使用 Java SDK 连接MQTT

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使用 Java SDK 连接MQTT

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Eclipse Paho Java Client 是用 Java 编写的 MQTT 客户端库（MQTT Java Client），可用于 JVM 或其他 Java 兼容平台（例如Android）。

Eclipse Paho Java Client 提供了MqttAsyncClient 和 MqttClient 异步和同步 API。

通过 Maven 安装 Paho Java
通过包管理工具 Maven 可以方便地安装 Paho Java 客户端库，截止目前最新版本安装如下：

<dependency>
  <groupId>org.eclipse.paho</groupId>
	<artifactId>org.eclipse.paho.client.mqttv3</artifactId>
	<version>1.2.2</version>
</dependency> 

Paho Java 使用示例 Java 体系中 Paho Java 是比较稳定、广泛应用的 MQTT 客户端库，本示例包含 Java
语言的 Paho Java 连接 EMQX Broker，并进行消息收发完整代码：

App.java

    package io.emqx;
    
    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.MqttMessage; import
    org.eclipse.paho.client.mqttv3.persist.MemoryPersistence;
    
    
    public class App {
    
    
        public static void main(String[] args) {
    
    
            String subTopic = "testtopic/#";
            String pubTopic = "testtopic/1";
            String content = "Hello World";
            int qos = 2;
            String broker = "tcp://broker.emqx.io:1883";
            String clientId = "emqx_test";
            MemoryPersistence persistence = new MemoryPersistence();
    
            try {
    
    
                MqttClient client = new MqttClient(broker, clientId, persistence);
    
                // MQTT 连接选项
                MqttConnectOptions connOpts = new MqttConnectOptions();
                connOpts.setUserName("emqx_test");
                connOpts.setPassword("emqx_test_password".toCharArray());
                // 保留会话
                connOpts.setCleanSession(true);
    
                // 设置回调
                client.setCallback(new PushCallback());
    
                // 建立连接
                System.out.println("Connecting to broker: " + broker);
                client.connect(connOpts);
    
                System.out.println("Connected");
                System.out.println("Publishing message: " + content);
    
                // 订阅
                client.subscribe(subTopic);
    
                // 消息发布所需参数
                MqttMessage message = new MqttMessage(content.getBytes());
                message.setQos(qos);
                client.publish(pubTopic, message);
                System.out.println("Message published");
    
                client.disconnect();
                System.out.println("Disconnected");
                client.close();
                System.exit(0);
            } catch (MqttException me) {
    
    
                System.out.println("reason " + me.getReasonCode());
                System.out.println("msg " + me.getMessage());
                System.out.println("loc " + me.getLocalizedMessage());
                System.out.println("cause " + me.getCause());
                System.out.println("excep " + me);
                me.printStackTrace();
            }
        } }

回调消息处理类 OnMessageCallback.java

package io.emqx;

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

public class OnMessageCallback implements MqttCallback {
    
    
    public void connectionLost(Throwable cause) {
    
    
        // 连接丢失后，一般在这里面进行重连
        System.out.println("连接断开，可以做重连");
    }

    public void messageArrived(String topic, MqttMessage message) throws Exception {
    
    
        // subscribe后得到的消息会执行到这里面
        System.out.println("接收消息主题:" + topic);
        System.out.println("接收消息Qos:" + message.getQos());
        System.out.println("接收消息内容:" + new String(message.getPayload()));
    }

    public void deliveryComplete(IMqttDeliveryToken token) {
    
    
        System.out.println("deliveryComplete---------" + token.isComplete());
    }
}