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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.handler.execution;
  
  import java.util.concurrent.Executor;
  import java.util.concurrent.ExecutorService;
  
  import org.jboss.netty.bootstrap.ServerBootstrap;
  import org.jboss.netty.channel.Channel;
  import org.jboss.netty.channel.ChannelDownstreamHandler;
  import org.jboss.netty.channel.ChannelEvent;
  import org.jboss.netty.channel.ChannelHandler;
  import org.jboss.netty.channel.ChannelHandler.Sharable;
  import org.jboss.netty.channel.ChannelHandlerContext;
  import org.jboss.netty.channel.ChannelPipeline;
  import org.jboss.netty.channel.ChannelPipelineFactory;
  import org.jboss.netty.channel.ChannelState;
  import org.jboss.netty.channel.ChannelStateEvent;
  import org.jboss.netty.channel.ChannelUpstreamHandler;
  import org.jboss.netty.channel.Channels;
  import org.jboss.netty.util.ExternalResourceReleasable;
  
  /**
   * Forwards an upstream {@link ChannelEvent} to an {@link Executor}.
   * <p>
   * {@link ExecutionHandler} is often used when your {@link ChannelHandler}
   * performs a blocking operation that takes long time or accesses a resource
   * which is not CPU-bound business logic such as DB access.  Running such
   * operations in a pipeline without an {@link ExecutionHandler} will result in
   * unwanted hiccup during I/O because an I/O thread cannot perform I/O until
   * your handler returns the control to the I/O thread.
   * <p>
   * In most cases, an {@link ExecutionHandler} is coupled with an
   * {@link OrderedMemoryAwareThreadPoolExecutor} because it guarantees the
   * correct event execution order and prevents an {@link OutOfMemoryError}
   * under load:
   * <pre>
   * public class DatabaseGatewayPipelineFactory implements {@link ChannelPipelineFactory} {
   *
   *     <b>private final {@link ExecutionHandler} executionHandler;</b>
   *
   *     public DatabaseGatewayPipelineFactory({@link ExecutionHandler} executionHandler) {
   *         this.executionHandler = executionHandler;
   *     }
   *
   *     public {@link ChannelPipeline} getPipeline() {
   *         return {@link Channels}.pipeline(
   *                 new DatabaseGatewayProtocolEncoder(),
   *                 new DatabaseGatewayProtocolDecoder(),
   *                 <b>executionHandler, // Must be shared</b>
   *                 new DatabaseQueryingHandler());
   *     }
   * }
   * ...
   *
   * public static void main(String[] args) {
   *     {@link ServerBootstrap} bootstrap = ...;
   *     ...
   *     <b>{@link ExecutionHandler} executionHandler = new {@link ExecutionHandler}(
   *             new {@link OrderedMemoryAwareThreadPoolExecutor}(16, 1048576, 1048576))
   *     bootstrap.setPipelineFactory(
   *             new DatabaseGatewayPipelineFactory(executionHandler));</b>
   *     ...
   *     bootstrap.bind(...);
   *     ...
   *
   *     while (!isServerReadyToShutDown()) {
   *         // ... wait ...
   *     }
   *
   *     bootstrap.releaseExternalResources();
   *     <b>executionHandler.releaseExternalResources();</b>
   * }
   * </pre>
   *
   * Please refer to {@link OrderedMemoryAwareThreadPoolExecutor} for the
   * detailed information about how the event order is guaranteed.
   *
   * <h3>SEDA (Staged Event-Driven Architecture)</h3>
   * You can implement an alternative thread model such as
   * <a href="http://en.wikipedia.org/wiki/Staged_event-driven_architecture">SEDA</a>
   * by adding more than one {@link ExecutionHandler} to the pipeline.
   *
   * <h3>Using other {@link Executor} implementation</h3>
   *
   * Although it's recommended to use {@link OrderedMemoryAwareThreadPoolExecutor},
  * you can use other {@link Executor} implementations.  However, you must note
  * that other {@link Executor} implementation might break your application
  * because they often do not maintain event execution order nor interact with
  * I/O threads to control the incoming traffic and avoid {@link OutOfMemoryError}.
  *
  * @apiviz.landmark
  * @apiviz.has java.util.concurrent.ThreadPoolExecutor
  */
 @Sharable
 public class ExecutionHandler implements ChannelUpstreamHandler, ChannelDownstreamHandler, ExternalResourceReleasable {
 
     private final Executor executor;
     private final boolean handleDownstream;
     private final boolean handleUpstream;
 
     /**
      * Creates a new instance with the specified {@link Executor}.
      * Specify an {@link OrderedMemoryAwareThreadPoolExecutor} if unsure.
      */
     public ExecutionHandler(Executor executor) {
         this(executor, false, true);
     }
 
     /**
      * Creates a new instance with the specified {@link Executor}.
      * Specify an {@link OrderedMemoryAwareThreadPoolExecutor} if unsure.
      */
     public ExecutionHandler(Executor executor, boolean handleDownstream, boolean handleUpstream) {
         if (executor == null) {
             throw new NullPointerException("executor");
         }
         if (!handleDownstream && !handleUpstream) {
             throw new IllegalArgumentException("You must handle at least handle one event type");
         }
         this.executor = executor;
         this.handleDownstream = handleDownstream;
         this.handleUpstream = handleUpstream;
     }
 
     /**
      * Returns the {@link Executor} which was specified with the constructor.
      */
     public Executor getExecutor() {
         return executor;
     }
 
     /**
      * Shuts down the {@link Executor} which was specified with the constructor
      * and wait for its termination.
      */
     public void releaseExternalResources() {
         Executor executor = getExecutor();
         if (executor instanceof ExecutorService) {
             ((ExecutorService) executor).shutdown();
         }
         if (executor instanceof ExternalResourceReleasable) {
             ((ExternalResourceReleasable) executor).releaseExternalResources();
         }
     }
 
     public void handleUpstream(
             ChannelHandlerContext context, ChannelEvent e) throws Exception {
         if (handleUpstream) {
             executor.execute(new ChannelUpstreamEventRunnable(context, e, executor));
         } else {
             context.sendUpstream(e);
         }
     }
 
     public void handleDownstream(
             ChannelHandlerContext ctx, ChannelEvent e) throws Exception {
         // check if the read was suspend
         if (!handleReadSuspend(ctx, e)) {
             if (handleDownstream) {
                 executor.execute(new ChannelDownstreamEventRunnable(ctx, e, executor));
             } else {
                 ctx.sendDownstream(e);
             }
         }
     }
 
     /**
      * Handle suspended reads
      */
     protected boolean handleReadSuspend(ChannelHandlerContext ctx, ChannelEvent e) {
         if (e instanceof ChannelStateEvent) {
             ChannelStateEvent cse = (ChannelStateEvent) e;
             if (cse.getState() == ChannelState.INTEREST_OPS &&
                 (((Integer) cse.getValue()).intValue() & Channel.OP_READ) != 0) {
 
                 // setReadable(true) requested
                 boolean readSuspended = ctx.getAttachment() != null;
                 if (readSuspended) {
                     // Drop the request silently if MemoryAwareThreadPool has
                     // set the flag.
                     e.getFuture().setSuccess();
                     return true;
                 }
             }
         }
 
         return false;
     }
 }