JDK 9: Creating a Java Runtime Image With Maven

At the moment, JDK 9 is only available as Early Access (EA) to let the community take a look how it works and what can be improved. Apart from all the news, for example about the modular system Jigsaw, there is one important question:

How can I create a Java runtime with Maven? But before we can begin, let's do a quick recap of runtime images.

Say you have a running Java runtime environment where all the parts that aren't really used by the current application have been removed. Based on the modularization of the Java platform itself, you can think of many parts that can be removed by a usual application. This, in the end, will give you the opportunity to deliver only the Java runtime image including your application. So this means in the target environment, you do not need to install a full blown JRE. This saves space in particular if you think about microservices.

There are a few things that need to be paid attention to. Everything written here is:

• Currently in a proof of concept state.

• Speculative.

So this means it might become an official Maven plugin. The intention is to get some feedback from interested volunteers — to see others working with JDK 9 and to learn about their experiences and to see if Maven needs to go in a different direction.

So let's start with the interesting part. Let's start with a simple multi-module project and see how this works. The example project currently contains three modules, mod-1, mod-2 , and mod-jar. The mod-jar is a usual JAR project that produces a JAR file as result.

The only important part is that this contains a module-info.java file, which is compiled into the JAR file. This JAR file is a modular JAR file because it contains a module-info.class file at the root location of the JAR.

Meanwhile, mod-1 and mod-2 are currently explicitly producing jmod files, which contain the module name in its file name. At the moment, those modules are placed into the target/jmods/ directory of the appropriate project.

So now we have summarized the different projects that produce either a JAR or jmod file, but to get a Java runtime image, we have to use the jlink command of JDK 9, which is the Java Linker that produces the runtime image. Then handling of jlink is currently put into the mod-jlink module, which contains a packaging of jlink.

The example project uses two newly created Maven plugins (maven-jmod-plugin and maven-jlink-plugin) that currently implement the handling for jmod and the jlink packaging type. They exist only as SNAPSHOTs that are available via the Apache Software Foundation repository. If you would like to use them, you have to configure them into your settings.xml or in your repository manager. One more thing is needed to be set up: toolchains to configure the environment, like where to find the JDK 9, etc.

If you run the whole project via:

mvn clean package
...
[INFO] ------------------------------------------------------------------------
[INFO] Building com.soebes.nine.one 1.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO]
[INFO] --- maven-clean-plugin:3.0.0:clean (default-clean) @ com.soebes.nine.one ---
[INFO] Deleting /Users/kama/jdk-9-test/maven-example/mod-1/target
[INFO]
[INFO] --- maven-toolchains-plugin:1.1:toolchain (default) @ com.soebes.nine.one ---
[INFO] Required toolchain: jdk [ vendor='oracle' version='1.9' ]
[INFO] Found matching toolchain for type jdk: JDK[/Library/Java/JavaVirtualMachines/jdk1.9.0_ea+172.jdk/Contents/Home]
[INFO]....
...
[INFO]
[INFO] --- maven-compiler-plugin:3.6.0:compile (default-compile) @ com.soebes.nine.one ---
[INFO] Toolchain in maven-compiler-plugin: JDK[/Library/Java/JavaVirtualMachines/jdk1.9.0_ea+172.jdk/Contents/Home]
[INFO] Changes detected - recompiling the module!
[INFO] Compiling 2 source files to /Users/kama/jdk-9-test/maven-example/mod-1/target/classes
[INFO]
...
...
[INFO]
[INFO] --- maven-jlink-plugin:3.0.0-SNAPSHOT:jlink (default-jlink) @ mod-jlink ---
[INFO] Toolchain in maven-jlink-plugin: jlink [ /Library/Java/JavaVirtualMachines/jdk1.9.0_ea+172.jdk/Contents/Home/bin/jlink ]
[INFO] The following dependencies will be linked into the runtime image:
[INFO]  -> com.soebes.examples.jdk9:com.soebes.nine.one:jmod:1.0-SNAPSHOT
[INFO]  -> com.soebes.examples.jdk9:com.soebes.nine.two:jmod:1.0-SNAPSHOT
[INFO]  -> com.soebes.examples.jdk9:com.soebes.nine.jar:jar:1.0-SNAPSHOT
[INFO] Building zip: /Users/kama/jdk-9-test/maven-example/mod-jlink/target/mod-jlink-1.0-SNAPSHOT.zip
[INFO]
[INFO] --- maven-site-plugin:3.6:attach-descriptor (attach-descriptor) @ mod-jlink ---
[INFO] Skipping because packaging 'jlink' is not pom.
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO]
[INFO] parent ............................................. SUCCESS [  0.931 s]
[INFO] com.soebes.nine.one ................................ SUCCESS [ 17.019 s]
[INFO] com.soebes.nine.two ................................ SUCCESS [  1.007 s]
[INFO] com.soebes.nine.jar ................................ SUCCESS [  0.926 s]
[INFO] mod-jlink .......................................... SUCCESS [  2.602 s]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 23.200 s
[INFO] Finished at: 2017-06-06T20:08:54+02:00
[INFO] Final Memory: 19M/385M
[INFO] ------------------------------------------------------------------------

