使用Java代码搭建一个MQTT客户端 - HelloWorld开发者社区

一个MQTT客户端可以做到：
1、发布其他客户端可能会订阅的信息
2、订阅其它客户端发布的消息
3、退订或删除应用程序的消息
4、断开与服务器连接

所以在使用Java代码进行搭建时，也要实现这几种功能。

在POM文件中导入坐标

<dependency>
    <groupId>org.springframework.integration</groupId>
      <artifactId>spring-integration-mqtt</artifactId>
      <version>6.0.2</version>
</dependency>

MQTT客户端（发布者）

1、创建一个SendMQTT类，对MQTT客户端进行初始化操作，创建连接。

public class SendMqtt {
    public static MqttClient mqttClient = null;
    private static MemoryPersistence memoryPersistence = null;
    private static MqttConnectOptions mqttConnectOptions = null;


    static {
        init("happyfan");
    }


    public static  void init(String clientId) {
        //初始化连接设置对象
        mqttConnectOptions = new MqttConnectOptions();
        //初始化MqttClient
        if(null != mqttConnectOptions) {
//            true可以安全地使用内存持久性作为客户端断开连接时清除的所有状态
            mqttConnectOptions.setCleanSession(true);
//            设置连接超时
            mqttConnectOptions.setConnectionTimeout(30);
//            设置持久化方式
            memoryPersistence = new MemoryPersistence();
            if(null != memoryPersistence && null != clientId) {
                try {
                    mqttClient = new MqttClient("tcp://127.0.0.1:1883", clientId,memoryPersistence);
                } catch (MqttException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }else {

            }
        }else {
            System.out.println("mqttConnectOptions对象为空");
        }
        System.out.println(mqttClient.isConnected());
        if(null != mqttClient) {
            if(!mqttClient.isConnected()) {
                try {
                    System.out.println("创建连接");
                    mqttClient.connect(mqttConnectOptions);
                } catch (MqttException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }

            }
        }else {
            System.out.println("mqttClient为空");
        }
        System.out.println(mqttClient.isConnected());
    }
}

2、实现与MQTT客户端断开连接以及重新连接的功能：

public class SendMqtt {
    public static MqttClient mqttClient = null;
    private static MemoryPersistence memoryPersistence = null;
    private static MqttConnectOptions mqttConnectOptions = null;


    static {
        init("happyfan");
    }


    public static  void init(String clientId) {
        //初始化连接设置对象
        mqttConnectOptions = new MqttConnectOptions();
        //初始化MqttClient
        if(null != mqttConnectOptions) {
//            true可以安全地使用内存持久性作为客户端断开连接时清除的所有状态
            mqttConnectOptions.setCleanSession(true);
//            设置连接超时
            mqttConnectOptions.setConnectionTimeout(30);
//            设置持久化方式
            memoryPersistence = new MemoryPersistence();
            if(null != memoryPersistence && null != clientId) {
                try {
                    mqttClient = new MqttClient("tcp://127.0.0.1:1883", clientId,memoryPersistence);
                } catch (MqttException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }else {

            }
        }else {
            System.out.println("mqttConnectOptions对象为空");
        }
        System.out.println(mqttClient.isConnected());
        if(null != mqttClient) {
            if(!mqttClient.isConnected()) {
                try {
                    System.out.println("创建连接");
                    mqttClient.connect(mqttConnectOptions);
                } catch (MqttException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }

            }
        }else {
            System.out.println("mqttClient为空");
        }
        System.out.println(mqttClient.isConnected());
    }
    //与MQTT服务器断开连接
    public void closeConnect() {
        //关闭存储方式
        if(null != memoryPersistence) {
            try {
                memoryPersistence.close();
            } catch (MqttPersistenceException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }else {
            System.out.println("memoryPersistence is null");
        }

