servletsIntroduction Life Cycle

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ServletServlet Introduction & Life CycleIntroduction & Life Cycle

Designed By:Designed By:




Prerequisitesrerequisites

For this section you are expected toFor this section you are expected toknow Java programming including:know Java programming including:

while while andand for for loops (vital)loops (vital)

if if (vital)(vital) Passing parameters to methods (vital)Passing parameters to methods (vital)

Arrays and Strings (important)Arrays and Strings (important)

TheThe Enumeration Enumeration class (important)class (important)

Threads andThreads and synchronized synchronized (useful)(useful)

ExceptionsExceptions try try andand catch catch (useful)(useful)




Serving Static HTMLerving Static HTML Web servers where designed to provideWeb servers where designed to provide

web browsers with HTML.web browsers with HTML. The HTML was read from text files byThe HTML was read from text files by

the server and sent to the client usingthe server and sent to the client using

HTTP. The web server does not modifyHTTP. The web server does not modify

the HTML. This is now termedthe HTML. This is now termed statictatic

HTML servingTML serving

ClientMachine

Server Machine

Web Browser

WebServer

HTML doGet or doPost

HTML

HTMLWeb Pageon disk


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Static HTML the Problemtatic HTML the Problem•Modern uses of web pages often require pages to bedynamically generated.

•For example a chat system would display any newmessages each time you hit refresh. This requiresthe web page the browser requests to be modified

each time someone adds a new message.•Until recently there was no easy way to do thisbecause

•web browsers were not designed to rewrite webpages

•web browsers are stateless so they process each

request in isolation without reference or memoryof what request came before it.




A Servlet does not run in a separate process.This removes the overhead of creating a newprocess for each request.

A Servlet stays in memory between requests. ACGI program (and probably also an extensiveruntime system or interpreter) needs to beloaded and started for each CGI request.

There is only a single instance which answers all There is only a single instance which answers allrequests concurrently. This saves memory and allows arequests concurrently. This saves memory and allows aServletServlet to easily manage persistent data.to easily manage persistent data.

A A ServletServlet can be run by acan be run by a ServletServlet Engine in a restrictiveEngine in a restrictiveSandboxSandbox (just like an Applet runs in a Web Browser's(just like an Applet runs in a Web Browser'sSandbox) which allows secure use of Sandbox) which allows secure use of untrusteduntrusted andandpotentially harmfulpotentially harmful ServletsServlets..

Browser 1

WebServer

Browser 2

Browser N

Perl 1

Perl 2

Perl N

Browser 1

WebServer

Browser 2

Browser N

Servlet

Whyhy servletservlets over CGIver CGI

script?cript?

http://www.novocode.com/doc/servlet-essentials/appendix.html#a_d_Sandbox










What can you build with Servlets?

Search Engines

Personalization Systems

E-Commerce Applications

Shopping Carts

Product Catalogs

Intranet Applications

Groupware Applications: bulletin boards, filesharing, etc.




Servletservlets - a solution forsolution for

dynamically generatingynamically generatingHTMLTML

Servletservlets are programs written in Java which runare programs written in Java which runon the web server and communicate using withon the web server and communicate using withthe web browser using HTTP and HTMLthe web browser using HTTP and HTML

ServletsServlets are compatible withare compatible with allll web browsersweb browsers

becausebecause servletsservlets communicate with the browsercommunicate with the browserusingusing onlynly HTML and HTTPHTML and HTTP

ServletsServlets are simple to write because they areare simple to write because they arewritten in standard Javawritten in standard Java

Most of the communication is done for you byMost of the communication is done for you bythethe ServletServlet class. No need to mess around withclass. No need to mess around withSockets, TCP/IP or Java serialisation.Sockets, TCP/IP or Java serialisation.




What is a What is a ServletServlet

Java technology for Common Gateway Interface Java technology for Common Gateway Interface

(CGI)(CGI) ServletsServlets areare Java programs Java programs that serve as anthat serve as an

intermediating layer between an HTTP requestintermediating layer between an HTTP requestof a client and applications in the Web serverof a client and applications in the Web server




A A ServletServlet is a Java class that extends the web server'sis a Java class that extends the web server'sservice capabilities.service capabilities.

When the client sends a request to the server the. When the client sends a request to the server the.Server can forward the request to aServer can forward the request to a ServletServlet and let itand let it

handle the request.handle the request. ServletsServlets provide open, crossprovide open, cross--server and OS APIs.server and OS APIs.

ServletsServlets have a clean design; This leads to simplicity.have a clean design; This leads to simplicity.

ServletsServlets live inside alive inside a ServletServlet Container.Container.




What Is ahat Is a Servletervlet Containerontainer Sometimes called aSometimes called a ServletServlet Engine.Engine.

