/*
 * Copyright (c) 2003, KNOPFLERFISH project
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following
 * conditions are met:
 *
 * - Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 *
 * - Redistributions in binary form must reproduce the above
 *   copyright notice, this list of conditions and the following
 *   disclaimer in the documentation and/or other materials
 *   provided with the distribution.
 *
 * - Neither the name of the KNOPFLERFISH project nor the names of its
 *   contributors may be used to endorse or promote products derived
 *   from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
 * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
 * OF THE POSSIBILITY OF SUCH DAMAGE.
 */

package org.knopflerfish.util;


import java.security.PrivilegedAction;
import java.security.PrivilegedExceptionAction;

/**
 * Utility class for handling common class loading cases, like wrapping
 * external libraries in the correct context class loader.
 */
public class ClassLoaderUtil {

  /**
   * Run the specified action in a specified ContextClassLoader.
   *
   * <p>
   * The <tt>doContextClassLoader</tt> sets the current thread's context 
   * class loader to the specified <tt>classloader</tt>, calls the 
   * action's <tt>run</tt>  method, resets the thread's context class 
   * loader and finally returns the resulting value.
   * </p>
   *
   * <p>
   * Example:
   * <pre>
   * Object result = ClassLoaderUtil
   *   .doContextClassLoader(Activator.getClass().getClassLoader(),
   *                         new PrivilegedAction() {
   *                           public Object run() {
   *                             // Use external library
   *                             // ...
   *                             return someresult;
   *                           }
   *                         };
   * </pre>
   * where <tt>Activator</tt> is the bundle activator, or any other
   * class loaded by the bundle class loader.
   * </p>
   *
   * @param classloader Class loader to be used as the thread's context 
   *                    class loader.
   * @param action Action code to run in the specified class loader.<br>
   *               The usage of the <tt>PrivilegedAction</tt> interface
   *               is to avoid creating a new interface with exactly the
   *               same methods. It does <b>not</b> imply that the code is run
   *               using <tt>AccessController.doPrivileged</tt>
   * @return       Value returned from the <tt>action.run</tt>
   */
  public static Object doContextClassLoader(ClassLoader classloader, 
					    PrivilegedAction action) {
    ClassLoader oldLoader = Thread.currentThread().getContextClassLoader();
    
    try {
      Thread.currentThread().setContextClassLoader(classloader);
      Object obj = action.run();
      return obj;
    } finally {
      Thread.currentThread().setContextClassLoader(oldLoader);
    }
  }

  /**
   * Run the specified action in a specified ContextClassLoader.
   *
   * <p>
   * As above, but accepts and rethrows exceptions from the action.
   * </p>
   *
   * @param classloader  Class loader to be used as the thread's context 
   *                     class loader.
   * @param action Action code to run in the specified class loader.<br>
   *               The usage of the <tt>PrivilegedAction</tt> interface
   *               is to avoid creating a new interface with exactly the
   *               same methods. It does <b>not</b> imply that the code is run
   *               using <tt>AccessController.doPrivileged</tt>
   * @return       Value returned from the <tt>action.run</tt>
   *
   * @throws Exception if <tt>action.run</tt> throws an exception, pass this 
   *                   upwards
   */
  public static Object doContextClassLoader(ClassLoader classloader, 
					    PrivilegedExceptionAction action) 
    throws Exception {

    ClassLoader oldLoader = Thread.currentThread().getContextClassLoader();
    
    try {
      Thread.currentThread().setContextClassLoader(classloader);
      Object obj = action.run();
      return obj;
    } finally {
      Thread.currentThread().setContextClassLoader(oldLoader);
    }
  }
}