        //关闭连接
        if(null != mqttClient) {
            if(mqttClient.isConnected()) {
                try {
                    mqttClient.disconnect();
                    mqttClient.close();
                } catch (MqttException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }else {
                System.out.println("mqttClient is not connect");
            }
        }else {
            System.out.println("mqttClient is null");
        }
    }
    //重新连接MQTT服务器
    public void reConnect() {
        if(null != mqttClient) {
            if(!mqttClient.isConnected()) {
                if(null != mqttConnectOptions) {
                    try {
                        mqttClient.connect(mqttConnectOptions);
                    } catch (MqttException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }else {
                    System.out.println("mqttConnectOptions is null");
                }
            }else {
                System.out.println("mqttClient is null or connect");
            }
        }else {
            init("happyfan");//其实就是重新回到初始化的方法中去尝试连接服务器
        }

    }
}

3、实现订阅、发布、清空主题：

public class SendMqtt {
    public static MqttClient mqttClient = null;
    private static MemoryPersistence memoryPersistence = null;
    private static MqttConnectOptions mqttConnectOptions = null;


    static {
        init("happyfan");
    }


    public static  void init(String clientId) {
        //初始化连接设置对象
        mqttConnectOptions = new MqttConnectOptions();
        //初始化MqttClient
        if(null != mqttConnectOptions) {
//            true可以安全地使用内存持久性作为客户端断开连接时清除的所有状态
            mqttConnectOptions.setCleanSession(true);
//            设置连接超时
            mqttConnectOptions.setConnectionTimeout(30);
//            设置持久化方式
            memoryPersistence = new MemoryPersistence();
            if(null != memoryPersistence && null != clientId) {
                try {
                    mqttClient = new MqttClient("tcp://127.0.0.1:1883", clientId,memoryPersistence);
                } catch (MqttException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }else {

            }
        }else {
            System.out.println("mqttConnectOptions对象为空");
        }
        System.out.println(mqttClient.isConnected());
        if(null != mqttClient) {
            if(!mqttClient.isConnected()) {
                try {
                    System.out.println("创建连接");
                    mqttClient.connect(mqttConnectOptions);
                } catch (MqttException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }

            }
        }else {
            System.out.println("mqttClient为空");
        }
        System.out.println(mqttClient.isConnected());
    }

    public void closeConnect() {
        //关闭存储方式
        if(null != memoryPersistence) {
            try {
                memoryPersistence.close();
            } catch (MqttPersistenceException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }else {
            System.out.println("memoryPersistence is null");
        }

        //关闭连接
        if(null != mqttClient) {
            if(mqttClient.isConnected()) {
                try {
                    mqttClient.disconnect();
                    mqttClient.close();
                } catch (MqttException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }else {
                System.out.println("mqttClient is not connect");
            }
        }else {
            System.out.println("mqttClient is null");
        }
    }
    
    //    重新连接
    public void reConnect() {
        if(null != mqttClient) {
            if(!mqttClient.isConnected()) {
                if(null != mqttConnectOptions) {
                    try {
                        mqttClient.connect(mqttConnectOptions);
                    } catch (MqttException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }else {
                    System.out.println("mqttConnectOptions is null");
                }
            }else {
                System.out.println("mqttClient is null or connect");
            }
        }else {
            init("happyfan");
        }

    }
    //    发布消息
    public void publishMessage(String pubTopic,String message,int qos) {
        if(null != mqttClient&& mqttClient.isConnected()) {
            System.out.println("发布消息   "+mqttClient.isConnected());
            System.out.println("id:"+mqttClient.getClientId());
            MqttMessage mqttMessage = new MqttMessage();
            mqttMessage.setQos(qos);
            mqttMessage.setPayload(message.getBytes());

            MqttTopic topic = mqttClient.getTopic(pubTopic);

            if(null != topic) {
                try {
                    MqttDeliveryToken publish = topic.publish(mqttMessage);
                    if(!publish.isComplete()) {
                        System.out.println("消息发布成功");
                    }
                } catch (MqttException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }

        }else {
            reConnect();
        }

    }
    //    订阅主题
    public void subTopic(String topic) {
        if(null != mqttClient&& mqttClient.isConnected()) {
            try {
                mqttClient.subscribe(topic, 1);
            } catch (MqttException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }else {
            System.out.println("mqttClient is error");
        }
    }


