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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.channel;
  
  import org.jboss.netty.util.internal.ConcurrentIdentityWeakKeyHashMap;
  
  import java.util.Collections;
  import java.util.Iterator;
  import java.util.Map.Entry;
  import java.util.concurrent.ConcurrentMap;
  
  /**
   * A global variable that is local to a {@link Channel}.  Think of this as a
   * variation of {@link ThreadLocal} whose key is a {@link Channel} rather than
   * a {@link Thread#currentThread()}.  One difference is that you always have to
   * specify the {@link Channel} to access the variable.
   * <p>
   * Alternatively, you might want to use the
   * {@link ChannelHandlerContext#setAttachment(Object) ChannelHandlerContext.attachment}
   * property, which performs better.
   * @apiviz.stereotype utility
   */
  public class ChannelLocal<T> implements Iterable<Entry<Channel, T>> {
  
      private final ConcurrentMap<Channel, T> map =
          new ConcurrentIdentityWeakKeyHashMap<Channel, T>();
  
      private final ChannelFutureListener remover = new ChannelFutureListener() {
          public void operationComplete(ChannelFuture future) throws Exception {
              remove(future.getChannel());
          }
      };
  
      private final boolean removeOnClose;
  
      /**
       * Creates a {@link Channel} local variable by calling {@link #ChannelLocal(boolean)} with
       * {@code false} as parameter
       */
      public ChannelLocal() {
          this(false);
      }
  
      /**
       * Creates a {@link Channel} local variable.
       *
       * @param removeOnClose if {@code true} the {@link ChannelLocal} will remove a
       *                      {@link Channel} from it own once the {@link Channel} was closed.
       */
      public ChannelLocal(boolean removeOnClose) {
          this.removeOnClose = removeOnClose;
      }
      /**
       * Returns the initial value of the variable.  By default, it returns
       * {@code null}.  Override it to change the initial value.
       *
       * @param channel the channel where this local variable is accessed with
       */
      protected T initialValue(Channel channel) {
          return null;
      }
  
      /**
       * Returns the value of this variable.
       */
      public T get(Channel channel) {
          if (channel == null) {
              throw new NullPointerException("channel");
          }
  
          T value = map.get(channel);
          if (value == null) {
              value = initialValue(channel);
              if (value != null) {
                  T oldValue = setIfAbsent(channel, value);
                  if (oldValue != null) {
                      value = oldValue;
                  }
              }
          }
          return value;
      }
  
      /**
       * Sets the value of this variable.
       *
      * @return the old value. {@code null} if there was no old value.
      */
     public T set(Channel channel, T value) {
         if (value == null) {
             return remove(channel);
         } else {
             if (channel == null) {
                 throw new NullPointerException("channel");
             }
             T old = map.put(channel, value);
             if (removeOnClose) {
                 channel.getCloseFuture().addListener(remover);
             }
             return old;
         }
     }
 
     /**
      * Sets the value of this variable only when no value was set.
      *
      * @return {@code null} if the specified value was set.
      *         An existing value if failed to set.
      */
     public T setIfAbsent(Channel channel, T value) {
         if (value == null) {
             return get(channel);
         } else {
             if (channel == null) {
                 throw new NullPointerException("channel");
             }
             T mapping = map.putIfAbsent(channel, value);
 
             if (removeOnClose && mapping == null) {
                 channel.getCloseFuture().addListener(remover);
             }
             return mapping;
         }
     }
 
     /**
      * Removes the variable and returns the removed value.  If no value was set,
      * this method returns the return value of {@link #initialValue(Channel)},
      * which is {@code null} by default.
      *
      * @return the removed value.
      *         {@linkplain #initialValue(Channel) an initial value} (by default
      *         {@code null}) if no value was set.
      */
     public T remove(Channel channel) {
         if (channel == null) {
             throw new NullPointerException("channel");
         }
         T removed = map.remove(channel);
         if (removed == null) {
             return initialValue(channel);
         } else {
             if (removeOnClose) {
                 channel.getCloseFuture().removeListener(remover);
             }
             return removed;
         }
     }
 
     /**
      * Returns a <strong>read-only</strong> {@link Iterator} that holds all {@link Entry}'s of this ChannelLocal
      */
     public Iterator<Entry<Channel, T>> iterator() {
         return Collections.unmodifiableSet(map.entrySet()).iterator();
     }
 }