TheThe ServletServlet Container providesContainer provides ServletsServlets

with the environment, configurationwith the environment, configurationand runtime needed to support theand runtime needed to support theServletServlet APIs:APIs: Load theLoad the ServletsServlets..

Initialize theInitialize the ServletsServlets..

Execute theExecute the ServletsServlets.. Destroy theDestroy the ServletsServlets..

ServletsServlets do not have a maindo not have a main…… TheThecontainer have itcontainer have it……




A Java A Java ServletServlet

Sending a

request and

receiving a

response




Who Backs Who Backs ServletsServlets??

ServletsServlets are backed by "anybody but Microsoft."are backed by "anybody but Microsoft." There is There is ServletsServlets support for virtually all commonsupport for virtually all common

servers (including Microsoft's).servers (including Microsoft's).

Supporting Supporting ServletsServlets can be done in one of two ways:can be done in one of two ways:

Supporting theSupporting the ServletServlet APIs in the web APIs in the web--server levelserver level

Adding a Adding a

ServletServlet

--Container addContainer add

--on to the webon to the web

--server.server.




Function Cycle of aFunction Cycle of a ServletServlet

1.) A user clicks submit of a1.) A user clicks submit of a

formform using ausing a servletservlet

2.) The data in the form is sent2.) The data in the form is senttoto the serverthe server

3.) The server finds the3.) The server finds the

appropriateappropriate servletservlet

4.) The4.) The servletservlet creates thecreates theHTMLHTML page based on thepage based on the

useruser’ ’ ss supplied informationsupplied information

5.) The created HTML page is then transmitted back to the user5.) The created HTML page is then transmitted back to the user6.) The requested information is displayed in the user6.) The requested information is displayed in the user’ ’ s webs web

browserbrowser

S l if C l




Servlet Life Cycle

Loaded in memory of the web server

Servlet resides in memory until called by the server

Computes requested information

Returns to idle state in memory

Servlet is destroyed




The Basic Structure of an The Basic Structure of an HttpServletHttpServlet

There are several methods that you commonly There are several methods that you commonly

override when you write anoverride when you write an HttpServletHttpServlet. These. Thesemethods aremethods are




void init(ServletConfig sc) throws ServletException;

void service(HttpServletRequest request, HttpServletResponseresponse)

throws ServletException, IOException;

void doGet(HttpServletRequest request, HttpServletResponseresponse)


void doPost(HttpServletRequest request, HttpServletResponseresponse)





The The ServletServlet LifecycleLifecycle




The The ServletServlet lifelife--cycle defines:cycle defines:

How aHow a ServletServlet is initialized.is initialized. How aHow a ServletServlet is serving users and how many times.is serving users and how many times.

How aHow a ServletServlet is destroyed.is destroyed. Concurrency management (multiConcurrency management (multi--threading threading

considerations).considerations).




In brief In brief ……the sequence of events..the sequence of events..

The sequence of events starts with a browser sending a The sequence of events starts with a browser sending a requestrequestto a Web server.to a Web server.

The server hands the request to a The server hands the request to a ServletServlet ContainerContainer.. The container loads the The container loads the servletservlet (if it isn't already loaded),(if it isn't already loaded),

instantiates a request and response objects, and then hands thesinstantiates a request and response objects, and then hands thesee

objects to theobjects to the servletservlet by calling first itsby calling first its init() methodinit() method, then its, then itsservice() methodservice() method, and lastly the, and lastly the destroy() methoddestroy() method..

The service() method will typically call one of the The service() method will typically call one of the doXXX doXXX ()()methods such asmethods such as doGetdoGet().().




ServletServlet InstantiationInstantiation The container instantiates a new The container instantiates a new ServletServlet in twoin two

occasions:occasions: A user request service from a A user request service from a ServletServlet that was notthat was not

instantiated yet.instantiated yet.

The The ServletServlet is in the list of startupis in the list of startup ServletsServlets and theand thecontainer is starting.container is starting.

A A ServletServlet is instantiated in the following manner:is instantiated in the following manner: The container loads the The container loads the ServletServlet’’ss class.class.

Using Using Class.newInstanceClass.newInstance() the container instantiates a() the container instantiates anew instance of thenew instance of the ServletServlet..




Servletervlet Initializationnitialization After the instantiation, the containerAfter the instantiation, the container

initializes theinitializes the ServletServlet.. The container casts the new instance to aThe container casts the new instance to a

ServletServlet object and initializes it by calling anobject and initializes it by calling an

init() method.init() method. If initialization fail theIf initialization fail the ServletServlet creation fail andcreation fail and

the new instance is garbage collected.the new instance is garbage collected.

TheThe ServletServlet initialization is done in ainitialization is done in a

single threaded manner.single threaded manner.




Init()Init()

The init method is called when the The init method is called when the servletservlet is firstis first

created;created; init(ServletConfiginit(ServletConfig )) oror init()init()

it is not called again for each user request.it is not called again for each user request.

