JLink Plugin Modules

This repository contains proof of concepts for dynamically loadable plugins for Java's jlink command.

Getting started

First of all current versions of the Java JDK do not export JLink's plugin API, so what we need to do is build a special version of the JDK with a very simple change applied.

Download/clone the JDK sources, for example from here: https://github.com/openjdk/jdk or here: https://github.com/openjdk/jdk-sandbox
Either manually edit the src/jdk.jlink/share/classes/module-info.java file by adding a exports jdk.tools.jlink.plugin; line, or apply a patch by running the following command:

patch -p1 <<EOF 
--- a/src/jdk.jlink/share/classes/module-info.java
+++ b/src/jdk.jlink/share/classes/module-info.java
@@ -55,6 +55,8 @@ module jdk.jlink {
    requires jdk.internal.opt;
    requires jdk.jdeps;

+    exports jdk.tools.jlink.plugin;
+
    uses jdk.tools.jlink.plugin.Plugin;

    provides java.util.spi.ToolProvider with
--
EOF

First you need to compile the JDK by following the instructions in their documentation, although it basically boils down to running:

$ bash configure
$ make images

in the root of the project.

If everything built correctly you should now have a JDK that supports loading and running external jlink plugins. For the remainder of this README we'll assume you're using that JDK to build any code or run any commands.

Building

In the root of the project simply run:

$ mvn clean package

This will build the plugins as well as the test applications.

Testing

To see if the plugins and the patched JDK were built correctly run the following:

$ jlink --module-path=lib --suggest-providers jdk.tools.jlink.plugin.Plugin

Suggested providers:
  dynamod provides jdk.tools.jlink.plugin.Plugin used by jdk.jlink
  jdk.jlink provides jdk.tools.jlink.plugin.Plugin used by jdk.jlink
  seal provides jdk.tools.jlink.plugin.Plugin used by jdk.jlink

The "dynamod" and "seal" plugins should show up in the output.

Another test is:

$ jlink -J--module-path=lib --list-plugins

List of available plugins:

Plugin Name: dyna-test
Plugin Class: plugin.dynamod.DynaModPlugin
Plugin Module: dynamod
Category: TRANSFORMER
Functional state: Functional.
Option: --dyna-test
Description: A test plugin to see if dynamic loading works

Plugin Name: seal
Plugin Class: plugin.seal.SealPlugin
Plugin Module: seal
Category: TRANSFORMER
Functional state: Functional.
Option: --seal=[comma separated list of module names]
Description: Analyzes classes and decided if they can be made final and/or marked sealed.

.
.
.

Which shows a list of all known plugins, their descriptions and arguments.

NB: As you might have noticed, this time we passed the --module-path using the -J option which sets it for the VM that jlink is using. This is actually necessary for jlink to actually access the plugin modules and use them.

DynaMod plugin

This is just a no-op plugin that can be used to test if the dynamic loading of plugins is working. It can also be used as a starting point to create new plugins.

For more details on how to use it see its README.

Seal plugin

The seal plugin is a plugin for the jlink command that analyses classes and tries to determine if the can be marked final or sealed.