One of the most exciting features in Java 9 are modular runtime images. Using the new jlink utility, you can create customized distributions which contain your app, its dependencies and just the JDK modules which it needs. For instance, a simple service based on the Undertow web server can be packaged into an image of just 25 MB, batteries included.

While that’s pretty cool already, it gets even nicer, as jlink provides a plug-in mechanism which allows to fine-tune the contents modular runtime images as they are created. There’s a set of jlink plug-ins coming with the JDK, e.g. for compressing image contents or removing debug symbols. But many more use cases may benefit from this API, for instance one could imagine plug-ins for removing un-used code or for performing byte code instrumentation of JPA entities. In the following, we’ll create a plug-in for adding an annotation index to the created image. At runtime, this index can then be used to discover annotations very efficiently, i.e. without loading classes and using reflection.

Sound great? For sure it does, there’s just one catch: the jlink plug-in API is not a supported part of the JDK as of Java 9. In fact, its packages are not even exported from the jdk.jlink module. This means some tricks are required to create custom plug-ins and run the jlink tool with these plug-ins enabled. The API may change in future Java versions, so any custom plug-in may break.

Nevertheless it’s definitely worth to explore the API and see what it can do. It’s my hope that it’ll be promoted to a public API eventually.

Creating the Plugin Implementation

Let’s begin by creating an implementation of the Plugin interface:

public class AddIndexPlugin implements {
    // ...

As the plugin package currently isn’t exported by the jdk.jlink module, this export must be added dynamically when compiling our custom Plugin implementation. To do so, the following compiler argument needs to be specified:


Each plug-in for jlink is enabled by a dedicated option which may have an optional value: --plugin-option=value. If there are additional arguments for that option, they are to be given like this: --plugin-option=value:arg-2=value-2:arg-3=value-3.... In the case of our plug-in for adding an annotation index, we’d like to enable the plug-in like so:

jlink --module-path path/to/modules \
    --add-modules some.module \
    --output path/to/image-dir \

The following options are given:

  • --add-modules: specifies the root modules to resolve

  • --module-path: specifies where to find the modules

  • --output: specifies where to create the runtime image

  • --add-index: the name of the option enabling our plug-in, its value com.example.b being the name of the module to which the index should be added; for-modules is an argument to that option, which receives a comma-separated list of modules which should be considered when creating the index

The following shows the implementation for exposing the plug-in name (which by default will also be used as the name of the option for enabling the plug-in) and receiving the plug-in configuration:

public class AddIndexPlugin implements Plugin {

    private static final String NAME = "add-index";
    private String targetModule;
    private List<String> modules;

    public String getName() {
        return NAME;

    public boolean hasArguments() {
        return true;

    public void configure(Map<String, String> config) {
        targetModule = config.get( NAME );
        String modulesToIndex = config.get( "for-modules" );
        this.modules = Arrays.asList( modulesToIndex.split( "," ) );

    // ...

hasArguments() must return true as our option has one more argument (for-modules) besides the option value itself. In configure() we receive the value for the plug-in option itself (indicating the target module for the index) and the value for the for-modules argument, indicating the name(s) of the modules to be indexed.

By implementing getDescription() and getArgumentsDescription(), a description of the plug-in and its arguments will be displayed when invoking jlink --list-plugins:

public String getDescription() {
    return "Adds an annotation index for one or more modules." + System.lineSeparator() +
            "<target-module>: name of the module which will host the index" + System.lineSeparator() +
            "<source-module-list>: comma-separated list of modules to include within the index";

public String getArgumentsDescription() {
    return "<target-module>:for-modules=<source-module-list>";

Finally, it’s time for implementing the actual logic of the plug-in itself. This is done within the transform() method, which receives a pool of resources to be added to the runtime image. This resource pool can be visited, building up a new pool via the passed ResourcePoolBuilder, while doing so. We may either pass on resources unmodified, alter or drop them or even add completely new resources. The contents of a resource pool entry can be access via content() which returns an InputStream, e.g. representing class file data.