So, it is used for oneSo, it is used for one--time initializations, just as withtime initializations, just as with

the init method of applets.the init method of applets.




Init()Init() …… A code for initialization that is called only once (e.g., creati A code for initialization that is called only once (e.g., creating ng

the tables of the database)the tables of the database)

Initialization parameters are server specificInitialization parameters are server specific Many times these parameters are used to specify theMany times these parameters are used to specify the

maximum or minimum number for yourmaximum or minimum number for your ServletServlet..

For example, if you have a banking account, there is typically For example, if you have a banking account, there is typically a minimum amount required to maintain the account. Thea minimum amount required to maintain the account. The

initial parameter could be set to this minimum number andinitial parameter could be set to this minimum number and

then yourthen your ServletServlet could access it to check the object's valuecould access it to check the object's value




Service Phaseervice Phase After theAfter the ServletServlet initialization, the container can ask theinitialization, the container can ask the

ServletServlet to serve incoming requests.to serve incoming requests.

The container will call theThe container will call the ServletServlet’ ’ ss service() method.service() method.

AA ServletServlet may handle many concurrent requests.may handle many concurrent requests.

By different threads, all of them running in the sameBy different threads, all of them running in the sameservice methods.service methods.

The service method should be thread safe.The service method should be thread safe.

MultithreadingMultithreading

By default theBy default the servletservlet need to be multithread safe!need to be multithread safe!

One may instruct the container to make theOne may instruct the container to make the servletservlet

“ “multithread safemultithread safe” ” by insuring that no two thread willby insuring that no two thread will

execute on a singleexecute on a single servletservlet instanceinstance By making theBy making the ServletServlet class implement theclass implement the

SingleThreadModelSingleThreadModel interface (deprecated)interface (deprecated)




singlethreadmodelsinglethreadmodel

javax.servletjavax.servlet

When we implement When we implement singlethreadmodelsinglethreadmodel it meansit means

that only one thread can access the servicethat only one thread can access the servicemethod at a timemethod at a time




Http MethodsHttp Methods

when u click on Submit on your JSP page...you when u click on Submit on your JSP page...you

will be directed to the method either will be directed to the method either doGetdoGet() or() or doPostdoPost() in the() in the servletservlet






The main diff in these methods The main diff in these methods

When you select GET as method in your form the data you When you select GET as method in your form the data you

pass from that JSP, will be appended to the URL with your pass from that JSP, will be appended to the URL with your

servletservlet address.......address.......

where if you select POST then, any kind of data will not be where if you select POST then, any kind of data will not be

appended to the URL........it will be passedappended to the URL........it will be passed directly(butdirectly(but uu

can not see appended data on the URL)can not see appended data on the URL)




Other MethodsOther Methods

HEADHEAD: Client sees only header of response to: Client sees only header of response to

determine size, etcdetermine size, etc…… PUTPUT: Place documents directly on server: Place documents directly on server

DELETEDELETE: Opposite of PUT: Opposite of PUT TRACE TRACE: Debugging aid returns to client: Debugging aid returns to client

contents of its requestcontents of its request

OPTIONSOPTIONS: what options available on server: what options available on server




HttpServletHttpServlet Request HandlingRequest Handling

GET request

service()

HttpServlet subclass

response

doGet()

doPost()

Web

Server

POST request

response




DestroyingDestroying ServletsServlets

Destroying:Destroying:

destroy() is adestroy() is a servletservlet lifelife--cycle method called by cycle method called by servletservlet container to kill the instance of thecontainer to kill the instance of the servletservlet..

destroy()destroy() methodmethod make sure all service threads completemake sure all service threads complete




End of End of servletservlet life cyclelife cycle

Servlet Architecture




A servlet is an object of the javax.servlet class.

It runs inside a Java Virtual Machine on a server host.

A servlet is an object. It is loaded and runs in an object calleda servlet engine, or a servlet container.

Every servlet must implement javax.servlet.Servlet interface

Most servlets implement the interface by extending one of these classes

– javax.servlet.GenericServlet

– javax.servlet.http. HttpServlet




Working with Working with ServletsServlets




ServletServlet PackagePackage

javax.servletjavax.servlet

The The ServletServlet interface defines methods thatinterface defines methods thatmanagemanage servletsservlets and their communication withand their communication with

clientsclients

Client Interaction: when it accepts call, receivesClient Interaction: when it accepts call, receives

two objects that implementstwo objects that implements

ServletRequestServletRequest

ServletResponseServletResponse




Architecture Architecture

YourOwnServlet

HttpServlet

Generic Servlet

Servlet

Generic Servlet & HTTP Servlet




GenericServletGenericServlet

service ( )Server

ClientClient

HTTPServletHTTPServlet

service ( )HTTPServer

BrowserBrowser

request

response

doGet( )

doPost( )

request

response

Difference betweenDifference between GenericServletGenericServlet




Difference betweenDifference between GenericServletGenericServlet

andand HttpServletHttpServlet GenericServletGenericServlet is foris for servletsservlets that might not usethat might not use

HTTP, like for instance FTPHTTP, like for instance FTP servlets.(futureservlets.(futuregrowth specs)growth specs)

GenericServletGenericServlet is protocol independent, whereasis protocol independent, whereas

HttpServletHttpServlet is protocol dependentis protocol dependent

InIn GenericServletsGenericServlets you cannot use Cookies oryou cannot use Cookies or

HttpSessionHttpSession..




