Plugin Framework for Java (PF4J)
A plugin is a way for a third party to extend the functionality of an application. A plugin implements extension points declared by application or other plugins. Also, a plugin can define extension points.
NOTE: Starting with version 0.9 you can define an extension directly in the application jar (you're not obligated to put the extension in a plugin - you can see this extension as a default/system extension). See WhazzupGreeting for a real example.
With PF4J you can easily transform a monolithic java application in a modular application.
PF4J is an open source (Apache license) lightweight (around 100 KB) plugin framework for java, with minimal dependencies (only slf4j-api) and very extensible (see PluginDescriptorFinder and ExtensionFinder).
Why Choose PF4J?
Practically, PF4J is a microframework that aims to keep the core simple but extensible. PF4J also provides a community-driven ecosystem of extensions.
No XML, only Java. You can mark any interface or abstract class as an extension point (with marker interface ExtensionPoint) and you specify that a class is an extension with @Extension annotation.
JarPluginManager, ZipPluginManager, DefaultPluginManager (it's a JarPluginManager + ZipPluginManager) or you can implement a custom plugin manager starting from AbstractPluginManager (implement only factory methods).ExtensionPoint interface).PLUGIN = a container for EXTENSION POINTS and EXTENSIONS + lifecycle methods (start, stop, delete)
A PLUGIN is similar with a MODULE from other systems. If you don't need lifecycle methods (hook methods for start, stop, delete) you are not forced to supply a plugin class (the PluginClass property from the plugin descriptor is optional). You only need to supply some description of plugin (id, version, author, ...) for a good tracking (your application wants to know who supplied the extensions or extensions points).
It's very simple to add pf4j in your application.
Define an extension point in your application/plugin using ExtensionPoint interface marker:
public interface Greeting extends ExtensionPoint {
String getGreeting();
}Create an extension using @Extension annotation:
@Extension
public class WelcomeGreeting implements Greeting {
public String getGreeting() {
return "Welcome";
}
}Create (it's optional) a Plugin class if you are interested in plugin's lifecycle events (start, stop, ...):
public class WelcomePlugin extends Plugin {
@Override
public void start() {
System.out.println("WelcomePlugin.start()");
}
@Override
public void stop() {
System.out.println("WelcomePlugin.stop()");
}
@Override
public void delete() {
System.out.println("WelcomePlugin.delete()");
}
}In above code we've created a plugin (welcome) that comes with one extension for the Greeting extension point.
You can distribute your plugin as a jar file (the simple solution). In this case add the plugin's metadata in MANIFEST.MF file of jar:
Manifest-Version: 1.0
Archiver-Version: Plexus Archiver
Created-By: Apache Maven
Built-By: decebal
Build-Jdk: 1.6.0_17
Plugin-Class: org.pf4j.demo.welcome.WelcomePlugin
Plugin-Dependencies: x, y, z
Plugin-Id: welcome-plugin
Plugin-Provider: Decebal Suiu
Plugin-Version: 0.0.1In the manifest above, we've described a plugin with the id of welcome-plugin (mandatory attribute). We've also defined a class org.pf4j.demo.welcome.WelcomePlugin (optional attribute), with version 0.0.1 (mandatory attribute) and with dependencies to plugins x, y, z (optional attribute).
Now you can play with plugins and extensions in your code:
…The output is:
>>> WelcomePF4J is very customizable and comes with a lot of goodies. Please read the documentation to discover yourself the power of this library.
Documentation is available on pf4j.org
Demo applications are available in demo folder
Many thanks to:
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