JavaOne 2016
Juggling Multiple Java Platforms and Jigsaw with Gradle
Cédric Champeau

Who am I

speaker {
    name 'Cédric Champeau'
    company 'Gradle Inc'
    oss 'Apache Groovy committer',
    successes 'Static type checker',
                    'Static compilation',
                    'Markup template engine',
        failures Stream.of(bugs),
        twitter '@CedricChampeau',
        github 'melix',
        extraDescription '''Groovy in Action 2 co-author
Misc OSS contribs (Gradle plugins, deck2pdf, jlangdetect, ...)'''

Groovy in Action 2


Coupon: groovyjo16 (42% off until Thursday)

What is Gradle?

Mechaphant Gradle

Emerge from Build Hell

  • Unified, fast, reproducible builds

  • Cross-platform

  • Language agnostic

  • A Build Tool + Cloud Services

Project Jigsaw

Project Jigsaw

  • Modularity for the Java platform

    • Reliable configuration (goodbye classpath)

    • Strong encapsulation (goodbye com.sun)

  • Enforce strict boundaries between modules

    • compile-time

    • run-time

Jigsaw modules

Declaring a module
module {
    requires org.baz.qux;

A module

  • Declares dependencies onto other modules

    • optionally transitive (public)

  • Declares a list of packages it exports

  • will be found on modulepath

Why it’s going to break your apps

  • Exports are package based

  • A non-exported package is not visible to consumers

  • 2 modules cannot export the same packages

  • 2 modules cannot have the same internal packages

Death to split packages.

How Gradle will help?

  • Find a smooth migration path

  • Prepare today, migrate when ready


  • Work in progress

  • Backport from the software model

  • Running Gradle on Jigsaw requires Gradle 3.0+

  • Works with JDK 9 b119

  • Breaks with JDK 9 b136

  • Running those samples require master

A simple project

Internal APIs

Internal APIs maintenance nightmare

The "Jigsaw" plugin

  • WIP

  • creates additional configurations for each platform

  • adds support for API

  • APIs enforced on JDK < 9 too


How to declare

Compilation fails!

Compile avoidance

Build scan

Compile avoidance benefits

  • Doesn’t recompile dependents if:

    • method body changes

    • parameter names change

    • method order changes

    • private method added/removed

  • In short: if API stable, do nothing

Declaring target platforms

Build scans

Running on multiple platforms

task run {}
platforms.targetPlatforms.each { platform ->
    def platformRun = task "${platform}Run"(type: JavaExec) {
        dependsOn "${platform}Jar"
        classpath = files(tasks."${platform}Jar".archivePath,
        executable = "${platforms.jdkFor(platform)}/bin/java"
        main = 'com.acme.core.Main'

Why does Gradle help?

  • Add targetPlatform 'java9'

  • Generate module descriptor automatically?

  • Profit!

Trying it out

Automatically generating

Generated file contents

module com.acme.utils {
   requires commons.lang3;
   exports com.acme.utils;

Multi-release jars

  • Good idea: provide different versions of classes depending on runtime

  • Bad idea: in general, dependencies will be different so should be different artifacts

  • But let’s give it a try…​

Creating a MRJAR

task multireleaseJar(type:Jar) {
    dependsOn compileJavaJava7, compileJavaJava9
    baseName = "${}-multi"
    from compileJavaJava7.destinationDir
    into('META-INF/versions/9') {
       from compileJavaJava9.destinationDir
    manifest.attributes('Multi-Release': 'true')

Fixing the run configurations

platforms.targetPlatforms.each { platform ->
    def platformRun = task "${platform}Run"(type: JavaExec) {
        dependsOn multireleaseJar
        classpath = files(multireleaseJar.archivePath,
        executable = "${platforms.jdkFor(platform)}/bin/java"
        main = 'com.acme.core.Main'

Create version specific classes

What’s next?

  • Making this a first class citizen in Gradle

  • Automatic toolchain selection

  • Toolchain provisioning

  • Support for modulepath

  • Support for jimage and jlink

Be involved

Free webinars


September 28th, 2016 - 11am PDT / 2 pm EDT

By Mark Vieira (Principal Engineer at Gradle Inc)

Learn how to use Gradle Cloud Services to substantially improve the process of troubleshooting build/test failures, performance tuning of your builds, and analyzing your dependencies and plugins.

The latest release of Gradle Cloud Services will be available end of September. We would like to invite you to a 20 minutes webinar featuring what is new on Wednesday, September 28.


Wed - Thurs October 26 - 27 8:30am - 12:30pm PDT

By Gary Hale (Principal Engineer at Gradle Inc)

Get hands-on training taught live by a Gradle core developer on the fundamentals features of Gradle in 8 hours.

We are very excited to announce a one-time offer to attend Gradle Inc’s Introduction to Gradle Training for FREE! For the last few years we have been charging $700-$900 per seat for our live, virtual Introduction to Gradle training. Don’t miss this chance to get a free seat to one our most popular training classes.

We’re hiring!


Thank you!