Session tracking is not possible, redirection is notSession tracking is not possible, redirection is not

possible.possible.

HttpHttp servletsservlets overridesoverrides doGetdoGet andand doPostdoPost methods.methods.

GenericGeneric servletservlet overidesoverides service method.service method.

genericgeneric

servletservlet

isis

superclasssuperclass

forfor

HttpServletHttpServlet

.

.

httpservlethttpservlet class can also have service()class can also have service() method.it'smethod.it's notnot

necessary that you can only havenecessary that you can only have doGetdoGet() and() and doPostdoPost()()

method defined.method defined.

GenericGeneric servletservlet is used for small data transfer whereasis used for small data transfer whereas

HttpServletHttpServlet is used for huge data transfer.is used for huge data transfer.

Writing aWriting a ServletServlet




Writing a Writing a ServletServlet

Your s Your servletservlets will usually extend the will usually extend the HttpServlet HttpServlet classclass. Which provides default implementations of . Which provides default implementations of

initinit, you only need to override this if you want to, you only need to override this if you want todo some initialisation such as open a databasedo some initialisation such as open a databaseconnection.connection.

destroydestroy, you only need to override this if you, you only need to override this if you want to do some cleaning up such as closing a want to do some cleaning up such as closing adatabase connection.database connection.

serviceservice, should not normally be overridden, should not normally be overriddenbecause it decides which of thebecause it decides which of the doX doX methodsmethodsshould be calledshould be called

i iW i i S lS l




Writing a Writing a ServletServlet....

doGetdoGet. HTTP Get is the default way a web. HTTP Get is the default way a web

browser makes a request and this causes thebrowser makes a request and this causes thedoGetdoGet method of themethod of the servletservlet to be invoked soto be invoked so

you would normally override this methodyou would normally override this method

doPostdoPost. If you don't respond to a. If you don't respond to a PostPost youyou

don't need to override this method.don't need to override this method.




HTTPHTTP Get or PostGet or Post??••The main differences between Get and Post areThe main differences between Get and Post are

* GET: data is sent as a query string client querydata is sent as a query string client query

is completely embedded in the URL sent to theis completely embedded in the URL sent to theserver. The amount of query information isserver. The amount of query information is

usually limited to 1KB. This information isusually limited to 1KB. This information is

stored in the request line. This is the defaultstored in the request line. This is the defaultmethod when you click on amethod when you click on a href href HTML link.HTML link.* POST: data is sent as part of the message body.data is sent as part of the message body.

Data is not visible in the URL and largeData is not visible in the URL and large

amounts of data can be sent to the server.amounts of data can be sent to the server.




ServletServlet doGetdoGet andand doPostdoPost

doGetdoGet andand doPostdoPost have identical signatures andhave identical signatures andreceive two parameters from the service method..receive two parameters from the service method..

public void public void doGet(HttpServletRequest doGet(HttpServletRequest request,request,HttpServletResponse HttpServletResponse response) throws response) throws IOException IOException , ,ServletException ServletException

public void public void doPut(HttpServletRequest doPut(HttpServletRequest request,request,HttpServletResponse HttpServletResponse response) throws response) throws IOException IOException , ,ServletException ServletException




•• HttpServletRequestHttpServletRequest is an Object containing informationis an Object containing informationabout the request from the web browser.about the request from the web browser.

Such as the login name of the user and any values in the headerSuch as the login name of the user and any values in the header field sentfield sentby the web browser.by the web browser. Any parameters sent as part of the request Any parameters sent as part of the request Any initialisation parameters set in the deployment descriptor Any initialisation parameters set in the deployment descriptor It also supplies any session and cookies (more on these later)It also supplies any session and cookies (more on these later)

•• HttpServletResponseHttpServletResponse is an Object which allows you to addis an Object which allows you to addcookies, add header values and redirect to a new URL, writecookies, add header values and redirect to a new URL, writeHTTP to the web browser and send error messages.HTTP to the web browser and send error messages.

