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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 org.fusesource.hawtbuf.Buffer;
import org.fusesource.hawtbuf.UTF8Buffer;
import org.fusesource.hawtdispatch.Task;

import java.util.concurrent.Callable;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicLong;

import static org.fusesource.hawtbuf.Buffer.utf8;

/**
 * 
 * A blocking Connection interface to MQTT.
 * 
 *
 * @author Hiram Chirino
 */
public class BlockingConnection {

    private final FutureConnection next;

    public BlockingConnection(FutureConnection next) {
        this.next = next;
    }

    public boolean isConnected() {
        return next.isConnected();
    }

    public void connect() throws Exception {
        this.next.connect().await();
    }

    public void disconnect() throws Exception {
        this.next.disconnect().await();
    }

    public void kill() throws Exception {
        this.next.kill().await();
    }

    public byte[] subscribe(final Topic[] topics) throws Exception {
        return this.next.subscribe(topics).await();
    }

    public void unsubscribe(final String[] topics) throws Exception {
        this.next.unsubscribe(topics).await();
    }

    public void unsubscribe(final UTF8Buffer[] topics) throws Exception {
        this.next.unsubscribe(topics).await();
    }

    public void publish(final UTF8Buffer topic, final Buffer payload, final QoS qos, final boolean retain) throws Exception {
        this.next.publish(topic, payload, qos, retain).await();
    }

    @Override
    protected Object clone() throws CloneNotSupportedException {
        return super.clone();    //To change body of overridden methods use File | Settings | File Templates.
    }

    public void publish(final String topic, final byte[] payload, final QoS qos, final boolean retain) throws Exception {
        publish(utf8(topic), new Buffer(payload), qos, retain);
    }

    public Message receive() throws Exception {
        return this.next.receive().await();
    }

    /**
     * @return null if the receive times out.
     */
    public Message receive(long amount, TimeUnit unit) throws Exception {
        Future receive = this.next.receive();
        try {
            Message message = receive.await(amount, unit);
            if( message!=null ) {
                message.blocking = true;
            }
            return message;
        } catch (TimeoutException e) {
            // Put it back on the queue..
            receive.then(new Callback() {
                public void onSuccess(final Message value) {
                    next.putBackMessage(value);
                }
                public void onFailure(Throwable value) {
                }
            });
            return null;
        }
    }

    public void setReceiveBuffer(final long receiveBuffer) throws InterruptedException {
        final CountDownLatch done = new CountDownLatch(1);
        next.getDispatchQueue().execute(new Runnable() {
            public void run() {
                try {
                    next.setReceiveBuffer(receiveBuffer);
                } finally {
                    done.countDown();
                }

            }
        });
        done.await();
    }

    public long getReceiveBuffer() throws InterruptedException {
        final CountDownLatch done = new CountDownLatch(1);
        final AtomicLong result = new AtomicLong();
        next.getDispatchQueue().execute(new Runnable() {
            public void run() {
                try {
                    result.set(next.getReceiveBuffer());
                } finally {
                    done.countDown();
                }

            }
        });
        done.await();
        return result.get();
    }

    public void resume() {
        next.resume();
    }

    public void suspend() {
        next.suspend();
    }
}