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   /*
    * Copyright 2012 The Netty Project
    *
    * The Netty Project licenses this file to you under the Apache License,
    * version 2.0 (the "License"); you may not use this file except in compliance
    * with the License. You may obtain a copy of the License at:
    *
    *   http://www.apache.org/licenses/LICENSE-2.0
    *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
   * License for the specific language governing permissions and limitations
   * under the License.
   */
  package org.jboss.netty.util;
  
  import java.util.Collections;
  import java.util.IdentityHashMap;
  import java.util.List;
  import java.util.Set;
  import java.util.concurrent.AbstractExecutorService;
  import java.util.concurrent.Executor;
  import java.util.concurrent.ExecutorService;
  import java.util.concurrent.RejectedExecutionException;
  import java.util.concurrent.TimeUnit;
  
  import org.jboss.netty.channel.ChannelFactory;
  import org.jboss.netty.channel.socket.nio.NioServerSocketChannelFactory;
  
  
  /**
   * A delegating {@link ExecutorService} with its own termination management.
   * <p>
   * {@link VirtualExecutorService} is used when you want to inject an
   * {@link ExecutorService} but you do not want to allow the explicit termination
   * of threads on shutdown request.  It is particularly useful when the
   * {@link ExecutorService} to inject is shared by different components and
   * the life cycle of the components depend on the termination of the injected
   * {@link ExecutorService}.
   *
   * <pre>
   * ExecutorService globalExecutor = ...;
   * ExecutorService virtualExecutor = new {@link VirtualExecutorService}(globalExecutor);
   *
   * {@link ChannelFactory} factory =
   *         new {@link NioServerSocketChannelFactory}(virtualExecutor, virtualExecutor);
   * ...
   *
   * // ChannelFactory.releaseExternalResources() shuts down the executor and
   * // interrupts the I/O threads to terminate all I/O tasks and to release all
   * // resources acquired by ChannelFactory.
   * factory.releaseExternalResources();
   *
   * // Note that globalExecutor is not shut down because VirtualExecutorService
   * // implements its own termination management. All threads which were acquired
   * // by ChannelFactory via VirtualExecutorService are returned to the pool.
   * assert !globalExecutor.isShutdown();
   * </pre>
   *
   * <h3>The differences from an ordinary {@link ExecutorService}</h3>
   *
   * A shutdown request ({@link #shutdown()} or {@link #shutdownNow()}) does not
   * shut down its parent {@link Executor} but simply sets its internal flag to
   * reject further execution request.
   * <p>
   * {@link #shutdownNow()} interrupts only the thread which is executing the
   * task executed via {@link VirtualExecutorService}.
   * <p>
   * {@link #awaitTermination(long, TimeUnit)} does not wait for real thread
   * termination but wait until {@link VirtualExecutorService} is shut down and
   * its active tasks are finished and the threads are returned to the parent
   * {@link Executor}.
   * @apiviz.landmark
   */
  public class VirtualExecutorService extends AbstractExecutorService {
  
      private final Executor e;
      private final ExecutorService s;
      final Object startStopLock = new Object();
      volatile boolean shutdown;
      final Set<Thread> activeThreads = new MapBackedSet<Thread>(new IdentityHashMap<Thread, Boolean>());
  
      /**
       * Creates a new instance with the specified parent {@link Executor}.
       */
      public VirtualExecutorService(Executor parent) {
          if (parent == null) {
              throw new NullPointerException("parent");
          }
  
          if (parent instanceof ExecutorService) {
              e = null;
              s = (ExecutorService) parent;
          } else {
              e = parent;
              s = null;
          }
      }
 
     public boolean isShutdown() {
         synchronized (startStopLock) {
             return shutdown;
         }
     }
 
     public boolean isTerminated() {
         synchronized (startStopLock) {
             return shutdown && activeThreads.isEmpty();
         }
     }
 
     public void shutdown() {
         synchronized (startStopLock) {
             if (shutdown) {
                 return;
             }
             shutdown = true;
         }
     }
 
     public List<Runnable> shutdownNow() {
         synchronized (startStopLock) {
             if (!isTerminated()) {
                 shutdown();
                 for (Thread t: activeThreads) {
                     t.interrupt();
                 }
             }
         }
 
         return Collections.emptyList();
     }
 
     public boolean awaitTermination(long timeout, TimeUnit unit)
             throws InterruptedException {
         synchronized (startStopLock) {
             if (!isTerminated()) {
                 startStopLock.wait(TimeUnit.MILLISECONDS.convert(timeout, unit));
             }
             return isTerminated();
         }
     }
 
     public void execute(Runnable command) {
         if (command == null) {
             throw new NullPointerException("command");
         }
 
         if (shutdown) {
             throw new RejectedExecutionException();
         }
 
         if (s != null) {
             s.execute(new ChildExecutorRunnable(command));
         } else {
             e.execute(new ChildExecutorRunnable(command));
         }
     }
 
     private class ChildExecutorRunnable implements Runnable {
 
         private final Runnable runnable;
 
         ChildExecutorRunnable(Runnable runnable) {
             this.runnable = runnable;
         }
 
         public void run() {
             Thread thread = Thread.currentThread();
             synchronized (startStopLock) {
                 activeThreads.add(thread);
             }
             try {
                 runnable.run();
             } finally {
                 synchronized (startStopLock) {
                     boolean removed = activeThreads.remove(thread);
                     assert removed;
                     if (isTerminated()) {
                         startStopLock.notifyAll();
                     }
                 }
             }
         }
     }
 }