ClientClient Server InteractionServer Interaction




ClientClient--Server InteractionServer Interaction

HTTP RequestHTTP Request (Sending request message to(Sending request message to

Server ):Server ):

• HTTP is a requestHTTP is a request--response protocolresponse protocol•• When a client sends a request, it consists of three parts:When a client sends a request, it consists of three parts:

•• request line:request line:

•• HTTP method type (GET or POST)HTTP method type (GET or POST)•• resource name (URL)resource name (URL)

•• protocol/versionprotocol/version e.g. POST /im51p/w7.htmle.g. POST /im51p/w7.html

HTTP/1/1HTTP/1/1

•• header variables: contains browser informationheader variables: contains browser information(optional)(optional)

•• message body: in POST method request information ismessage body: in POST method request information is

stored here (optional)stored here (optional)




HTTP ResponseHTTP Response (Sending response message to client):(Sending response message to client):

•• The response sent by the server also consists of three parts: The response sent by the server also consists of three parts:

•• response line (server protocol and status code)response line (server protocol and status code)

•• header variables (server and response information)header variables (server and response information)

•• message body (response, such as HTML)message body (response, such as HTML)

ServletServlet OutputOutput




public void doGet(HttpServletRequest request,

HttpServletResponse response)throws IOException, ServletException

{response.setContentType("text/html");PrintWriter out = response.getWriter();

out.println("<HTML>");out.println("<HEAD><TITLE>First Servlet

program</TITLE></HEAD>");

out.println("<BODY>");out.println("<H1>Welcome to Servlets</H1>");out.println("</BODY>");out.println("</HTML>");out.close();

}

<HTML><HEAD><TITLE>First Servlet Program</TITLE></HEAD><BODY><H1>Welcome to Servlets</H1></BODY></HTML>

The servlet must get thecommunications channel from the

response Object and set the

encoding before sending anything tothe client

Request containsany parameters,

Cookies andSessions send by

the client

Response allows

you to add cookiesand send data tothe client

FirstFirst ServletServlet ExampleExample




import java.io.*; import javax.servlet.*; import javax.servlet.http.*;

public class ServWelcome extends HttpServlet

{ public void doGet( HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException

{ response.setContentType("text/html");

PrintWriter out = response.getWriter();

out.println(" <HTML>");

out.println(" <HEAD><TITLE>First Servlet Program</TITLE></HEAD>"); out.println(" <BODY>"); out.println(" <H1>Welcome to Servlets</H1>"); out.println(" </BODY>"); out.println("< /HTML>");

out.close();}

}

Servlets are not part of the

standard SDK, they are part of the J2EE

Servlets normally extendHttpServlet

Details of the HTTP requestfrom the client

The response to be sent to theclient

Set the response type totext/html (this is normal)

This HTMLtext is sent to

the clientDon't forget to

close theconnection

with the client

ServletServlet Life Cycle Managed by the Server and ContainerLife Cycle Managed by the Server and Container




Call to methodsdoGet(), doPost() in theuser defined servlet class

// load the servlet class and setup request, response objects

Servlet servlet = …ServletRequest request = …ServletResponse response = …

// initialise servletServletConfig config = servlet.getServletConfig();servlet.init(config);

// service zero or more requestswhile ( …more requests… )// invoke servlet to process requestservlet.service(request, service);

// cleanup after serviceservlet.destroy();

Unlike CGI a single servlet can service several requests. In CGIa separate process for each request. Servlets therefore reduce

Scalability ofScalability of ServletsServlets




Scalability of Scalability of ServletsServlets

There are several costs associated with loading andThere are several costs associated with loading andexecuting user supplied code in the web server.executing user supplied code in the web server.

Compiling and loadingCompiling and loading

Handling sequential requestsHandling sequential requests

Handling concurrent requestsHandling concurrent requests TheThe servletservlet is only recompiled by the web browser if itis only recompiled by the web browser if it

has changed otherwise the already compiled .class file ishas changed otherwise the already compiled .class file isloaded =>loaded => Faster response timesaster response times because thebecause the servletservletdoes not need to be recompileddoes not need to be recompiled

TheThe servletservlet can be kept in memory for a long time tocan be kept in memory for a long time toservice many sequential requests =>service many sequential requests => Faster responseaster responsetimesimes because thebecause the servletservlet does not need to be reloadeddoes not need to be reloaded

Only one copy of theOnly one copy of the servletservlet is held in memory even if is held in memory even if

there are multiple concurrent requests for it =>there are multiple concurrent requests for it => lessessmemory usageemory usage foror concurrentoncurrent requestsequests and noand no need toeed toload another copyoad another copy of theof the servletservlet and create a newand create a newprocess to run it.process to run it.

