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Using jlink to cross-compile minimal JREs
16 January 2023
jlink is a JDK tool to create bespoke, minimal JREs for your applications.
Let’s try it with a “Hello, world!” program:
class Main {
public static void main(String... args) {
System.out.println("Hello, world!");
}
}
My laptop is an M1 Mac and I have downloaded the Azul Zulu JDK 19 build for it.
With the JDK I can both compile Java and then run the resulting program.
$ mkdir out
$ zulu19.30.11-ca-jdk19.0.1-macosx_aarch64/bin/javac -d out in/Main.java
$ zulu19.30.11-ca-jdk19.0.1-macosx_aarch64/bin/java -cp out Main
Hello, world!
Azul Zulu also provides a JRE that I can use to run compiled programs.
$ zulu19.30.11-ca-jre19.0.1-macosx_aarch64/bin/java -cp out Main
Hello, world!
Note the slight change in folder name (“jdk” → “jre”).
If we were shipping this to end-users it would be an easy win for binary size.
$ du -hs zulu*
329M zulu19.30.11-ca-jdk19.0.1-macosx_aarch64
136M zulu19.30.11-ca-jre19.0.1-macosx_aarch64
But 136MiB just for “Hello, world”? Don’t tell Reddit or Hacker News!
Thankfully, jlink is here to help us build a minimal JRE with only what we need.
Given our program, a sibling tool, jdeps, lists the Java modules which are required.
$ zulu19.30.11-ca-jdk19.0.1-macosx_aarch64/bin/jdeps \
--print-module-deps \
out/Main.class
java.base
Our program is so simple that it only needs the “base” module.
Now with jlink we can produce a minimal JRE.
$ zulu19.30.11-ca-jdk19.0.1-macosx_aarch64/bin/jlink \
--compress 2 \
--strip-debug \
--no-header-files \
--no-man-pages \
--output zulu-hello-jre \
--add-modules java.base
$ du -hs zulu*
28M zulu-hello-jre
329M zulu19.30.11-ca-jdk19.0.1-macosx_aarch64
136M zulu19.30.11-ca-jre19.0.1-macosx_aarch64
28MiB won’t win any language wars, but it’s a massive 80% savings over the full JRE.
$ zulu-hello-jre/bin/java -cp out Main
Hello, world!
We can ship it to our client and call it a day, right?
$ tar -czf hello.tgz zulu-hello-jre out
$ scp hello.tgz jw@server:
hello.tgz 100% 14MB 2.0MB/s 00:07
$ ssh jw@server "tar xzf hello.tgz && zulu-hello-jre/bin/java -cp out Main"
bash: zulu-hello-jre/bin/java: cannot execute binary file: Exec format error
Nope!
While the Java bytecode we compiled is platform independent, the JRE is specific to each platform and my server runs Linux x64.
Thankfully, jlink can operate on JDKs for different platforms.
Let’s download the Linux x64 JDK and point jlink at its Java modules using --module-path.
$ zulu19.30.11-ca-jdk19.0.1-macosx_aarch64/bin/jlink \
--compress 2 \
--strip-debug \
--no-header-files \
--no-man-pages \
--output zulu-hello-jre-linux-x64 \
--module-path zulu19.30.11-ca-jdk19.0.1-linux_x64/jmods
--add-modules java.base
$ du -hs zulu*
28M zulu-hello-jre
36M zulu-hello-jre-linux-x64
338M zulu19.30.11-ca-jdk19.0.1-linux_x64
329M zulu19.30.11-ca-jdk19.0.1-macosx_aarch64
136M zulu19.30.11-ca-jre19.0.1-macosx_aarch64
The Linux x64 JRE is a little larger than the one for my ARM Mac, but it’s still small compared to the full-size JRE.
Does it work on the client?
$ tar -czf hello-linux.tgz zulu-hello-jre-linux-x64 out
$ scp hello-linux.tgz jw@server:
hello.tgz 100% 16MB 2.1MB/s 00:08
$ ssh jw@server "tar xzf hello-linux.tgz && zulu-hello-jre-linux-x64/bin/java -cp out Main"
Hello, world!
It works! Now we can grab JDKs for any architecture for any platform and use our host jlink to effectively cross-compile minimal JREs for each target.
This is a great solution for multi-architecture Docker containers, desktop clients like JetBrains Compose UI, shipping to devices where you can’t fit a full JDK, and more.
Be sure to explore all the options on jdeps and jlink for ways to keep your runtimes small.
— Jake Wharton