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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.

Features/Benefits

With PF4J you can easily transform a monolithic java application in a modular application.
PF4J is an open source (Apache license) lightweight (around 50 KB) plugin framework for java, with minimal dependencies (only slf4j-api) and very extensible (see PluginDescriptorFinder and ExtensionFinder).

Practically PF4J is a microframework and the aim is to keep the core simple but extensible. I try to create a little ecosystem (extensions) based on this core with the help of the comunity.
For now are available these extensions:

No XML, only Java.

You can mark any interface or abstract class as an extension point (with marker interface ExtensionPoint) and you specified that an class is an extension with @Extension annotation.

Components

  • Plugin is the base class for all plugins types. Each plugin is loaded into a separate class loader to avoid conflicts.
  • PluginManager is used for all aspects of plugins management (loading, starting, stopping). You can use a built-in implementation as DefaultPluginManager or you can implement a custom plugin manager starting from AbstractPluginManager (implement only factory methods).
  • PluginLoader loads all information (classes) needed by a plugin.
  • ExtensionPoint is a point in the application where custom code can be invoked. It’s a java interface marker.
    Any java interface or abstract class can be marked as an extension point (implements ExtensionPoint interface).
  • Extension is an implementation of an extension point. It’s a java annotation on a class.