Create Java runtime image on one platform for another using Jlink

Question

I created runtime image using jlink on my Linux machine. And I see linux folder under the include folder. Does it mean that I can use this runtime image only for Linux platform? If yes, are there any ways to create runtime images on one platform for another (e.g. on Linux for Windows and vice versa)

Granth

Answer 1

The include directory is for header files, such as jni.h, that are needed when compiling C/C++ code that uses JNI and other native interfaces. It's nothing to do with jlink.

The jlink tool can create a run-time image for another platform (cross targeting). You need to download two JDKs to do this. One for the platform where you run jlink, the other for the target platform. Run jlink with --module-path $TARGET/jmods where $TARGET is the directory where you've unzipped the JDK for the target platform.

Outpouring

Answer 2

Being generally unable to add anything to Alan Bateman's answers in terms of information, I'll offer a working example. This example illustrates using jlink on Mac OS and then running the binary on Ubuntu in a Docker container.

The salient points are as follows.

Given two simple modules, we compile on Mac OS:

javac -d build/modules \
--module-source-path src \
`find src -name "*.java"`

jar --create --file=lib/[email protected] \
-C build/modules/net.codetojoy.db . 

jar --create --file=lib/[email protected] \
-C build/modules/net.codetojoy.service . 

Assuming that the Linux 64 JDK is unpacked in a local directory (specified as command-line arg), we call jlink (on Mac OS in this example). JAVA_HOME is the crux of the solution:

# $1 is ./jdk9_linux_64/jdk-9.0.1
JAVA_HOME=$1 

rm -rf serviceapp 

jlink --module-path $JAVA_HOME/jmods:build/modules \
--add-modules net.codetojoy.service \
--output serviceapp

Then, assuming we've pulled the ubuntu image for Docker, we can execute the following in a Docker terminal (i.e. Linux):

docker run --rm -v $(pwd):/data ubuntu /data/serviceapp/bin/java net.codetojoy.service.impl.UserServiceImpl

TRACER : hello from UserServiceImpl

To re-iterate this feature of Java 9/jlink: Linux does not have Java installed and the Linux binary was built on Mac OS.

Outnumber

Using JAVA_HOME as the variable name is unnecessarily confusing. Call it LINUX_JAVA_HOME or UNPACKED_LINUX_JAVA or WHATEVER. – Architectural

What is the functional purpose of the docker for the average reader? Why not just extract the target JDK to a local directory? Also, please fix the improper use of the reserved $JAVA_HOME variable per comment above. – Parra

Using docker is overcomplicating things unnecessarily here. – Supersession

On the other hand, using docker here shows you how to test your newly created binary on the same machine without using a VM, rebooting or anything else. – Rueful