CompilingCompiling




CompilingCompiling

In order to compile aIn order to compile a servletservlet, you may need to add to your, you may need to add to your

CLATHPATH definition the following:CLATHPATH definition the following:

setenv setenv CLASSPATHCLASSPATH ${CLASSPATH}:${CLASSPATH}:

//usr/local/java/apache/jakartausr/local/java/apache/jakarta--tomcat/lib/ant.jartomcat/lib/ant.jar::

//usr/local/java/apache/jakartausr/local/java/apache/jakarta--tomcat/lib/jasper.jartomcat/lib/jasper.jar:://usr/local/java/apache/jakartausr/local/java/apache/jakarta--tomcat/lib/jaxp.jartomcat/lib/jaxp.jar::

//usr/local/java/apache/jakartausr/local/java/apache/jakarta--tomcat/lib/parser.jartomcat/lib/parser.jar::

//usr/local/java/apache/jakartausr/local/java/apache/jakarta--tomcat/lib/servlet.jartomcat/lib/servlet.jar::

//usr/local/java/apache/jakartausr/local/java/apache/jakarta--tomcat/ lib/tomcat/ lib/ webserver.jar webserver.jar

Calling theCalling the ServletServlet





Calling theCalling the servletservlet is done from the Webis done from the Web

browser:browser:http://http://host:port/servlet/ServletNamehost:port/servlet/ServletName

For servlets that are positioned under

webapps/ROOT/WEB-INF/classes






Calling theCalling the servletservlet is done from the Webis done from the Web

browser:browser:http://http://host:port/dirName/servlet/ServletNamehost:port/dirName/servlet/ServletName

For servlets that are positioned under

dir_path/WEB-INF/classesand dir_path is mapped to dirName

PackagesPackages




PackagesPackages

Add Add packege packege packageName packageName to the java code toto the java code to

create a packagecreate a package Put the classes files underPut the classes files under

tomcat_home/webapps/ROOT/WEBtomcat_home/webapps/ROOT/WEB--

INF/classes/packageNameINF/classes/packageName

Call theCall the servletservlet with with

http://http://host:port/servlet/packageName.servletNahost:port/servlet/packageName.servletNameme




The The ServletServlet API API

WhatWhat’’s in the APIss in the APIs




What What s in the APIss in the APIs

The The ServletServlet APIs defines: APIs defines:

A A servletservlet lifecyclelifecycle – – how ahow a servletservlet is created, usedis created, usedand destroyed.and destroyed.

A programmer interface A programmer interface – – how ahow a servletservlet looks likelooks like

to the container and how the container looks like toto the container and how the container looks like tothethe servletservlet..

The The ServletServlet APIs is what the programmer APIs is what the programmershould care aboutshould care about

A Destroy ExampleA Destroy Example




A Destroy Example A Destroy Example

public void public void destroy()destroy() {{

/* Check to see whether there are still/* Check to see whether there are still

* service methods running,* service methods running,* and if there are, tell them to stop.* and if there are, tell them to stop.*/*/if (if (numServicesnumServices() > 0) {() > 0) {

setShuttingDown(truesetShuttingDown(true););

}}

/* Wait for the service methods to stop. *//* Wait for the service methods to stop. */ while(numServices while(numServices() > 0) {() > 0) {

try {try {

Thread.sleep(interval Thread.sleep(interval););} catch (} catch (InterruptedExceptionInterruptedException e) {}e) {}

}}}}

ServletServlet DestructionDestruction




A container may destroy the A container may destroy the ServletServlet because:because: It wants to preserve resources (this is not common).It wants to preserve resources (this is not common).

The container is shutting down. The container is shutting down.

In both cases the container will call theIn both cases the container will call the ServletServlet’’ss destroy()destroy()method.method.

Note that this means that a certainNote that this means that a certain ServletServlet will usually stay will usually stay ““alivealive”” for a very long time and is going to serve many users.for a very long time and is going to serve many users.

After the destruction the After the destruction the ServletServlet is garbage collected.is garbage collected.

SupportingSupporting ServletsServlets




SupportingSupporting ServletsServlets

The Web server must support The Web server must support servletsservlets::

Apache Apache Tomcat Tomcat SunSun’’ss JavaServer JavaServer Web Development Kit ( Web Development Kit ( JSWDK JSWDK ) )

Allaire Allaire Jrun Jrun – – anan engeineengeine that can be added to IIS,that can be added to IIS,

PWS, old Apache Web servers etcPWS, old Apache Web servers etc……

SunSun’’s Java Web Servers Java Web Server

……

““ListeningListening”” to a Shutdownto a Shutdown




ListeningListening to a Shutdownto a Shutdown

public void doPost(...) {...

for(i = 0; ((i < numberOfThingsToDo)

&& !isShuttingDown()); i++) {try {

partOfLongRunningOperation(i);} catch (InterruptedException e) {}

}}




Passing Multiple Parameters toPassing Multiple Parameters to ServletsServlets

The name of theparameter

The value of the

parameter

The name of theparameter

The value of the

parameter

To pass more than one parameter separate each one To pass more than one parameter separate each one

with a & with a &

<<a a

href href

="http:// localhost:8080/ /kingj1/servlet/param ="http:// localhost:8080/ /kingj1/servlet/param

? ?

