库:Paho,这个的社区比较活跃

安装java_paho

没有使用maven,所以手动下载的jar包

根据需要下载下载地址:https://repo.eclipse.org/content/repositories/paho-releases/org/eclipse/paho/

我下载的是org.eclipse.paho.client.mqttv3 -1.2.2版本

代码实现

    ///服务器地址
      String broker = "tcp://localhost:1883";
///clientid在同一个emqx服务器必须唯一
        String clientId = "tn_link2";
///本地消息的持久化实例
        MemoryPersistence persistence = new MemoryPersistence();
        try {
            MqttClient sampleClient = new MqttClient(broker, clientId, persistence);
            MqttConnectOptions connOpts = new MqttConnectOptions();
            connOpts.setCleanSession(true);
            System.out.println("Connecting to broker:" + broker);
            sampleClient.connect(connOpts);
            System.out.println("Connected");
            String topic = "pubdata";
            System.out.println("Subscribe to topic:" + topic);
            //订阅主题
            sampleClient.subscribe(topic);
            //设置了一个回调实例,消息转发过来的时候调用该实例的方法
            sampleClient.setCallback(new MqttCallback() {
                public void messageArrived(String topic, MqttMessage message) throws Exception {
                    String theMsg = MessageFormat.format("{0} topic is {1}", new String(message.getPayload(),"utf-8"),topic);
                    System.out.println(theMsg);               
                }
                public void deliveryComplete(IMqttDeliveryToken token) {
                }
                public void connectionLost(Throwable throwable) {
                }
            });
           String content = "Message from MqttPublishSample";
            int qos = 2;
            System.out.println("Publishing message:" + content);
            MqttMessage message = new MqttMessage(content.getBytes());
            //可以指定也可不指定
            message.setQos(qos);
          //发布消息
            sampleClient.publish(topic, message);
            System.out.println("Message published");

        } 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();
        }

经测试是可以连接到emqx服务器