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When trying to use PF4J i created the necessary parts as outlined in

https://github.com/pf4j/pf4j

1. an Interface that extends the ExtensionPoint


2. a Plugin


3. Jar with Manifest


4. Plugin load and activation

Why is the List of clickHandlers empty?

I have tested this with a JUnit test where I can debug the other parts which seem to work fine. See debug log below.

I have also looked at https://github.com/pf4j/pf4j/issues/21 and activated the Eclipse annotation processing with no positive effect.

1. Interface that extends the Extension Point

public interface ClickHandler extends ExtensionPoint 
...

2. a Plugin

public class MBClickHandlerPlugin extends Plugin 

 /**
 * construct me
 * @param wrapper
 */
 public MBClickHandlerPlugin(PluginWrapper wrapper) 
 super(wrapper);
 

 @Extension
 public static class MBClickHandler implements ClickHandler 


 

3. Jar with Manifest

unzip -q -c target/com.bitplan.mb-0.0.1.jar META-INF/MANIFEST.MF
Manifest-Version: 1.0
Plugin-Dependencies: 
Plugin-Id: com.bitplan.mb
Built-By: wf
Plugin-Provider: BITPlan GmbH
Plugin-Version: 0.0.1
Plugin-Class: com.bitplan.mb.MBClickHandlerPlugin
Created-By: Apache Maven 3.5.2
Build-Jdk: 1.8.0_152

4. Plugin load and activation

 /**
 * activate the plugins requested on the command line
 */
 public void activatePlugins() 
 pluginManager = new DefaultPluginManager();

 for (String plugin : plugins) 
 Path pluginPath = Paths.get(plugin);
 pluginManager.loadPlugin(pluginPath);
 
 pluginManager.startPlugins();

 List<ClickHandler> clickHandlers = pluginManager
 .getExtensions(ClickHandler.class);
 for (ClickHandler clickHandler : clickHandlers) 
 installClickHandler(clickHandler);
 
 

Debug log

22 [main] DEBUG org.pf4j.CompoundPluginDescriptorFinder - Try to continue with the next finder
22 [main] DEBUG org.pf4j.CompoundPluginDescriptorFinder - 'org.pf4j.ManifestPluginDescriptorFinder@73d4cc9e' is applicable for plugin '/Users/wf/Documents/workspace/com.bitplan.mb/target/com.bitplan.mb-0.0.1.jar'
24 [main] DEBUG org.pf4j.AbstractPluginManager - Found descriptor PluginDescriptor [pluginId=com.bitplan.mb, pluginClass=com.bitplan.mb.MBClickHandlerPlugin, version=0.0.1, provider=BITPlan GmbH, dependencies=, description=, requires=*, license=null]
24 [main] DEBUG org.pf4j.AbstractPluginManager - Class 'com.bitplan.mb.MBClickHandlerPlugin' for plugin '/Users/wf/Documents/workspace/com.bitplan.mb/target/com.bitplan.mb-0.0.1.jar'
24 [main] DEBUG org.pf4j.AbstractPluginManager - Loading plugin '/Users/wf/Documents/workspace/com.bitplan.mb/target/com.bitplan.mb-0.0.1.jar'
24 [main] DEBUG org.pf4j.CompoundPluginLoader - 'org.pf4j.DefaultPluginLoader@6366ebe0' is not applicable for plugin '/Users/wf/Documents/workspace/com.bitplan.mb/target/com.bitplan.mb-0.0.1.jar'
24 [main] DEBUG org.pf4j.CompoundPluginLoader - 'org.pf4j.JarPluginLoader@44f75083' is applicable for plugin '/Users/wf/Documents/workspace/com.bitplan.mb/target/com.bitplan.mb-0.0.1.jar'
25 [main] DEBUG org.pf4j.PluginClassLoader - Add 'file:/Users/wf/Documents/workspace/com.bitplan.mb/target/com.bitplan.mb-0.0.1.jar'
25 [main] DEBUG org.pf4j.AbstractPluginManager - Loaded plugin '/Users/wf/Documents/workspace/com.bitplan.mb/target/com.bitplan.mb-0.0.1.jar' with class loader 'org.pf4j.PluginClassLoader@43d7741f'
25 [main] DEBUG org.pf4j.AbstractPluginManager - Creating wrapper for plugin '/Users/wf/Documents/workspace/com.bitplan.mb/target/com.bitplan.mb-0.0.1.jar'
25 [main] DEBUG org.pf4j.AbstractPluginManager - Created wrapper 'PluginWrapper [descriptor=PluginDescriptor [pluginId=com.bitplan.mb, pluginClass=com.bitplan.mb.MBClickHandlerPlugin, version=0.0.1, provider=BITPlan GmbH, dependencies=, description=, requires=*, license=null], pluginPath=/Users/wf/Documents/workspace/com.bitplan.mb/target/com.bitplan.mb-0.0.1.jar]' for plugin '/Users/wf/Documents/workspace/com.bitplan.mb/target/com.bitplan.mb-0.0.1.jar'
26 [main] DEBUG org.pf4j.DependencyResolver - Graph: 
 com.bitplan.mb -> 
26 [main] DEBUG org.pf4j.DependencyResolver - Plugins order: [com.bitplan.mb]
27 [main] INFO org.pf4j.AbstractPluginManager - Plugin 'com.bitplan.mb@0.0.1' resolved
27 [main] INFO org.pf4j.AbstractPluginManager - Start plugin 'com.bitplan.mb@0.0.1'
27 [main] DEBUG org.pf4j.DefaultPluginFactory - Create instance for plugin 'com.bitplan.mb.MBClickHandlerPlugin'
28 [main] DEBUG org.pf4j.AbstractExtensionFinder - Finding extensions of extension point 'com.bitplan.uml2mxgraph.ClickHandler'
28 [main] DEBUG org.pf4j.LegacyExtensionFinder - Reading extensions storages from classpath
28 [main] DEBUG org.pf4j.AbstractExtensionFinder - No extensions found
28 [main] DEBUG org.pf4j.LegacyExtensionFinder - Reading extensions storages from plugins
28 [main] DEBUG org.pf4j.LegacyExtensionFinder - Reading extensions storage from plugin 'com.bitplan.mb'
28 [main] DEBUG org.pf4j.LegacyExtensionFinder - Cannot find 'META-INF/extensions.idx'
28 [main] DEBUG org.pf4j.AbstractExtensionFinder - No extensions found
28 [main] DEBUG org.pf4j.AbstractExtensionFinder - Finding extensions of extension point 'com.bitplan.uml2mxgraph.ClickHandler' for plugin 'null'
28 [main] DEBUG org.pf4j.AbstractExtensionFinder - Finding extensions of extension point 'com.bitplan.uml2mxgraph.ClickHandler' for plugin 'com.bitplan.mb'
29 [main] DEBUG org.pf4j.AbstractExtensionFinder - Found 0 extensions for extension point 'com.bitplan.uml2mxgraph.ClickHandler

