content: /**
   * Licensed to the Apache Software Foundation (ASF) under one or more
   * contributor license agreements.  See the NOTICE file distributed with
   * this work for additional information regarding copyright ownership.
   * The ASF 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.apache.activemq.util;
  
  
  import java.util.ArrayList;
  import java.util.concurrent.*;
  
  /**
   * <p>
   *   Used to implement callback based result passing of a promised computation.
   *   Can be converted to a future using the future() method.
   * </p>
   *
   * @author <a href="http://hiramchirino.com">Hiram Chirino</a>
   */
  public class Promise<T> extends PromiseCallback<T> {
  
      ArrayList<PromiseCallback<T>> callbacks = new ArrayList<PromiseCallback<T>>(1);
      T value;
      Throwable error;
      Future<T> future=null;
  
      private class PromiseFuture extends PromiseCallback<T> implements Future<T> {
          CountDownLatch latch = new CountDownLatch(1);
  
          public boolean cancel(boolean mayInterruptIfRunning) {
              return false;
          }
  
          public boolean isCancelled() {
              return false;
          }
  
          public boolean isDone() {
              return latch.getCount() == 0;
          }
  
          public T get() throws InterruptedException, ExecutionException {
              latch.await();
              return value();
          }
  
          public T get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException {
              if (latch.await(timeout, unit)) {
                  return value();
              } else {
                  throw new TimeoutException();
              }
          }
  
          public void onComplete(T value, Throwable error) {
              latch.countDown();
          }
  
          private T value() throws ExecutionException {
              if( error!=null ) {
                  throw new ExecutionException(error);
              }
              return value;
          }
      }
  
      public Future<T> future() {
          if( future == null ) {
              PromiseFuture future = new PromiseFuture();
              watch(future);
              this.future = future;
          }
          return future;
      }
  
      public void watch(PromiseCallback<T> callback) {
          if (callback == null)
              throw new IllegalArgumentException("callback cannot be null");
          boolean queued = false;
          synchronized (this) {
              if (callbacks != null) {
                  callbacks.add(callback);
                  queued = true;
              }
          }
          if (!queued) {
              callback.onComplete(value, error);
          }
      }
  
      @Override
      public void onComplete(T value, Throwable error) {
          if( value!=null && error !=null ) {
              throw new IllegalArgumentException("You can not have both a vaule and error");
          }
          ArrayList<PromiseCallback<T>> callbacks;
          synchronized (this) {
              callbacks = this.callbacks;
              if (callbacks != null) {
                  this.value = value;
                  this.error = error;
                  this.callbacks = null;
              }
          }
          if (callbacks != null) {
              for (PromiseCallback callback : callbacks) {
                  callback.onComplete(this.value, this.error);
              }
          }
      }
  
  }
  