在《MQTT的學習之Mosquitto安裝&使用（1）》一文末尾，我已經模擬了發布-訂閱模式，只是那時在服務器直接模擬的，並不是java代碼模擬的。下面貼出Java代碼

1、首先引入依賴包：

<!-- mosquitto依賴 -->
        <dependency>
            <groupId>org.eclipse.paho</groupId>
            <artifactId>org.eclipse.paho.client.mqttv3</artifactId>
            <
 
version>1.1.0</version>
        </dependency>

2、代碼

ClientMQTT：

package org.mqtt.model;


import org.eclipse.paho.client.mqttv3.MqttClient;
import org.eclipse.paho.client.mqttv3.MqttConnectOptions;
import org.eclipse.paho.client.mqttv3.MqttException;
import org.eclipse.paho.client.mqttv3.MqttTopic;
 
import org.eclipse.paho.client.mqttv3.persist.MemoryPersistence;

public class ClientMQTT {

   public static final String HOST = "tcp://192.168.238.133:1883";
   public static final String TOPIC = "sensor";
   private static final String clientid = "client11";
   private MqttClient client;
   private
 
 MqttConnectOptions options;
   private String userName = "admin";
   private String passWord = "password";

   private void start() {
       try {
           // host為主機名，clientid即連接MQTT的客戶端ID，一般以唯一標識符表示，MemoryPersistence設置clientid的保存形式，默認為以內存保存
           client = new MqttClient(HOST, clientid, new MemoryPersistence());
           // MQTT的連接設置
           options = new MqttConnectOptions();
           // 設置是否清空session,這裏如果設置為false表示服務器會保留客戶端的連接記錄，這裏設置為true表示每次連接到服務器都以新的身份連接
           options.setCleanSession(true);
           // 設置連接的用戶名
           options.setUserName(userName);
           // 設置連接的密碼
           options.setPassword(passWord.toCharArray());
           // 設置超時時間 單位為秒
           options.setConnectionTimeout(10);
           // 設置會話心跳時間 單位為秒 服務器會每隔1.5*20秒的時間向客戶端發送個消息判斷客戶端是否在線，但這個方法並沒有重連的機制
           options.setKeepAliveInterval(20);
           // 設置回調
           client.setCallback(new PushCallback());
           MqttTopic topic = client.getTopic(TOPIC);
           //setWill方法，如果項目中需要知道客戶端是否掉線可以調用該方法。設置最終端口的通知消息
           options.setWill(topic, "close".getBytes(), 2, true);

           client.connect(options);
           //訂閱消息
           int[] Qos  = {1};
           String[] topic1 = {TOPIC};
           client.subscribe(topic1, Qos);

       } catch (Exception e) {
           e.printStackTrace();
       }
   }

   public static void main(String[] args) throws MqttException {
       ClientMQTT client = new ClientMQTT();
       client.start();
   }
}

PushCallback：

package org.mqtt.model;

import org.eclipse.paho.client.mqttv3.IMqttDeliveryToken;
import org.eclipse.paho.client.mqttv3.MqttCallback;
import org.eclipse.paho.client.mqttv3.MqttMessage;

/**
 * 發布消息的回調類
 * 
 * 必須實現MqttCallback的接口並實現對應的相關接口方法 ?CallBack 類將實現
 * MqttCallBack。每個客戶機標識都需要一個回調實例。在此示例中
 * ，構造函數傳遞客戶機標識以另存為實例數據。在回調中，將它用來標識已經啟動了該回調的哪個實例。 ?必須在回調類中實現三個方法：
 * 
 * public void messageArrived(MqttTopic topic, MqttMessage message) 接收已經預訂的發布。
 * 
 * public void connectionLost(Throwable cause) 在斷開連接時調用。
 * 
 * public void deliveryComplete(MqttDeliveryToken token)) 接收到已經發布的 QoS 1 或 QoS 2
 * 消息的傳遞令牌時調用。 ?由 MqttClient.connect 激活此回調。
 * 
 */
public class PushCallback implements MqttCallback {

    public void connectionLost(Throwable cause) {
        // 連接丟失後，一般在這裏面進行重連
        System.out.println("連接斷開，可以做重連");
    }
 
