PF4J : Custom PluginManager

To create a custom plugin manager you could:

implements PluginManager interface (create a plugin manager from scratch)
modifies some aspects/behaviors of built-in implementations (DefaultPluginManager, JarPluginManager)
extends AbstractPluginManager class

JarPluginManager is a PluginManager that loads plugin from a jar file. Actually, a plugin is a fat jar, a jar which contains classes from all the libraries, on which your project depends and, of course, the classes of current project. AbstractPluginManager adds some glue that help you to create quickly a plugin manager. All you need to do is to implement some factory methods. PF4J uses in many places the factory method pattern to implement the dependency injection (DI) concept in a manually mode. See below the abstract methods for AbstractPluginManager:

public abstract class AbstractPluginManager implements PluginManager {

    protected abstract PluginRepository createPluginRepository();
    protected abstract PluginFactory createPluginFactory();
    protected abstract ExtensionFactory createExtensionFactory();
    protected abstract PluginDescriptorFinder createPluginDescriptorFinder();
    protected abstract ExtensionFinder createExtensionFinder();
    protected abstract PluginStatusProvider createPluginStatusProvider();
    protected abstract PluginLoader createPluginLoader();

    // other non abstract methods

}

DefaultPluginManager contributes with “default” components (DefaultExtensionFactory, DefaultPluginFactory, DefaultPluginLoader, …) to AbstractPluginManager.
Most of the times it’s enough to extends DefaultPluginManager and to supply your custom components.

Starting with version 2.0 it’s possible to coexist multiple plugins types (jar, zip, directory) in the same PluginManager. For example, DefaultPluginManager works out of the box with zip and jar plugins. The idea is that DefaultPluginManager uses a compound version for:

PluginDescriptorFinder (CompoundPluginDescriptorFinder)
PluginLoader (CompoundPluginLoader)
PluginRepository (CompoundPluginRepository)

public class DefaultPluginManager extends AbstractPluginManager {
   
    ...
    
    @Override
    protected PluginDescriptorFinder createPluginDescriptorFinder() {
        return new CompoundPluginDescriptorFinder()
            .add(new PropertiesPluginDescriptorFinder())
            .add(new ManifestPluginDescriptorFinder());
    }
    
    @Override
    protected PluginRepository createPluginRepository() {
        return new CompoundPluginRepository()
            .add(new DefaultPluginRepository(getPluginsRoot(), isDevelopment()))
            .add(new JarPluginRepository(getPluginsRoot()));
    }
    
    @Override
    protected PluginLoader createPluginLoader() {
        return new CompoundPluginLoader()
            .add(new DefaultPluginLoader(this, pluginClasspath))
            .add(new JarPluginLoader(this));
    }

}

So, it’s very easy to add new strategies for plugin descriptor finder, plugin loader and plugin repository.

PF4J Plugin Framework for Java

Custom PluginManager