Creating plugins

Defining the ContainerManager or the classloader manager

The entry point of the API is the ContainerManager. It allows you to define what is the Shared classloader and to create children:

try (final ContainerManager manager = new ContainerManager( (1)
    ContainerManager.DependenciesResolutionConfiguration.builder() (2)
        .resolver(new MvnDependencyListLocalRepositoryResolver("META-INF/talend/dependencies.list"))
        .rootRepositoryLocation(new File(System.getProperty("user.home"), ".m2/repository"))
        .create(),
    ContainerManager.ClassLoaderConfiguration.builder() (3)
        .parent(getClass().getClassLoader())
        .classesFilter(name -> true)
        .parentClassesFilter(name -> true)
        .create())) {

    // create plugins

}
1 The ContainerManager is AutoCloseable, which allows you to use it in a try or finally block if needed.
This manager has two main configuration entries:

  • how to resolve dependencies for plugins from the plugin file/location

  • how to configure the classloaders (what is the parent classloader, how to handle the parent first/last delegation, and so on).

    It is recommended to keep the manager running if you can reuse plugins in order to avoid recreating classloaders and to mutualize them.
2 DependenciesResolutionConfiguration allows you to pass a custom Resolver which is used to build the plugin classloaders.
Currently, the library only provides MvnDependencyListLocalRepositoryResolver, which reads the output of mvn dependencies:list. Add it to the plugin jar to resolve the dependencies from a local maven repository.
Note that SNAPSHOT are only resolved based on their name and not from the metadata (only useful in development).
To continue the comparison with a Servlet server, you can implement an unpacked war resolver.
3 ClassLoaderConfiguration configures the behavior of the whole container/plugin pair, including:

  • What the shared classloader is

  • Which classes are loaded from the shared loader first (intended to be used for API which should not be loaded from the plugin loader)

  • Which classes are loaded from the parent classloader. This can be useful to prevent loading a "common" library from the parent classloader. For instance, it can be neat for guava, commons-lang3, an so on).

Creating plugins

Once you have defined a manager, you can create plugins:

final Container plugin1 = manager.create( (1)
    "plugin-id", (2)
    new File("/plugin/myplugin1.jar")); (3)
1 To create a plugin Container, use the create method of the manager.
2 Give an explicit ID to the plugin. You can choose to bypass it. In that case, the manager uses the jar name.
3 Specify the plugin root jar.

To create the plugin container, the Resolver resolves the dependencies needed for the plugin, then the manager creates the plugin classloader and registers the plugin Container.

Defining a listener for the plugin registration

Some actions are needed when a plugin is registered or unregistered. For that purpose, you can use ContainerListener:

public class MyListener implements ContainerListener {
    @Override
    public void onCreate(final Container container) {
        System.out.println("Container #" + container.getId() + " started.");
    }

    @Override
    public void onClose(final Container container) {
        System.out.println("Container #" + container.getId() + " stopped.");
    }
}

Plugins are directly registered on the manager:

final ContainerManager manager = getContainerManager();
final ContainerListener myListener = new MyListener();

manager.registerListener(myListener); (1)
// do something
manager.unregisterListener(myListener); (2)
1 registerListener is used to add the listener going forward. However, it does not get any event for already created containers.
2 You can remove a listener at any time by using unregisterListener.
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