    @Override
    public void deliveryComplete(IMqttDeliveryToken arg0) {
        // publish後會執行到這裏
        System.out.println("deliveryComplete---------" + arg0.isComplete());
    }


    @Override
    public void messageArrived(String arg0, MqttMessage arg1) throws Exception {
        System.out.println("接收消息主題:" + arg0);
        System.out.println("接收消息Qos:" + arg1.getQos());
        System.out.println("接收消息內容:" + new String(arg1.getPayload()));
    }

}

ServerMQTT：

package org.mqtt.model;

/**
 * Created by Administrator on 17-2-10.
 */

import org.eclipse.paho.client.mqttv3.MqttClient;
import org.eclipse.paho.client.mqttv3.MqttConnectOptions;
import org.eclipse.paho.client.mqttv3.MqttDeliveryToken;
import org.eclipse.paho.client.mqttv3.MqttException;
import org.eclipse.paho.client.mqttv3.MqttMessage;
import org.eclipse.paho.client.mqttv3.MqttPersistenceException;
import org.eclipse.paho.client.mqttv3.MqttTopic;
import org.eclipse.paho.client.mqttv3.persist.MemoryPersistence;

public class ServerMQTT {

    //tcp://MQTT安裝的服務器地址:MQTT定義的端口號
    public static final String HOST = "tcp://192.168.238.133:1883";
    //public static final String HOST = "tcp://10.10.10.201:1883";
    
    //定義一個主題
    public static final String TOPIC = "sensor";
    //定義MQTT的ID，可以在MQTT服務配置中指定
    private static final String clientid = "server11";

    private MqttClient client;
    private MqttTopic topic11;
    private String userName = "mosquitto";
    private String passWord = "";

    private MqttMessage message;

    /**
     * 構造函數
     * @throws MqttException
     */
    public ServerMQTT() throws MqttException {
        // MemoryPersistence設置clientid的保存形式，默認為以內存保存
        client = new MqttClient(HOST, clientid, new MemoryPersistence());
        connect();
    }

    /**
     *  用來連接服務器
     */
    private void connect() {
        MqttConnectOptions options = new MqttConnectOptions();
        options.setCleanSession(false);
        options.setUserName(userName);
        options.setPassword(passWord.toCharArray());
        // 設置超時時間
        options.setConnectionTimeout(10);
        // 設置會話心跳時間
        options.setKeepAliveInterval(20);
        try {
            client.setCallback(new PushCallback());
            client.connect(options);

            topic11 = client.getTopic(TOPIC);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    /**
     *
     * @param topic
     * @param message
     * @throws MqttPersistenceException
     * @throws MqttException
     */
    public void publish(MqttTopic topic , MqttMessage message) throws MqttPersistenceException,
            MqttException {
        MqttDeliveryToken token = topic.publish(message);
        token.waitForCompletion();
        System.out.println("message is published completely! "
                + token.isComplete());
    }

    /**
     *  啟動入口
     * @param args
     * @throws MqttException
     */
    public static void main(String[] args) throws MqttException {
        ServerMQTT server = new ServerMQTT();

        server.message = new MqttMessage();
        server.message.setQos(1);
        server.message.setRetained(true);
  //      server.message.setPayload("hello,topic11".getBytes());
     
        for(int i = 0; i < 1000000;i++){
             server.message.setPayload(("hello,see" + i).getBytes());
             server.publish(server.topic11 , server.message);
        }
       // server.publish(server.topic11 , server.message);
        System.out.println(server.message.isRetained() + "------ratained狀態");
    }
}

經過測試：ClientMQTT可以實時接收到ServerMQTT發送的數據

MQTT的學習之Mosquitto發布-訂閱（2）