'

Workaround #1

use a customized PluginManager

pluginManager = new JarPluginManager(this.getClass().getClassLoader());

from the class that will use the plugin to make sure the same classloader is used

JarPluginManager source code:

import java.nio.file.Path;

import org.pf4j.DefaultPluginManager;
import org.pf4j.JarPluginLoader;
import org.pf4j.ManifestPluginDescriptorFinder;
import org.pf4j.PluginClassLoader;
import org.pf4j.PluginDescriptor;
import org.pf4j.PluginDescriptorFinder;
import org.pf4j.PluginLoader;
import org.pf4j.PluginManager;

/**
 * see https://github.com/pf4j/pf4j/issues/249 see
 * https://pf4j.org/doc/class-loading.html
 * 
 * @author wf
 *
 */
public class JarPluginManager extends DefaultPluginManager 
 public static class ParentClassLoaderJarPluginLoader extends JarPluginLoader 
 static ClassLoader parentClassLoader;

 /**
 * 
 * @param pluginManager
 */
 public ParentClassLoaderJarPluginLoader(PluginManager pluginManager) 
 super(pluginManager);
 

 static PluginClassLoader pluginClassLoader;

 @Override
 public ClassLoader loadPlugin(Path pluginPath,
 PluginDescriptor pluginDescriptor) 
 if (pluginClassLoader == null) 
 boolean parentFirst=true;
 pluginClassLoader = new PluginClassLoader(pluginManager,
 pluginDescriptor, parentClassLoader,parentFirst);
 
 pluginClassLoader.addFile(pluginPath.toFile());

 return pluginClassLoader;
 
 

 /**
 * construct me with the given classloader
 * @param classLoader
 */
 public JarPluginManager(ClassLoader classLoader) 
 ParentClassLoaderJarPluginLoader.parentClassLoader=classLoader;
 //System.setProperty("pf4j.mode", RuntimeMode.DEPLOYMENT.toString());
 //System.setProperty("pf4j.mode", RuntimeMode.DEVELOPMENT.toString());
 

 @Override
 protected PluginLoader createPluginLoader() 
 // load only jar plugins
 return new ParentClassLoaderJarPluginLoader(this);
 

 @Override
 protected PluginDescriptorFinder createPluginDescriptorFinder() 
 // read plugin descriptor from jar's manifest
 return new ManifestPluginDescriptorFinder();
 

Workaround #2
If the extensions.idx file is not created there is something wrong with your annotation processing. You might want to fix the source of the problem but it is also possible to try to work around it:

• https://groups.google.com/forum/#!topic/pf4j/nn20axJHpfI
  pointed me to creating the META-INF/extensions.idx file manually and making sure there is a no args constructor for the static inner class. With this change things work.




• Watch out for setting the classname correctly in the extensions.idx
  file - otherwise you'll end up with a null entry in the handler list




• Watch out for having a null argument constructor otherwise you'll endup with an exception

@Extension
public static class MBClickHandler implements ClickHandler {
 /**
 * constructor with no argument
 */
 public MBClickHandler() 

 

src/main/resources/META-INF/extensions.idx

com.bitplan.mb.MBClickHandlerPlugin$MBClickHandler

Code to check

1. Correct name for extension.idx entry

MBClickHandler ch=new MBClickHandler();
File extFile=new File("src/main/resources/META-INF/extensions.idx");
String extidx=FileUtils.readFileToString(extFile,"UTF-8");
assertEquals(extidx,ch.getClass().getName());

2. 
   checking the extensions

List<PluginWrapper> startedPlugins = pluginManager.getStartedPlugins();

 for (PluginWrapper plugin : startedPlugins) 
 String pluginId = plugin.getDescriptor().getPluginId();
 System.out.println(String.format("Extensions added by plugin '%s':", pluginId));
 Set<String> extensionClassNames = pluginManager.getExtensionClassNames(pluginId);
 for (String extension : extensionClassNames) 
 System.out.println(" " + extension);