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The ActiveMQ Message Broker implementation
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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);
            }
        }
    }

}