JLink plugin

The JLink plugin can be used for Java projects where you would want to deploy a Java runtime specific for your application or install a minimal Java runtime.

Keys

This plugin provides the following unique keys.

jlink-modules

Type: list of strings

The exhaustive list of modules to include in the OpenJDK image. If set, this key overrides all other keys for this plugin.

jlink-jars

Type: list of strings

List of paths to your application’s JAR files. If not specified, the plugin will find all JAR files in the staging area.

jlink-extra-modules

Type: list of strings

Additional modules to include in the OpenJDK image.

jlink-multi-release

Type: int or string

Default: “base”

OpenJDK release version to use for multi-release JARs.

Dependencies

The plugin expects OpenJDK to be available on the system and to contain the jlink executable. OpenJDK can be defined as a build-package in the part using jlink plugin. Another alternative is to define another part with the name jlink-deps, and declare that the part using the jlink plugin comes after the jlink-deps part through the after key.

If the system has multiple OpenJDK installations available, one must be selected by setting the JAVA_HOME environment variable.

parts:
  runtime:
    plugin: jlink
    build-packages:
      - openjdk-21-jdk
    build-environment:
      - JAVA_HOME: /usr/jvm/java-21-openjdk-${CRAFT_ARCH_BUILD_FOR}

The user is expected to stage OpenJDK dependencies either by installing an appropriate OpenJDK slice:

parts:
  runtime:
    plugin: jlink
    build-packages:
      - openjdk-21-jdk
    after:
      - deps

    deps:
      plugin: nil
      stage-packages:
        - openjdk-21-jre-headless_security
      stage:
        - -usr/lib/jvm

Or, by installing the dependencies directly:

parts:
  runtime:
    plugin: jlink
    build-packages:
      - openjdk-21-jdk
    after:
      - deps

  deps:
    plugin: nil
    stage-packages:
      - libc6_libs
      - libgcc-s1_libs
      - libstdc++6_libs
      - zlib1g_libs
      - libnss3_libs

How it works

If the jlink-modules key is set, the plugin runs jlink to create an OpenJDK image with the listed modules.

Otherwise, the plugin performs the following actions:

1. Find all JAR files in the staging area or selects jars specified in jlink-jars.

2. Unpack JAR files to the temporary location and concatenate all embedded jars into jdeps class path.

3. Run jdeps to discover Java modules required for the staged jars.

4. Run jlink to create a runtime image from the build JDK.