api  
定义插件接口 Greeting.java
import org.pf4j.ExtensionPoint;
public interface Greeting extends ExtensionPoint {
    String getGreeting();
}



core  插件实现 ，打包成jar ,放到指定的插件目录 ，由宿主程序加载运行
plugin1   插件1实现

WelcomeGreeting.java

import org.pf4j.Extension;
import dyyx.pf4jdemo.api.Greeting;
@Extension
public class WelcomeGreeting implements Greeting{	
	public String getGreeting() {
        return "Welcome";
    }
}


WelcomePlugin.java

import org.pf4j.Plugin;
import org.pf4j.PluginWrapper;

public class WelcomePlugin extends Plugin{
	 public WelcomePlugin(PluginWrapper wrapper) {
	        super(wrapper);
	    }

	    @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()");
	    }
}

打包插件 
mvn clean package

maven-jar-plugin 指定 MANIFEST 文件

 Maven 打包 定制 manifest  

把 生成的jar包放到宿主应用指定的插件目录下


app
宿主应用 ，从指定目录加载插件并运行

import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.List;

import org.pf4j.DefaultPluginManager;
import org.pf4j.PluginManager;
import org.pf4j.PluginWrapper;

import dyyx.pf4jdemo.api.Greeting;

public class PluginDemo{
	
	public static void main(String[] args) throws Exception{		
		System.out.println("hello,pf4j");
		
		// create the plugin manager
		// or "new ZipPluginManager() / new DefaultPluginManager()"
		// 不要带 file:// 前缀
		String pluginRootDir = "/PATH/pf4jdemo/app/plugins";
		Path pluginRoot = Paths.get(pluginRootDir);
		System.out.println("pluginRoot="+pluginRoot);
	    // PluginManager pluginManager = new JarPluginManager(pluginRoot); 
		
		PluginManager pluginManager = new DefaultPluginManager(pluginRoot); 
		
	    // start and load all plugins of application
	    pluginManager.loadPlugins();
	    pluginManager.startPlugins();
	    
	    List pluginWrappers = pluginManager.getPlugins();
	    for(PluginWrapper item:pluginWrappers) {
	    	System.out.println("PluginWrapper="+item);
	    }
	    System.out.println();
	    List resolvedPlugins = pluginManager.getResolvedPlugins();
	    for(PluginWrapper item:resolvedPlugins) {
	    	System.out.println("resolvedPlugins.PluginWrapper="+item);
	    }
	    
	    // retrieve all extensions for "Greeting" extension point
	    List greetings = pluginManager.getExtensions(Greeting.class);
	    
	    System.out.println("greetings plugin num "+greetings.size());
	    
	    for (Greeting greeting : greetings) {
	        System.out.println(greeting + "#" + greeting.getGreeting());
	    }
	    
	    // stop and unload all plugins
	    pluginManager.stopPlugins();
	    pluginManager.unloadPlugins();
	    
	    // Unknown pluginId welcome-plugin
	    // pluginManager.deletePlugin("welcome-plugin");
	    
	    Thread.sleep(3000);
		
	}

}





pluginRoot=/PATH/pf4jdemo/app/plugins
SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.
WelcomePlugin.start()
Welcome2Plugin.start()
PluginWrapper=PluginWrapper [descriptor=PluginDescriptor [pluginId=welcome-plugin, pluginClass=dyyx.pf4jdemo.core.plugin1.WelcomePlugin, version=1.0.0, provider=dugang, dependencies=[], description=, requires=*, license=null], pluginPath=/PATH/pf4jdemo/app/plugins/pf4jdemo-plugin1-1.0.0.jar]
PluginWrapper=PluginWrapper [descriptor=PluginDescriptor [pluginId=welcome-plugin2, pluginClass=dyyx.pf4jdemo.core.plugin2.Welcome2Plugin, version=1.0.0, provider=dugang, dependencies=[], description=, requires=*, license=null], pluginPath=/PATH/pf4jdemo/app/plugins/pf4jdemo-plugin2-1.0.0.jar]
resolvedPlugins.PluginWrapper=PluginWrapper [descriptor=PluginDescriptor [pluginId=welcome-plugin, pluginClass=dyyx.pf4jdemo.core.plugin1.WelcomePlugin, version=1.0.0, provider=dugang, dependencies=[], description=, requires=*, license=null], pluginPath=/PATH/pf4jdemo-plugin1-1.0.0.jar]
resolvedPlugins.PluginWrapper=PluginWrapper [descriptor=PluginDescriptor [pluginId=welcome-plugin2, pluginClass=dyyx.pf4jdemo.core.plugin2.Welcome2Plugin, version=1.0.0, provider=dugang, dependencies=[], description=, requires=*, license=null], pluginPath=/PATH/pf4jdemo/app/plugins/pf4jdemo-plugin2-1.0.0.jar]
greetings plugin num 2
dyyx.pf4jdemo.core.plugin1.WelcomeGreeting@41975e01#Welcome
dyyx.pf4jdemo.core.plugin2.Welcome2Greeting@c2e1f26#Welcome2
Welcome2Plugin.stop()
WelcomePlugin.stop()



完整代码
https://gitee.com/dyyx/hellocode/tree/master/demo/pf4jdemo