In our case we traverse the input pool and pass on all resources as is. Classes in any of the configured source modules are added to an annotation index. This index is created with help of the Jandex library, which also is used by the WildFly application server to speed up annotation retrieval.

Ultimately, the annotation index is added as an additional resource within the configured target module (note that any exception handling has been omitted from this snippet for the sake of brevity):

public ResourcePool transform(ResourcePool in, ResourcePoolBuilder out) {
    Indexer indexer = new Indexer();

        e -> {
            // is it a class file in any of the source modules?
            if ( addToIndex( e ) ) {
                indexer.index( e.content() );

            return e;

    // write the index to a byte array and add it to /META-INF/jandex.idx in the target module
    byte[] index = writeToOutputStream( indexer ).toByteArray();
    out.add( ResourcePoolEntry.create( "/" + targetModule + "/META-INF/jandex.idx", index ) );


private boolean addToIndex(ResourcePoolEntry entry) {
    if ( !entry.path().endsWith( "class" ) ) {
        return false;

    for ( String moduleToIndex : modules ) {
        if ( entry.path().startsWith( "/" + moduleToIndex ) ) {
            return true;

    return false;

private ByteArrayOutputStream writeToOutputStream(Indexer indexer) {
    ByteArrayOutputStream outStream = new ByteArrayOutputStream();
    Index index = indexer.complete();
    IndexWriter writer = new IndexWriter( outStream );


    return outStream;

Having created the jlink plug-in, it’s time to see it in action. This is where things become a bit tricky, though. Remember the --add-exports option used for compilation above? The same will be needed when running jlink, but unfortunately this option isn’t supported by the tool.

Update: As I learned from Alan Bateman, all the VM options actually can be passed to jlink (and other JDK tools) in the form of -J-javaoption, e.g. -J--module-path=.... Therefore the wrapper described in the following actually isn’t needed. I’m leaving the section here for future reference, but you can safely skip it and continue reading here.

So how can we make sure that the module containing our plug-in can "see" the Plugin interface at runtime? The new ToolProvider SPI of Java 9 (which we’ve discussed on this blog recently) comes in handy for that. This SPI allows to call all the JDK tools programmatically, running within the same process. We can leverage that SPI by creating a simple wrapper around jlink, whose only purpose is to receive additional arguments which aren’t supported by the jlink binary itself:

public class JLinkWrapper {

    public static void main(String[] args) {
        Optional<ToolProvider> jlink = ToolProvider.findFirst( "jlink" );


By calling this wrapper instead of of jlink directly, also other options such as -Xdebug could be passed, if needed.

Trick 2: The Java Agent

With the wrapper around jlink in place, one might think that passing the --add-exports option (well, -J--add-exports) will eventually get us to our goal of calling jlink with our custom plug-in. There’s one more issue to resolve though, and this is related to how plug-ins are discovered by jlink. This is done using the service loader mechanism, so the natural way for exposing our plug-in would be to add the following line to the descriptor of our module:

provides with org.hibernate.demos.jlink.plugins.AddIndexPlugin;

Unfortunately, this doesn’t work, though. The reason being that the service binding (i.e. processing of the provides clause above) is done by the platform before it handles the --add-exports option. This results in an error during service binding, as Plugin isn’t exported to our module and we therefore cannot provide an implementation for it.

So it seems that we’re back to square 1. There’s one loophole, though, which is the Java Agent API. By implementing and registering a small agent, we get a bit more flexibility for amending module exports and provided services. This allows us to perform the required steps in the correct order:

public class JLinkPluginRegistrationAgent {

    public static void premain(String agentArgs, Instrumentation inst) throws Exception {
        Module jlinkModule = ModuleLayer.boot().findModule( "jdk.jlink" ).get();
        Module addIndexModule = ModuleLayer.boot().findModule( "org.hibernate.demos.jlink" ).get();

        Map<String, Set<Module>> extraExports = new HashMap<>();
        extraExports.put( "", Collections.singleton( addIndexModule ) );

        // alter jdk.jlink to export its API to the module with our indexing plug-in
                jlinkModule, Collections.emptySet(), extraExports, Collections.emptyMap(),
                Collections.emptySet(), Collections.emptyMap()

