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1.MQTT（消息队列遥测传输协议），是一种基于 发布/订阅 （publish/subscribe）模式的"轻量级"通讯协议， 该协议构建于TCP/IP协议上 。MQTT最大优点在于，可以以极少的代码和有限的带宽，为连接远程设备提供实时可靠的消息服务。作为一种低开销、低带宽占用的即时通讯协议，使其在物联网、小型设备、移动应用等方面有较广泛的应用。

MQTT协议中有三种身份：发布者（Publish）、服务器（Broker）、订阅者（Subscribe）

2.api操作：
Connect：与服务器进行MQTT连接；
Disconnect：与服务器断开MQTT连接；
Subscribe：订阅某个主题；
UnSubscribe：取消订阅某个主题；
Publish：发布消息；

3.安卓客户端示例：

添加依赖：

    implementation 'org.eclipse.paho:org.eclipse.paho.client.mqttv3:1.2.2'implementation 'org.eclipse.paho:org.eclipse.paho.android.service:1.1.1'

添加权限

<uses-permission android:name="android.permission.INTERNET" />

java代码

//创建实例
//brokerUrl为实例，clientid是唯一值
MqttAndroidClient mqttAndroidClient = new MqttAndroidClient(context, brokerUrl, clientId);//设置监听订阅消息的回调mqttAndroidClient.setCallback(mqttCallback);mMqttConnectOptions = new MqttConnectOptions();//设置是否清除缓存mMqttConnectOptions.setCleanSession(true);//设置超时时间，单位：秒mMqttConnectOptions.setConnectionTimeout(60);//设置心跳包发送间隔，单位：秒mMqttConnectOptions.setKeepAliveInterval(60);//设置用户名mMqttConnectOptions.setUserName(userName);//设置密码mMqttConnectOptions.setPassword(password.toCharArray());//是否重连mMqttConnectOptions.setAutomaticReconnect(false);//同时发布的最大消息数量mMqttConnectOptions.setMaxInflight(10000);//建立连接mqttAndroidClient.connect(mMqttConnectOptions, null, iMqttActionListener);private final MqttCallback mqttCallback = new MqttCallback() {//这里处理收到的数据@Overridepublic void messageArrived(String topic, MqttMessage message) {}@Overridepublic void deliveryComplete(IMqttDeliveryToken arg0) {Log.d(TAG, "deliveryComplete: ");}@Overridepublic void connectionLost(Throwable arg0) {Log.e(TAG, "connectionLost:" + arg0.getMessage());Log.e(TAG, "onFailure: " + mqttAndroidClient.isConnected());doReconnect();}};private final IMqttActionListener iMqttActionListener = new IMqttActionListener() {@Overridepublic void onSuccess(IMqttToken arg0) {//建立订阅try {mqttAndroidClient.subscribe(topic, topicQos, null,new IMqttActionListener() {@Overridepublic void onSuccess(IMqttToken asyncActionToken) {Log.d(TAG, "Subscribed topic successfully: " + asyncActionToken.toString());}@Overridepublic void onFailure(IMqttToken asyncActionToken,Throwable exception) {Log.d(TAG, "Failed to subscribe to a topic: " + asyncActionToken.toString());}});} catch (MqttException e) {Log.d(TAG, "Subscribe to the topic: " + e);e.printStackTrace();}}@Overridepublic void onFailure(IMqttToken arg0, Throwable arg1) {arg1.printStackTrace();Log.e(TAG, "iMqttActionListener: arg0：" + arg0.toString() + " arg1:" + arg1.getMessage());Log.e(TAG, "onFailure: " + mqttAndroidClient.isConnected());doReconnect();}};MqttMessage message = new MqttMessage();message.setPayload(payload.getBytes());message.setQos(0);//publish到mqttmqttAndroidClient.publish(mTopic, message, null, new IMqttActionListener() {@Overridepublic void onSuccess(IMqttToken asyncActionToken) {Log.i(TAG, "publish succeed!");listener.onState(1);}@Overridepublic void onFailure(IMqttToken asyncActionToken, Throwable exception) {Log.i(TAG, "publish failed!");listener.onState(2);}});