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ProjectJigsaw– Modularityatlanguagelevel
Kamil Korzekwa@kamkorz | blog.kamkor.me April14,2016
ProjectJigsaw
• AddsmodularitytotheJavaplatform• Hopefully comingtoJDK9inMarch2017• WillcausecompatibilityissuesforapplicationsthatuseJDKinternalAPIs(JEP260)
HowisitdifferentfromtoolslikeMaven?
Jigsawislanguagelevelmechanism!
What’sajigsawmodule?
• A groupingofcode• ForexampleJavapackages
• Cancontainotherdatasuchas• resources• configurationfiles• nativecode(forexamplewhenusingJNI)
• Definedinthemodule-info.java file• Fileisplacedintherootofthemodulefolder
• Canbepackagedasjar
module-info.java file
• Modulehasasymbolicname(thatonlylookslikeapackage)• Moduleexportspackages(publicAPI)toothermodules• Modulerequiresdependenciestoothermodules
module me.kamkor.foo {
exports me.kamkor.foo.api;
exports me.kamkor.foo;
requires java.sql;
}
Current naming convention
Sameasmodulename,confusing
enough?
public!=accessible(fundamentalchangetoJava)
• public
JDK9• publictoeveryonemodule me.kamkor.foo {
exports me.kamkor.foo.api;}• public,butonlytospecificmodulesmodule me.kamkor.foo {
exports me.kamkor.foo.api tome.kamkor.bar,me.kamkor.magic;
}• publiconlywithinamodule
PRE-JDK9
Impliedreadabilitymodule me.kamkor.foo {
requires java.sql;}
• Codeintheme.kamkor.foo module:
final Driver driver = DriverManager.getDriver(url);final Logger logger = driver.getParentLogger();logger.info(“Connection acquired”); Logger is exported in
the java.logging module
Driver is exported in the java.sql module
Impliedreadability// assumed configuration (fake)
module java.sql { requires java.logging;
..}
Impliedreadabilitymodule java.sql {
requires public java.logging;
}
Impliedreadability
• Ifyouusetypesfromrequiredmoduleinyourexportedtypes,thenconsiderexposingthismoduletoyourusersusingrequirespublic
• Ifyouusetypesfromrequiredmoduleonlyinyourinternalcode,thendonotexposethismoduletoyourusers
“Fun”withclasspath
bin/greeter/GreetingProvider.class
bin/starwarsgreeter/GreetingProvider.class
bin/app/GreeterApp.classUsing default packagefor the sake of simplicity
“Fun”withclasspath
$ java -classpath bin/greeter:bin/app GreeterApp
Hello World
$ java -classpath bin/starwarsgreeter:bin/app GreeterApp
May the force be with you!
$ java -classpath bin/greeter:bin/starwarsgreeter:bin/app GreeterApp
Hello World
Classpath hell– shadowing
• Defaultclassloader loadsthefirstmatchingclassitfinds
"Theorderinwhichyouspecifymultipleclasspathentriesisimportant.TheJavainterpreterwilllookforclassesinthedirectoriesintheordertheyappearintheclasspathvariable.”https://docs.oracle.com/javase/8/docs/technotes/tools/windows/classpath.html
Classpath hell– realworldexamplesofshadowing• Differentlibrariesontheclasspath containclasswiththe
samefullyqualifiedname• Twodifferentversionsofthesamelibraryareonthe
classpath• Library isrenamedandaccidentallyaddedtotheclasspath
twice
Modulepath totherescue!
Classpath vsmodulepath
• Flatstructure• Allowstolocateindividualtypes• Defaultclassloader loadsthefirstmatchingclassitfindsontheclasspath
Modulepath• Graphstructure• Allowstolocatemodulesratherthanindividualtypes• Javaisawareaboutrelationshipsbetweenthemodules• Candiscoverpotentialproblemssooner
Classpath
ShadowingfixedwithJigsawmodulepath
• CompilingJavaapplicationwithtwomodulesonthemodulepath thatexportthesamepackage
src/me.kamkor.foo/module-info.java:1: error:
module me.kamkor.foo reads package me.kamkor.beta from both me.kamkor.bar1 and me.kamkor.bar2
Compile error
ShadowingfixedwithJigsawmodulepath
• RunningJavaapplicationwithtwomodulesonthemodulepath thathavethesamename
Error occurred during initialization of VM java.lang.module.ResolutionException: Two versions of module me.kamkor.bar found in mods Error when trying to
run application
Moduleversionselection
• Versionselectionisleftforthedependency-resolutionmechanismssuchasMaven,Gradle,sbtetc.