p1 p1= = hellohello& & p2 p2 = = goodbye goodbye ">"> click here for a parameter example! </a>click here for a parameter example! </a>

Getting a list of ParametersGetting a list of Parameters




Gett g a st o Pa a ete sg

You can get a list of all the parameters passed by using You can get a list of all the parameters passed by using

getParameterNames getParameterNames ()() which returns an which returns an Enumeration Object Enumeration Object

Enumeration p=Enumeration p=request.getParameterNames request.getParameterNames (); ();

while ( while (

p.hasMoreElements p.hasMoreElements

())())

{{

String n=( String n=( String)p.nextElement String)p.nextElement (); ();

String v=String v=request.getParameter(n request.getParameter(n ); );

out.println("parameter out.println("parameter name is "+n+" parameter value" +v+"<name is "+n+" parameter value" +v+"<br br >"); >"); }}

Using forms to Pass ParametersUsing forms to Pass Parameters




gg

The second way to pass parameters to an The second way to pass parameters to an servletservlet isis

to useto use HTML formsHTML forms..<FORM METHOD="<FORM METHOD="GETGET""

ACTION="http://localhost:8080/servlet/param" ACTION="http://localhost:8080/servlet/param"

>>Enter your Name: <INPUT TYPE=TEXTEnter your Name: <INPUT TYPE=TEXTNAME="NAME="myNamemyName">">

<INPUT TYPE="SUBMIT" VALUE="Send<INPUT TYPE="SUBMIT" VALUE="SendName">Name">

</FORM></FORM>

Name of parametervalue of parameteris whatever is typed

into the text box

This will cause the doGet method in the servlet tobe invoked




Using Choice boxesUsing Choice boxes The choice box allows one of a set of choices to be The choice box allows one of a set of choices to be

selectedselected

Just like a TEXT input the result of the selection is Just like a TEXT input the result of the selection isstored in a parameter and passed to your applicationstored in a parameter and passed to your application

Using Choice boxesUsing Choice boxes




Using Choice boxesUsing Choice boxes The code is a little more complex than a TEXT field. The code is a little more complex than a TEXT field.<FORM METHOD="POST" <FORM METHOD="POST"

ACTION="http://localhost:8080/servlet/cw?action=submit"> ACTION="http://localhost:8080/servlet/cw?action=submit">

<<br br ><><br br >question 0 what grade do you want on this coursework? >question 0 what grade do you want on this coursework? <<br br > F > F

<input type="radio" name="q0" value=0 ><input type="radio" name="q0" value=0 >

<<br br >> resit resit

<input type="radio" name="q0" value=1 ><input type="radio" name="q0" value=1 ><<br br > fail > fail

<input type="radio" name="q0" value=2 ><input type="radio" name="q0" value=2 >

<<br br > retake > retake

<input type="radio" name="q0" value=3 ><input type="radio" name="q0" value=3 ><INPUT TYPE="SUBMIT" VALUE="Mark"><INPUT TYPE="SUBMIT" VALUE="Mark">

</form></form>

All the choices associatedwith the same choice box

have the same name!

Each possible choice shouldhave a different value.

When the submit button is

pressed the parameter q0 willhave the value 0 or 1 or 2 or 3




Using Choice boxesUsing Choice boxes You may have one than one independent choice You may have one than one independent choice

box in the same form. The result of each select isbox in the same form. The result of each select ispassed as separate parameters.passed as separate parameters.

Using Choice boxesUsing Choice boxes




Using Choice boxesUs g C o ce bo es In this case one choice is called q0 and the second q1In this case one choice is called q0 and the second q1<FORM METHOD="POST" <FORM METHOD="POST"

ACTION="http://www3.unl.ac.uk:8186/kingj1/servlet/cw?action=subm ACTION="http://www3.unl.ac.uk:8186/kingj1/servlet/cw?action=subm it">it">

<<br br ><><br br >question 0 what grade do you want on this coursework? >question 0 what grade do you want on this coursework? <<br br > F > F

<input type="radio" name=" <input type="radio" name=" q0 q0 " value=0 >" value=0 >

<<br br >> resit resit

<input type="radio" name=" <input type="radio" name=" q0 q0 " value=1 >" value=1 ><<br br ><><br br >question 0 what grade do you want on the exam? >question 0 what grade do you want on the exam?

