetokenwithoursecretwhichisaglobal

variableinthesamefile*/

tokenString,_:=token.SignedString(mySigningKey)

/*Finally,writethetokentothebrowserwindow*/

w.Write([]byte(tokenString))

}else{

w.Write([]byte("Authenticationfailed"))

Thenextstepistovalidateatoken.

//ValidateTokenwillvalidatethetoken

funcValidateToken(myTokenstring)(bool,string){

token,err:=jwt.Parse(myToken,func(token*jwt.Token)(interface{},error){

return[]byte(mySigningKey),nil

if(err!=nil)||(!token.Valid){

returnfalse,""

returntoken.Valid,token.Claims["username"].(string)

BuildinganAPI

We'llcalltheParsemethodonthetokenwhichwereceiveasaparameterinthefunctioncall.Thetoken.Validfieldisaboolenvariablewhichistrueifthetokenisvalidandfalseotherwise.

MakinganAPIcallMakinganAPIcallisanalogoustoournormalview.

BuildinganAPI

//GetCategoryFuncAPIwillreturnthecategoriesfortheuser

//dependsontheIDthatweget,ifwegetall,thenreturnall

//categoriesoftheuser

funcGetCategoryFuncAPI(whttp.ResponseWriter,r*http.Request){

ifr.Method=="GET"{

varerrerror

varmessagestring

varstatustypes.Status

//getthecustomHTTPheadercalledToken

token:=r.Header["Token"][0]

w.Header().Set("Content-Type","application/json;charset=UTF-8")

IsTokenValid,username:=ValidateToken(token)

//Whenthetokenisnotvalidshowthe

//defaulterrorJSONdocument

if!IsTokenValid{

status=types.Status

StatusCode:http.StatusInternalServerError,

Message:message

w.WriteHeader(http.StatusInternalServerError)

//thefollowingstatementwillwritetheJSONdocumentto

//theHTTPResponseWriterobject.

err=json.NewEncoder(w).Encode(status)

iferr!=nil{

panic(err)

return

log.Println("tokenisvalid"+username+"isloggedin")

categories:=db.GetCategories(username)

w.Header().Set("Content-Type","application/json;charset=UTF-8")

w.WriteHeader(http.StatusOK)

err=json.NewEncoder(w).Encode(categories)

iferr!=nil{

panic(err)

DuringanAPIcall,wesenddatainJSONformat,forthat,weneedtosetourcontent-typeasapplication/json,bydoingthis,evenawebbrowserwilldetectthatitisgettingaJSONdocument.WhenweneedtowriteaJSONdocumenttotheresponsewriterobject,weuse

BuildinganAPI

thejson.NewEncoder(w).Encode(categories)method,wherecategoriesisourJSONdocument.

FormattingaJSONdocumentThis,below,isourTasksstruct,whichwillbepopulatedasaJSONdocumentwhenwerunourserver.Asyoumightknow,wecan'tuseCapitalletterasthefirstletterinaJSONtitle,byconventiontheyshouldallbesmallletters.Gohasaspecialwayoflettingusdothat.Theexampleisbelow,whenwewritejson:"id",wearetellingGothatthenameofthisfieldinaJSONrenderingshouldbeidandnotId.Thereisanotherspecialsyntaxcalledomitempty,insomeJSOndocumentsyoumightwantsomefieldtonotbedisplayed.Itsohappensthattherearefieldswhichyouwouldwanttodisappearwhentheirvaluesaren'tpresent,theymaynotbeimportantorit'dbetooclunkytohavethemasNULLinallJSONdocuments.

typeTaskstruct{

Idint`json:"id"`

Titlestring`json:"title"`

Contentstring`json:"content"`

Createdstring`json:"created"`

Prioritystring`json:"priority"`

Categorystring`json:"category"`

Refererstring`json:"referer,omitempty"`

Comments[]Comment`json:"comments,omitempty"`

IsOverduebool`json:"isoverdue,omitempty"`

TestingAPIWe'lluseFirefoxandRestClientextensiontotestourAPI.RestClientallowsustosendvariousrequeststoourAPIserver,ifyouareonChrome,POSTmanisthebestalternative.

ForRestClienttosendFormdata,setacustomheaderName:Content-TypeValue:application/x-www-form-urlencoded

Otherwiseyou'llbesendingblankPOSTrequestsallthetime.Theserverneedstounderstandthecontenttypeofthedataitisgettingformtheclient.

Tosendtheactualformdata,example:wehavethreefields,username,passwordandname.Thenwewriteitinthebodysectionlikethis:

username=thewhitetulip&password=password&name=thewhitetulip

BuildinganAPI

AlsosetacustomHTTPheaderbytheName:tokenValue:thetokenwhichyougetin/api/get-token/call

BuildinganAPI

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