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mqtt-client provides an ASL 2.0 licensed API to MQTT. It takes care of automatically reconnecting to your MQTT server and restoring your client session if any network failures occur. Applications can use a blocking API style, a futures based API, or a callback/continuations passing API style.

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/**
 * Copyright (C) 2010-2012, FuseSource Corp.  All rights reserved.
 *
 *     http://fusesource.com
 *
 * Licensed 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.fusesource.mqtt.client;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;

/**
 * 
 * 
 *
 * @author Hiram Chirino
 */
public class Promise implements Callback, Future {

    private final CountDownLatch latch = new CountDownLatch(1);
    private Callback next;
    private Throwable error;
    private T value;

    public void onFailure(Throwable value) {
        Callback callback = null;
        synchronized(this)  {
            error = value;
            latch.countDown();
            callback = next;
        }
        if( callback!=null ) {
            callback.onFailure(value);
        }
    }

    public void onSuccess(T value) {
        Callback callback = null;
        synchronized(this)  {
            this.value = value;
            latch.countDown();
            callback = next;
        }
        if( callback!=null ) {
            callback.onSuccess(value);
        }
    }

    public void then(Callback callback) {
        boolean fire = false;
        synchronized(this)  {
            next = callback;
            if( latch.getCount() == 0 ) {
                fire = true;
            }
        }
        if( fire ) {
            if( error!=null ) {
                callback.onFailure(error);
            } else {
                callback.onSuccess(value);
            }
        }
    }

    public T await(long amount, TimeUnit unit) throws Exception {
        if( latch.await(amount, unit) ) {
            return get();
        } else {
            throw new TimeoutException();
        }
    }

    public T await() throws Exception {
        latch.await();
        return get();
    }

    private T get() throws Exception {
        Throwable e = error;
        if( e !=null ) {
            if( e instanceof RuntimeException ) {
                throw (RuntimeException) e;
            } else if( e instanceof Exception) {
                throw (Exception) e;
            } else if( e instanceof Error) {
                throw (Error) e;
            } else {
                // don't expect to hit this case.
                throw new RuntimeException(e);
            }
        }
        return value;
    }

}