<<br br > F > F

<input type="radio" name=" <input type="radio" name=" q1 q1 " value=0 >" value=0 >

<<br br >> resit resit <input type="radio" name=" <input type="radio" name=" q1 q1 " value=1 >" value=1 >

<INPUT TYPE="SUBMIT" VALUE="Mark"><INPUT TYPE="SUBMIT" VALUE="Mark">

</form></form>

Thread Unsafe Thread Unsafeimportimport java.iojava.io.*;.*;




Read thenmodify then

write

importp java.ioj . ;;

importimport javax.servletjavax.servlet.*;.*;importimport javax.servlet.httpjavax.servlet.http.*;.*;public class multi extendspublic class multi extends HttpServletHttpServlet {{intint count=0;count=0;public voidpublic void doGet(HttpServletRequestdoGet(HttpServletRequest request,request, HttpServletResponseHttpServletResponse response)response)

throwsthrows IOExceptionIOException,, ServletExceptionServletException {{response.setContentType("textresponse.setContentType("text/html");/html");PrintWriterPrintWriter out =out = response.getWriterresponse.getWriter();();out.printlnout.println("<HTML>");("<HTML>");out.printlnout.println("<HEAD><TITLE> Thread UNSAFE example </TITLE>("<HEAD><TITLE> Thread UNSAFE example </TITLE>

</HEAD> ");</HEAD> ");out.printlnout.println("<BODY>");("<BODY>");out.printlnout.println("<H1> old count is "+count+"</H1>");("<H1> old count is "+count+"</H1>");count=count+1;count=count+1;try { Thread.sleep(4000); } catch (Exception e) {try { Thread.sleep(4000); } catch (Exception e) { }}

out.printlnout.println("<H1> new count is "+count+"</H1>");("<H1> new count is "+count+"</H1>");out.printlnout.println("</BODY>");("</BODY>");out.printlnout.println("</HTML>");("</HTML>");

}}}}

Thread Safe Thread Safe




A good way to make sure A good way to make sure servletsservlets are thread safe is to have noare thread safe is to have noglobal data. But this is not always practical.global data. But this is not always practical.

A partial solution is to store data on a per client basis. But t A partial solution is to store data on a per client basis. But thishisrequiresrequires sessionssessions oror cookies.cookies. It does not work if one clientIt does not work if one clientmachine has two instances of the web browser accessing themachine has two instances of the web browser accessing theservletservlet concurrently concurrently

The best method is to mark the sections of code that access The best method is to mark the sections of code that accessglobal data asglobal data as synchronized.synchronized. This allows the sections that do This allows the sections that donot access global data to run concurrently not access global data to run concurrently

Thread Safe Thread Safe Enclose sections that use global data inEnclose sections that use global data in synchronizedsynchronized sectionssections




Can not beexecutedconcurrently

Enclose sections that use global data inEnclose sections that use global data in synchronized synchronized sectionssectionsimportimport java.iojava.io.*;.*;importimport javax.servletjavax.servlet.*;.*;importimport javax.servlet.httpjavax.servlet.http.*;.*;public class multi extendspublic class multi extends HttpServletHttpServlet {{intint count=0;count=0;Object s=new Object();Object s=new Object();public voidpublic void doGet(HttpServletRequestdoGet(HttpServletRequest request,request, HttpServletResponseHttpServletResponse response) throwsresponse) throws

IOExceptionIOException,, ServletExceptionServletException {{response.setContentType("textresponse.setContentType("text/html");/html");

PrintWriterPrintWriter out =out = response.getWriterresponse.getWriter();();out.printlnout.println("<HTML>");("<HTML>");out.printlnout.println("<HEAD><TITLE> Thread UNSAFE example </TITLE> </HEAD> ");("<HEAD><TITLE> Thread UNSAFE example </TITLE> </HEAD> ");out.printlnout.println("<BODY>");("<BODY>");synchronized (s) {synchronized (s) {

out.printlnout.println("<H1> old count is "+count+"</H1>");("<H1> old count is "+count+"</H1>");count=count+1;count=count+1;try { Thread.sleep(4000); } catch (Exception e) {try { Thread.sleep(4000); } catch (Exception e) { }}out.printlnout.println("<H1> new count is "+count+"</H1>");("<H1> new count is "+count+"</H1>"); }}out.printlnout.println("</BODY>");("</BODY>");

out.printlnout.println("</HTML>");("</HTML>");}}

}}

Servlets vs. JSP




• JSPs– Look like standard HTML or XHTML

• Normally include HTML or XHTML markup

– Known as fixed-template data– HTML codes can be written easily

– Used when content is mostly fixed-template data• Small amounts of content generated dynamically

• Servlets– HTML codes have to be written to the PrintWriter or

OutputStream

– Used when small amount of content is fixed-template data

• Most content generated dynamically

SummarySummary




The The servletservlet life cycle?life cycle?

EachEach servletservlet has the same life cycle:has the same life cycle:

A server loads and initializes the A server loads and initializes the servletservlet (init())(init())

The The servletservlet handles zero or more client requests (service())handles zero or more client requests (service())

The server removes the The server removes the servletservlet (destroy()) (some servers do(destroy()) (some servers do

this step only when they shut down)this step only when they shut down)




End of End of ServletsServlets BasicsBasics

andand

ServletsServlets Life CycleLife Cycle

Documents

servletsIntroduction Life Cycle