MQTT fusesource 的 Callback API 消息发布的实现

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当QoS level 1的时候发布消息时，Server会回调通知客户端。fusesource 有实现带callback的发布方式。

所有QoS level 1都要在可变头部中附加一个16位的消息ID。

SUBSCRIBE和UNSUBSCRIBE消息使用QoS level 1。

针对消息的发布，Qos level 1，意味着消息至少被传输一次。

发送者若在一段时间内接收不到PUBACK消息，发送者需要打开DUB标记为1，然后重新发送PUBLISH消息。因此会导致接收方可能会收到两次PUBLISH消息。针对客户端发布消息到服务器的消息流：

ClientMessage and directionServerQoS = 1
DUP = 0
Message ID = x

Action: Store message

PUBLISH
---------->

Actions:

Store message
Publish message to subscribers
Delete message

Reception: >=1

Action: Discard messagePUBACK
<----------Message ID = x

针对服务器发布到订阅者的消息流：

ServerMessage and directionSubscriberQoS = 1
DUP = 0
Message ID = xPUBLISH
---------->

Actions:

Store message
Make message available

Reception: >=1

PUBACK
<----------Message ID = x

发布者（客户端/服务器）若因种种异常接收不到PUBACK消息，会再次重新发送PUBLISH消息，同时设置DUP标记为1。接收者以服务器为例，这可能会导致服务器收到重复消息，按照流程，broker（服务器）发布消息到订阅者（会导致订阅者接收到重复消息），然后发送一条PUBACK确认消息到发布者。

在业务层面，或许可以弥补MQTT协议的不足之处：重试的消息ID一定要一致接收方一定判断当前接收的消息ID是否已经接受过

但一样不能够完全确保，消息一定到达了。

import org.fusesource.hawtbuf.Buffer;import org.fusesource.hawtbuf.UTF8Buffer;import org.fusesource.mqtt.client.*;import org.slf4j.Logger;import org.slf4j.LoggerFactory;import java.net.URISyntaxException;/** * Created by tiantao on 14-12-2. */public class MQTTCallbackServer {    private static final Logger LOG = LoggerFactory.getLogger(MQTTCallbackServer.class);    private final static String CONNECTION_STRING = "tcp://localhost:1883";    private final static boolean CLEAN_START = true;    private final static short KEEP_ALIVE = 30;// 低耗网络，但是又需要及时获取数据，心跳30s    public final  static long RECONNECTION_ATTEMPT_MAX=6;    public final  static long RECONNECTION_DELAY=2000;    public final static int SEND_BUFFER_SIZE=2*1024*1024;//发送最大缓冲为2M    public static void main(String[] args)   {        //创建MQTT对象        MQTT mqtt = new MQTT();        try {            //设置mqtt broker的ip和端口            mqtt.setHost("localhost", 1883);            //连接前清空会话信息            mqtt.setCleanSession(CLEAN_START);            //设置重新连接的次数            mqtt.setReconnectAttemptsMax(RECONNECTION_ATTEMPT_MAX);            //设置重连的间隔时间            mqtt.setReconnectDelay(RECONNECTION_DELAY);            //设置心跳时间            mqtt.setKeepAlive(KEEP_ALIVE);            //设置缓冲的大小            mqtt.setSendBufferSize(SEND_BUFFER_SIZE);            //获取mqtt的连接对象BlockingConnection            final CallbackConnection connection = mqtt.callbackConnection();            //添加连接的监听事件            connection.listener(new Listener() {                public void onDisconnected() {                }                public void onConnected() {                }                public void onPublish(UTF8Buffer topic, Buffer payload, Runnable ack) {                    // You can now process a received message from a topic.                    // Once process execute the ack runnable.                    ack.run();                    System.out.println("topic"+topic.toString()+"="+new String(payload.getData()));                }                public void onFailure(Throwable value) {                }            });            //添加连接事件            connection.connect(new Callback<Void>() {                /**                 * 连接失败的操作                 */                public void onFailure(Throwable value) {                    // If we could not connect to the server.                    System.out.println("MQTTCallbackServer.CallbackConnection.connect.onFailure"+"连接失败......"+value.getMessage());                    value.printStackTrace();                }                /**                 * 连接成功的操作                 * @param v                 */                public void onSuccess(Void v) {                    int count=1;                                 // 用于发布消息，目前手机段不需要向服务端发送消息                    //主题的内容                    final String message="hello "+count+"chinese people !";                    final String topic = "tokudu/aedb222fdf736c94";                    System.out.println("MQTTCallbackServer  publish  topic="+topic+" message :"+message);                    connection.publish(topic, message.getBytes(), QoS.AT_LEAST_ONCE, false, new Callback<Void>() {                        public void onSuccess(Void v) {                            System.out.println("OK");                            // the pubish operation completed successfully.                        }                        public void onFailure(Throwable value) {                            value.printStackTrace();                        }                    });                    try {                        Thread.sleep(2000);                    } catch (InterruptedException e) {                        // TODO Auto-generated catch block                        e.printStackTrace();                    }                }            });        } catch (URISyntaxException e) {            // TODO Auto-generated catch block            e.printStackTrace();        } catch (Exception e) {            // TODO Auto-generated catch block            e.printStackTrace();        }finally{        }    }}

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