    //    清空主题
    public void cleanTopic(String topic) {
        if(null != mqttClient&& !mqttClient.isConnected()) {
            try {
                mqttClient.unsubscribe(topic);
            } catch (MqttException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }else {
            System.out.println("mqttClient is error");
        }
    }

}

在使用发布消息的功能时，只需要调用这个类中的publishMessage()方法即可。指定一个发布的主题78:21:84:B8:EB:0C(随便自己的)，使用MQTTX尝试一下能不能接收到这个主题的消息（MQTTX也得订阅这个主题）。
使用Java代码搭建一个MQTT客户端
可以看到，MQTTX以及成功的接收到了这个消息，这样一个MQTT客户端(发布者)就搭建完成了。

MQTT客户端（订阅者）

1、创建一个MqttReceive类，对MQTT客户端进行初始化操作，并且实现接收消息的功能（把主题订阅上，创建回调函数）：

@Service
public class MqttReceive {
    @Autowired
    private  MqttReceiveCallback mqttReceiveCallback;

    private static int QoS = 1;//通讯的质量，最高是2
    private static String Host = "tcp://127.0.0.1:1883";
    private static MemoryPersistence memoryPersistence = null;
    private static MqttConnectOptions mqttConnectOptions = null;
    private static MqttClient mqttClient  = null;

    public  void init(String clientId) {
        mqttConnectOptions = new MqttConnectOptions();
        memoryPersistence = new MemoryPersistence();

        if(null != memoryPersistence && null != clientId && null != Host) {
            try {
                mqttClient = new MqttClient(Host, clientId, memoryPersistence);
            } catch (MqttException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }

        if(null != mqttConnectOptions) {
            mqttConnectOptions.setCleanSession(true);
            mqttConnectOptions.setConnectionTimeout(30);
            mqttConnectOptions.setKeepAliveInterval(45);
            if(null != mqttClient && !mqttClient.isConnected()) {
                //这里可以自己new一个回调函数，比如new MqttReceiveCallback()。我这里使用自动装配，让Spring容器来管理bean与bean的依赖
               mqttClient.setCallback(mqttReceiveCallback);
                try {
                    System.out.println(mqttReceiveCallback);
                    System.out.println("尝试连接");
                    mqttClient.connect();
                } catch (MqttException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }
    }
    public  void receive(String topic) {
        int[] Qos = {QoS};
        String[] topics = {topic};

        if(null != mqttClient && mqttClient.isConnected()) {
            if(null!=topics && null!=Qos && topics.length>0 && Qos.length>0) {
                try {
                    System.out.println("订阅主题");

                    mqttClient.subscribe(topics, Qos);
                } catch (MqttException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }else {
            System.out.println("初始化");
            init("HAPPYFAN");
            receive(topic);
        }
    }
}

在使用时，只要调用这个类中的 receive(String topic)方法即可。当MQTT客户端接收到订阅的主题(topic)的消息时，传输的信息就会到回调函数中（就是在MqttReceive类中，自己new的那个回调函数，要确保一致类名和new出来的回调函数一致）。
2、创建回调函数

@Service
public class MqttReceiveCallback implements MqttCallback {

    @Override
    public void messageArrived(String topic, MqttMessage message) throws Exception {
        String msg = new String(message.getPayload());
        System.out.println("Client 接收消息主题 : " + topic);
        System.out.println("Client 接收消息Qos : " + message.getQos());
        System.out.println("Client 接收消息内容 : " + msg);
    }
    @Override
    public void deliveryComplete(IMqttDeliveryToken token) {
    }
    @Override
    public void connectionLost(Throwable cause) {
    }

}

调用receive(String topic)方法，指定一个接收消息的主题(也是随自己的，我这里写的就是receive)，尝试能不能接收到这个主题的信息。
使用Java代码搭建一个MQTT客户端
可以看到，再idea控制台中成功的打印出了接收主题的消息、质量、以及内容。一个MQTT客户端(订阅者)就搭建完成了。

在POM文件中导入坐标

MQTT客户端（发布者）

MQTT客户端（订阅者）

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