java/src/main/com/google/apphosting/utils/config/PluginLoader.java

   1 // Copyright 2010 Google Inc. All Rights Reserved.
   2
   3 package com.google.apphosting.utils.config;
   4
   5 import java.io.File;
   6 import java.net.MalformedURLException;
   7 import java.net.URL;
   8 import java.net.URLClassLoader;
   9 import java.util.ArrayList;
  10 import java.util.ServiceLoader;
  11 import java.util.logging.Logger;
  12
  13 /**
  14  * Utility for loading plugins in AppConfig and DevAppserver.
  15  *
  16  */
  17 public class PluginLoader {
  18
  19   /**
  20    * Name of the system property used to specify the plugin search path.
  21    */
  22   public final static String PLUGIN_PATH = "com.google.appengine.plugin.path";
  23   private final static Logger logger = Logger.getLogger(PluginLoader.class.getName());
  24
  25   private static ClassLoader loader = null;
  26
  27   /**
  28    * Searches for plugins of the specified type.
  29    */
  30   public static <T> Iterable<T> loadPlugins(Class<T> pluginClass) {
  31     return ServiceLoader.load(pluginClass, getPluginClassLoader());
  32   }
  33
  34   private synchronized static ClassLoader getPluginClassLoader() {
  35     if (loader == null) {
  36       ClassLoader parent = PluginLoader.class.getClassLoader();
  37       String path = System.getProperty(PLUGIN_PATH);
  38       if (path == null) {
  39         loader = parent;
  40       } else {
  41         String[] paths = path.split(File.pathSeparator);
  42         ArrayList<URL> urls = new ArrayList<URL>(paths.length);
  43         for (int i = 0; i < paths.length; ++i) {
  44           try {
  45             urls.add(new File(paths[i]).toURI().toURL());
  46           } catch (MalformedURLException ex) {
  47             logger.severe("Skipping invalid plugin path " + paths[i]);
  48           }
  49         }
  50         loader = new URLClassLoader(urls.toArray(new URL[urls.size()]), parent);
  51       }
  52     }
  53     return loader;
  54   }
  55
  56 }