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.

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 35KB) plugin framework for java, with minimal dependencies and very extensible (see PluginDescriptorFinder and ExtensionFinder).

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).
- **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.

Artifacts
-------------------
- PF4J `pf4j` (jar)
- PF4J Demo `pf4j-demo` (executable jar)

Using Maven
-------------------
In your pom.xml you must define the dependencies to PF4J artifacts with:

```xml
<dependency>
    <groupId>ro.fortsoft.pf4j</groupId>
    <artifactId>pf4j</artifactId>
    <version>${pf4j.version}</version>
</dependency>    
```

where ${pf4j.version} is the last pf4j version.

You may want to check for the latest released version using [Maven Search](http://search.maven.org/#search%7Cga%7C1%7Cpf4j)

How to use
-------------------
It's very simple to add pf4j in your application:

    public static void main(String[] args) {
        ...
        
        PluginManager pluginManager = new DefaultPluginManager();
        pluginManager.loadPlugins();
        pluginManager.startPlugins();

        ...
    }

In above code, I created a **DefaultPluginManager** (it's the default implementation for
**PluginManager** interface) that loads and starts all active(resolved) plugins.  
Each available plugin is loaded using a **PluginClassLoader**.   
The **PluginClassLoader** contains only classes found in _classes_ and _lib_ folders of plugin and runtime classes and libraries of the required plugins. 
The plugins are stored in a folder. You can specify the plugins folder in the constructor of DefaultPluginManager. If the plugins folder is not specified 
than the location is returned by `System.getProperty("pf4j.pluginsDir", "plugins")`.

The structure of plugins folder is:
* plugin1.zip (or plugin1 folder)
* plugin2.zip (or plugin2 folder)

In plugins folder you can put a plugin as folder or archive file (zip).
A plugin folder has this structure:
* `classes` folder
* `lib` folder (optional - if the plugin used third party libraries)

The plugin manager searches plugins metadata using a **PluginDescriptorFinder**.   
**DefaultPluginDescriptorFinder** lookups plugins descriptors in MANIFEST.MF file.
In this case the `classes/META-INF/MANIFEST.MF` file looks like:

    Manifest-Version: 1.0
    Archiver-Version: Plexus Archiver
    Created-By: Apache Maven
    Built-By: decebal
    Build-Jdk: 1.6.0_17
    Plugin-Class: ro.fortsoft.pf4j.demo.welcome.WelcomePlugin
    Plugin-Dependencies: x, y, z
    Plugin-Id: welcome-plugin
    Plugin-Provider: Decebal Suiu
    Plugin-Version: 0.0.1

In above manifest I described a plugin with id `welcome-plugin`, with class `ro.fortsoft.pf4j.demo.welcome.WelcomePlugin`, with version `0.0.1` and with dependencies 
to plugins `x, y, z`.

You can define an extension point in your application using **ExtensionPoint** interface marker.

    public interface Greeting extends ExtensionPoint {

        public String getGreeting();

    }

Another important internal component is **ExtensionFinder** that describes how plugin manager discovers extensions for extensions points.   
**DefaultExtensionFinder** looks up extensions using **Extension** annotation.

    public class WelcomePlugin extends Plugin {

        public WelcomePlugin(PluginWrapper wrapper) {
            super(wrapper);
        }

        @Extension
        public static class WelcomeGreeting implements Greeting {

            public String getGreeting() {
                return "Welcome";
            }

        }

    }

In above code I supply an extension for the `Greeting` extension point.

You can retrieve all extensions for an extension point with:

    List<Greeting> greetings = pluginManager.getExtensions(Greeting.class);
    for (Greeting greeting : greetings) {
        System.out.println(">>> " + greeting.getGreeting());
    }

The output is:

    >>> Welcome
    >>> Hello

You can inject your custom component (for example PluginDescriptorFinder, ExtensionFinder) in DefaultPluginManager just override createXXX methods (factory method pattern).

Example:

    protected PluginDescriptorFinder createPluginDescriptorFinder() {
        return new PropertiesPluginDescriptorFinder();
    }
    
and in plugin respository you must have a plugin.properties file with the below content:

    plugin.class=ro.fortsoft.pf4j.demo.welcome.WelcomePlugin
    plugin.dependencies=x, y, z
    plugin.id=welcome-plugin
    plugin.provider=Decebal Suiu
    plugin.version=0.0.1
    

For more information please see the demo sources.

Enable/Disable plugins
-------------------
In theory, it's a relation **1:N** between an extension point and the extensions for this extension point.   
This works well, except for when you develop multiple plugins for this extension point as different options for your clients to decide on which one to use.  
In this situation you wish a possibility to disable all but one extension.   
For example I have an extension point for sending mail (EmailSender interface) with two extensions: one based on Sendgrid and another
based on Amazon Simple Email Service.   
The first extension is located in Plugin1 and the second extension is located in Plugin2.   
I want to go only with one extension ( **1:1** relation between extension point and extensions) and to achieve this I have two options:  
1) uninstall Plugin1 or Plugin2 (remove folder pluginX.zip and pluginX from plugins folder)  
2) disable Plugin1 or Plugin2  

For option two you must create a simple file **enabled.txt** or **disabled.txt** in your plugins folder.   
The content for **enabled.txt** is similar with:

    ########################################
    # - load only these plugins
    # - add one plugin id on each line
    # - put this file in plugins folder
    ########################################
    welcome-plugin

The content for **disabled.txt** is similar with:

    ########################################
    # - load all plugins except these
    # - add one plugin id on each line
    # - put this file in plugins folder
    ########################################
    welcome-plugin

All comment lines (line that start with # character) are ignored.   
If a file with enabled.txt exists than disabled.txt is ignored. See enabled.txt and disabled.txt from the demo folder. 

Demo
-------------------
I have a tiny demo application. The demo application is in demo folder.
In demo/api folder I declared an extension point (_Greeting_).  
In demo/plugin* I implemented two plugins: plugin1, plugin2 (each plugin adds an extension for _Greeting_).  

To run the demo application use:  
 
    ./run-demo.sh

License
--------------
Copyright 2012 Decebal Suiu
 
Licensed under the Apache License, Version 2.0 (the "License"); you may not use this work except in compliance with
the License. You may obtain a copy of the License in the LICENSE file, or at:
 
http://www.apache.org/licenses/LICENSE-2.0
 
Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
specific language governing permissions and limitations under the License.