UnifiedClassLoader.java

/*
 * Copyright (C) 2009 eXo Platform SAS.
 *
 * This is free software; you can redistribute it and/or modify it
 * under the terms of the GNU Lesser General Public License as
 * published by the Free Software Foundation; either version 2.1 of
 * the License, or (at your option) any later version.
 *
 * This software is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this software; if not, write to the Free
 * Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
 * 02110-1301 USA, or see the FSF site: http://www.fsf.org.
 */
package org.exoplatform.container;

import org.exoplatform.commons.utils.SecurityHelper;

import java.io.IOException;
import java.net.URL;
import java.security.PrivilegedAction;
import java.util.Collections;
import java.util.Enumeration;
import java.util.LinkedHashSet;
import java.util.Set;

/**
 * This class is used to merge different {@link ClassLoader} to create one single one.
 * This ClassLoader is used only for the resources the class will be loaded from the 
 * ContextClassLoader. For each resources, it will always consider that the {@link ClassLoader}
 * with the highest priority has always right, in other words for example in the method
 * getResource, it will try to get the resource in the {@link ClassLoader} of the highest
 * priority, if it cans not find it, it will try the {@link ClassLoader} with the second highest
 * priority and so on. The priority of the {@link ClassLoader} is the order given in the 
 * constructor, the last {@link ClassLoader} is the one with the highest priority.
 * 
 * Created by The eXo Platform SAS
 * Author : Nicolas Filotto
 *          nicolas.filotto@exoplatform.com
 * 18 sept. 2009  
 */
class UnifiedClassLoader extends ClassLoader
{

   /**
    * The list of all the {@link ClassLoader} to merge together
    */
   private final ClassLoader[] cls;

   /**
    * @param cls the list of all the {@link ClassLoader} to merge ordered by priority. The last 
    * {@link ClassLoader} has highest priority. 
    */
   UnifiedClassLoader(ClassLoader... cls)
   {
      super(Thread.currentThread().getContextClassLoader());

      if (cls == null || cls.length == 0)
      {
         throw new IllegalArgumentException("The array of ClassLoader cannot be empty");
      }
      this.cls = cls;
   }

   /**
    * Allows to override the list of {@link ClassLoader} if it is a dynamic list
    * @return the list of the {@link ClassLoader} to merge
    */
   protected ClassLoader[] getClassLoaders()
   {
      return cls;
   }

   /**
    * {@inheritDoc}
    */
   @Override
   public URL getResource(String name)
   {
      final ClassLoader[] cls = getClassLoaders();
      for (int i = cls.length - 1; i >= 0; i--)
      {
         final ClassLoader cl = cls[i];
         URL url = cl.getResource(name);
         if (url != null)
         {
            return url;
         }
      }
      return null;
   }

   /**
    * {@inheritDoc}
    */
   @Override
   public Enumeration<URL> getResources(String name) throws IOException
   {
      final ClassLoader[] cls = getClassLoaders();
      final Set<URL> urls = new LinkedHashSet<URL>();
      for (int i = 0, length = cls.length; i < length; i++)
      {
         Enumeration<URL> eUrls = cls[i].getResources(name);
         if (eUrls != null && eUrls.hasMoreElements())
         {
            // Prevent duplicates
            urls.addAll(Collections.list(eUrls));
         }
      }
      return Collections.enumeration(urls);
   }

   static protected UnifiedClassLoader createUnifiedClassLoaderInPrivilegedMode(final ClassLoader... cls)
   {
      return SecurityHelper.doPrivilegedAction(new PrivilegedAction<UnifiedClassLoader>()
      {
         public UnifiedClassLoader run()
         {
            return new UnifiedClassLoader(cls);
         }
      });
   }
}