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/**
 * 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.*;

/**
 * 

* Used to implement callback based result passing of a promised computation. * Can be converted to a future using the future() method. *

* * @author Hiram Chirino */ public class Promise extends PromiseCallback { ArrayList> callbacks = new ArrayList>(1); T value; Throwable error; Future future=null; private class PromiseFuture extends PromiseCallback implements Future { 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 future() { if( future == null ) { PromiseFuture future = new PromiseFuture(); watch(future); this.future = future; } return future; } public void watch(PromiseCallback 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> 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); } } } }




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