If everything works, then you will find the Java runtime image:

cd mod-jlink/target/jlink
~/jdk-9-test/maven-example/mod-jlink/target/jlink (master)$ ls -la
total 8
drwxr-xr-x   8 kama  staff  272 Jun  6 00:10 .
drwxr-xr-x   4 kama  staff  136 Jun  6 00:10 ..
drwxr-xr-x   4 kama  staff  136 Jun  6 00:10 bin
drwxr-xr-x   4 kama  staff  136 Jun  6 00:10 conf
drwxr-xr-x   7 kama  staff  238 Jun  6 00:10 include
drwxr-xr-x   3 kama  staff  102 Jun  6 00:10 legal
drwxr-xr-x  19 kama  staff  646 Jun  6 00:10 lib
-rw-r--r--   1 kama  staff   97 Jun  6 00:10 release

You can check the Java runtime first via:

~/jdk-9-test/maven-example/mod-jlink/target/jlink (master)$ bin/java --list-modules
com.soebes.nine.jar
com.soebes.nine.one@1.0-SNAPSHOT
com.soebes.nine.two@1.0-SNAPSHOT
java.base@9-ea

which shows that this JRE only contains those modules which are needed. Of course, you can check the size of generated image on the hard drive:

~/jdk-9-test/maven-example/mod-jlink/target/jlink (master)$ du -h -d 1 .
144K    ./bin
 76K    ./conf
184K    ./include
 44K    ./legal
 35M    ./lib
 35M    .

So this means the whole Java runtime image is only 35 MiB in size. You can check against a full-fledged JRE and compare that with the 35 MiB.

To test different modules in that Java runtime image, you can call them like this:

~/jdk-9-test/maven-example/mod-jlink/target/jlink (master)$ bin/java com.soebes.example.nine.jar.Main
Main from Java 9 (jar)
~/jdk-9-test/maven-example/mod-jlink/target/jlink (master)$ bin/java com.soebes.example.nine.one.Main
Main one from Java 9
~/jdk-9-test/maven-example/mod-jlink/target/jlink (master)$ bin/java com.soebes.example.nine.two.Main
Main two from Java 9

Conclusion

This is the first step of Maven to support of JDK 9 tools like jlink and jmod. Other tools, like maven-jdeps-plugin, have existed for a longer time.

This is currently a proof of concept and not an official statement. This is intended to get feedback from others working with JDK 9 and see if we are on the right track.

Apart from that, you see the opportunities that JDK 9 with its runtime images is offering. Based on the current implementation of the plugins, it would not be very complicated to migrate an existing application (only command line apps??) to a Java runtime image.