http://openjdk.java.net/projects/jigsaw/goals-reqs/03#version-selection
Multiplemoduleversions• Jigsawallowsonlyoneversionofmoduleinsingleconfiguration• Dependencyresolutionmechanismmustpickone
• Modulepath versionhelljustlikeclasspath versionhell
http://openjdk.java.net/projects/jigsaw/goals-reqs/03#version-selection
Circulardependenciesmodule me.kamkor.foo {
requires me.kamkor.bar;}
module me.kamkor.bar {requires me.kamkor.foo;
}
./src/me.kamkor.foo/module-info.java:2: error: cyclic dependence involving me.kamkor.bar requires me.kamkor.bar;
Compile error
BetterJDKwiththeuseofProjectJigsaw
JDKisBIG!
• 4000+classesinrt.jar(classes.jar onMacs)• java.util.List etc.
• Allloadedonstartbythebootstrapclassloader• Doesyourapplicationreallyneedallofthem?
(source:Liguori,R.,Liguori, P.:Java8PocketGuide)
JDKwillbesplitintomodules
http://openjdk.java.net/jeps/200
JDKStructure(simplified)
binjava
javac
jrebin java
lib rt.jar
lib tools.jar
JDK9
binjava
javacconf
jmods
lib
Pre-JDK9
jre directoryrt.jartools.jar
jlink – TheJavaLinker
“Createatoolthatcanassembleandoptimizeasetofmodulesandtheirdependenciesintoacustomrun-timeimageasdefinedinJEP220.”http://openjdk.java.net/jeps/282
jlink – TheJavaLinker
$ jlink --modulepath $JAVA_HOME/jmods:mlib \--addmods me.kamkor.greeter --output executable \--strip-debug --compress=2
$ tree –L 1 executable
executable
bin
conf
lib
jlink – TheJavaLinker
$ du -h -d 0 executable20M executable
$ executable/bin/me.kamkor.greeterGreetings World!
jlink – TheJavaLinker
$ executable/bin/java -listmodsjava.base@9-ea
Version is just for information purposes
Compatibility&Migration
• Applicationscanstilluseclasspath• JDKwillbesplitintomodules.ApplicationsmaybeaffectediftheyusedinternalAPIsoftheJDK(JEP260)• ApplicationscanbesplitintoJigsawmodulesinsmallsteps.Migrationguide:• http://openjdk.java.net/projects/jigsaw/spec/sotms/#compatibility--migration
ProjectJigsawsummary• ImprovedmaintainabilityofJDKanduserapplications
• Strongerencapsulation(modulesexportpublicAPIs)• Reliableconfigurationwithmodulepath thatreplaceserrorproneclasspathmechanism
• Improvedsecurityofapplications• LessAPIsmaketheattacksurfacesmaller
• ImprovedJavaplatformscalabilityandflexibility• ConfigurablemodularJDK• Run-timeimages(jlink – theJavaLinkertool)
• Improvedperformanceofapplications• Startupperformanceimprovements(lessclassestoloadbythebootstrapclassloader)
ImpactofProjectJigsaw• IhopethatitwillspeedupthedevelopmentoftheJDK• Willitbewidelyadoptedbythedevelopers?• Spring5willsupportJigsawoutofthebox• SpringwilldoeverythingitcantoallowyoutouseJigsaw• Springjars(spring-mvc,spring-jdbc etc.)willbeconfiguredasJigsawmodules• http://www.infoq.com/presentations/spring-framework-5
Usefulresources• https://github.com/AdoptOpenJDK/jdk9-jigsaw• http://openjdk.java.net/projects/jigsaw/spec/sotms/• http://openjdk.java.net/projects/jigsaw/
Thankyou,questions?