        Class<?> pluginClass = jlinkModule.getClassLoader()
                .loadClass( "" );
        Class<?> addIndexPluginClass = addIndexModule.getClassLoader()
                .loadClass( "org.hibernate.demos.jlink.plugins.AddIndexPlugin" );

        Map<Class<?>, List<Class<?>>> extraProvides = new HashMap<>();
        extraProvides.put( pluginClass, Collections.singletonList( addIndexPluginClass ) );

        // alter the module with the indexing plug-in so it provides the plug-in as a service
                addIndexModule, Collections.emptySet(), Collections.emptyMap(),
                Collections.emptyMap(), Collections.emptySet(), extraProvides

In the agent’s premain() method we first amend the exports of the jdk.jlink module so that it exports the package to the module with our custom plug-in. Then we amend the descriptor of this module to provide an implementation of the Plugin service contract.

And with that we finally can run jlink with our custom plug-in:

$JAVA_HOME/bin/jlink -J-javaagent:path/to/jlink-agent.jar \
    -J--module-path=path/to/modules \
    -J--add-modules=org.hibernate.demos.jlink \
    --module-path=$JAVA_HOME/jmods/:path/to/modules \
    --add-modules com.example.b \
    --output integrationtest/target/jlink-image \

All the -J arguments are passed to the VM launched by jlink, whereas all the other arguments apply to the jlink tool itself.

Update: The following invocation applied to the JLinkWrapper class, which actually isn’t needed as pointed out before. Just invoke jlink directly as shown above.

And with that we finally can run jlink (via our wrapper) with our custom plug-in:

java -javaagent:path/to/jlink-agent.jar \
    --module-path path/to/modules \
    --module org.hibernate.demos.jlink/org.hibernate.demos.jlink.JLinkWrapper \
    --module-path $JAVA_HOME/jmods/:path/to/modules \
    --add-modules com.example.b \
    --output path/to/jlink-image \

Note that JLinkWrapper is the executed main class here. All arguments after --module are passed to its main() method, which in turn passes them on to the programmatically launched jlink tool.

Using the Index

As a last step let’s take a look at some code within the com.example.b module, which makes use of the generated annotation index:

public class Main {

    public static void main(String[] args) throws Exception {
        try (InputStream input = Main.class.getResourceAsStream( "/META-INF/jandex.idx" ) ) {
            IndexReader reader = new IndexReader( input );
            Index index =;

            List<AnnotationInstance> entityInstances = index.getAnnotations(
                DotName.createSimple( "com.example.a.Entity" )

            for (AnnotationInstance annotationInstance : entityInstances) {
                System.out.println( );

Here we open the index which has been generated and injected via our custom plug-in and simply print out all classes annotated with @Entity. Of course this requires that the Jandex library is part of the runtime image, too. As Jandex isn’t a Java 9 module yet at this point, we can use ModiTect for adding a descriptor on the fly (ModiTect is a side-project I’ve started to facilitate the work with Java 9 modules and modular runtime images).


In this post we’ve demonstrated how to create custom plug-ins for the jlink tool introduced in Java 9.

jlink allows you to create fully self-contained runtime images containing your application, its dependencies and the JDK modules it requires. The plug-in API of jlink allows to adjust runtime images as they are created, e.g. by adding an annotation index as show in the example above. But there are many more potential use cases for that API, so it’d be great if that API were promoted to an offically supported public API in one of the next JDK releases.

For sure it’d be very helpful to let the JDK team know about your use cases for this API and your findings from explorations as the one conducted here.

You can find a complete example with the annotation index plug-in in the Hibernate demos repository. The project contains a module for the plug-in, a module for the agent, one example module which will be added to the index, one example module which will contain the generated index and one integration test module. The integration test first runs jlink with the annotation index plug-in and then runs the generated runtime image, asserting the standard output of the second example module, which should contain the names of an expected set of "entities" contained in the first example module.

Many thanks to Alan Bateman and Remi Forax for their help while exploring the jlink API!

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