MQTT源码解析

一、MQTT的创建和connect流程

1.Android端实现mqtt，首先会new一个MqttAndroidClient，并传入需要的参数。

首先，MqttAndroidClient是如何创建的呢？

public MqttAndroidClient(Context context, String serverURI,
			String clientId, MqttClientPersistence persistence, Ack ackType) {
		myContext = context;
		this.serverURI = serverURI;
		this.clientId = clientId;
		this.persistence = persistence;
		messageAck = ackType;
	}

MqttAndroidClient本身是继承Broadcast的子类，他的构造器需要传入上面的参数，必须传入的为

public MqttAndroidClient(Context context, String serverURI,
			String clientId) {
		this(context, serverURI, clientId, null, Ack.AUTO_ACK);
	}

2.connect的核心方法：

@Override
	public IMqttToken connect(MqttConnectOptions options, Object userContext,
			IMqttActionListener callback) throws MqttException {

		IMqttToken token = new MqttTokenAndroid(this, userContext,
				callback);

		connectOptions = options;
		connectToken = token;

		/*
		 * The actual connection depends on the service, which we start and bind
		 * to here, but which we can't actually use until the serviceConnection
		 * onServiceConnected() method has run (asynchronously), so the
		 * connection itself takes place in the onServiceConnected() method
		 */
		if (mqttService == null) { // First time - must bind to the service
			Intent serviceStartIntent = new Intent();
			serviceStartIntent.setClassName(myContext, SERVICE_NAME);
			Object service = myContext.startService(serviceStartIntent);
			if (service == null) {
				IMqttActionListener listener = token.getActionCallback();
				if (listener != null) {
					listener.onFailure(token, new RuntimeException(
							"cannot start service " + SERVICE_NAME));
				}
			}

			// We bind with BIND_SERVICE_FLAG (0), leaving us the manage the lifecycle
			// until the last time it is stopped by a call to stopService()
			myContext.bindService(serviceStartIntent, serviceConnection,
					Context.BIND_AUTO_CREATE);

			if (!receiverRegistered) registerReceiver(this);
		}
		else {
			pool.execute(new Runnable() {

				@Override
				public void run() {
					doConnect();
					
					//Register receiver to show shoulder tap.
					if (!receiverRegistered) registerReceiver(MqttAndroidClient.this);
				}

			});
		}

		return token;
	}

返回一个IMqttToken对象，这里首先new一个MqttTokenAndroid对象，然后去判断是否存在MqttService实例。

没有的话会创建一个，如果创建失败直接调用IMqttActionListener的onFailure回调。

成功的话绑定一个bindService，传入ServiceConnection参数对象。

而ServiceConnection的成功回调中有个doConnect()方法，里面会调用MqttService的connect方法从而建立Mqtt的连接。

而在MqttService的connect方法里面，会创建一个MqttConnection对象，调用他的connect方法。

而MqttConnection的connect方法里面会判断是否有MqttAsyncClient实例，如果没有会创建他的实例并调用他的connect方法

// if myClient is null, then create a new connection
			else {
				alarmPingSender = new AlarmPingSender(service);
				myClient = new MqttAsyncClien