public class MainActivity extends Activity {private static final String TAG = "MainActivity";private Handler mHandler;private Button btnSendeToMainThread;private static final int MSG_SUB_TO_MAIN= 100;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);btnSendeToMainThread = (Button) findViewById(R.id.btn_sendto_mainthread);// 1.创建Handler，并重写handleMessage方法mHandler = new Handler() {@Overridepublic void handleMessage(Message msg) {super.handleMessage(msg);// 处理消息switch (msg.what) {case MSG_SUB_TO_MAIN:// 打印出处理消息的线程名和Message.objLog.e(TAG, "接收到消息： " + Thread.currentThread().getName() + ","+ msg.obj);break;default:break;}}};btnSendeToMainThread.setOnClickListener(new View.OnClickListener() {@Overridepublic void onClick(View v) {// 创建一个子线程，在子线程中发送消息new Thread(new Runnable() {@Overridepublic void run() {Message msg = Message.obtain();msg.what = MSG_SUB_TO_MAIN;msg.obj = "这是一个来自子线程的消息";// 2.发送消息mHandler.sendMessage(msg);}}).start();}});}
}

boolean enqueueMessage(Message msg, long when) {//即不允许添加同步消息屏障，实现异步消息了。if (msg.target == null) {throw new IllegalArgumentException("Message must have a target.");}if (msg.isInUse()) {throw new IllegalStateException(msg + " This message is already in use.");}synchronized (this) {//. . . . . .msg.markInUse();msg.when = when;Message p = mMessages;boolean needWake;//当队列为空，第一次发来的 message，或者新发来的 message 的时间更小的情况，就走if块，添加到链表头部。if (p == null || when == 0 || when < p.when) {// New head, wake up the event queue if blocked.msg.next = p;mMessages = msg;//如果此时Looper处于阻塞状态，则唤醒。并循环执行 message 的读取。needWake = mBlocked;} else {// Inserted within the middle of the queue.  Usually we don't have to wake// up the event queue unless there is a barrier at the head of the queue// and the message is the earliest asynchronous message in the queue.// 将发来的 message 插入到 queue 队列的中间或者尾部。// 这种情况下，我们不需要唤醒事件队列。除非有一个消息屏障在队列的头部，并且它是队列中最早的异步消息。// 判断当前发送来的 message 是否是一个异步消息，如果是异步消息，并且队列 queue头部含有 消息屏障// p.target = null, 同时当前 queue是处于阻塞状态。则设为 “需要唤醒队列”。 //这里的 p 消息，也有可能是 postAsyncBarrier()中传到 消息队列里的，因此此处需要处理异步消息的情况needWake = mBlocked && p.target == null && msg.isAsynchronous();Message prev;// 执行一个 for循环，并通过时间的对比，将新发来的消息，插入到队列中适当的位置。for (;;) {prev = p;p = p.next;if (p == null || when < p.when) {break;}//如果队列头部有了 target=null 的一个消息屏障，并且当前发送过来的 message 是一个异步消息。// 如果添加 message 之前，队列中还有其他异步消息等待处理，则就不唤醒 queue。否则就唤醒。if (needWake && p.isAsynchronous()) {needWake = false;}}msg.next = p;prev.next = msg;}// We can assume mPtr != 0 because mQuitting is false.if (needWake) {nativeWake(mPtr);}}return true;
}

2. 源码——Looper

public static void loop() {boolean slowDeliveryDetected = false;for (;;) {Message msg = queue.next(); // might blockif (msg == null) {// No message indicates that the message queue is quitting.return;}try {// 将事件交由 Handler处理msg.target.dispatchMessage(msg);dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;} finally {if (traceTag != 0) {Trace.traceEnd(traceTag);}}// 回收msg，以便 Message.obtain() 实现复用。msg.recycleUnchecked();}
}

3. 源码——MessageQueue

Message next() {// mPtr，Native层的 MessageQueue。final long ptr = mPtr;if (ptr == 0) {return null;}int nextPollTimeoutMillis = 0;for (;;) {// Native层设置阻塞时间—延迟消息的时间nativePollOnce(ptr, nextPollTimeoutMillis);synchronized (this) {// Try to retrieve the next message.  Return if found.final long now = SystemClock.uptimeMillis();Message prevMsg = null;Message msg = mMessages; // 获取链表头部信息if (msg != null && msg.target == null) {// Stalled by a barrier.  Find the next asynchronous message in the queue.// 循环遍历，查找第一个 异步消息。会直接忽略 queue 中的其他消息。do {prevMsg = msg;msg = msg.next;} while (msg != null && !msg.isAsynchronous());}if (msg != null) {if (now < msg.when) {// Next message is not ready.  Set a timeout to wake up when it is ready.// 根据延迟消息，设置阻塞时间nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);} else {// 返回一个即时消息，或者返回一个已到时间的 延迟消息。mBlocked = false; // 设置为 queue未阻塞状态if (prevMsg != null) {prevMsg.next = msg.next;} else {mMessages = msg.next;}msg.next = null;msg.markInUse();return msg;}} else {// queue 队列中没有消息时，会返回 -1，表示 nativePollOnce() 会一直阻塞 nextPollTimeoutMillis = -1;}}}
}

4. 延迟消息Handler.postDelay(new Runnable(), 2000)

@Override
protected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);final Handler handler = new Handler() {@Overridepublic void handleMessage(Message msg) {super.handleMessage(msg);System.out.println("打印: " + msg.what);switch (msg.what){case 999://第四步 异步消息执行完之后必需，移除消息屏障，否则会一直阻塞 queue。removeSyncBarrier();break;}}};new Thread(new Runnable() {int max = 0;@Overridepublic void run() {while (max != 10) {handler.sendEmptyMessageDelayed(max, 5000 + 500 * max);max++;}}}).start();new Thread(new Runnable() {@Overridepublic void run() {Message message = Message.obtain();message.what = 999;// 第一步 设置 message为 异步消息message.setAsynchronous(true);// 第二步 设置消息屏障postSyncBarrier();// 第三步 发送消息handler.sendMessageDelayed(message, 0);}}).start();
}private int token = 0;//设置消息屏障时返回的token，用于删除消息屏障时使用。// 反射执行投递同步屏障
public void postSyncBarrier() {Method method = null;try {method = MessageQueue.class.getDeclaredMethod("postSyncBarrier");token = (int) method.invoke(Looper.getMainLooper().getQueue());} catch (NoSuchMethodException e) {e.printStackTrace();} catch (IllegalAccessException e) {e.printStackTrace();} catch (InvocationTargetException e) {e.printStackTrace();}
}// 反射执行移除同步屏障
public void removeSyncBarrier() {Method method = null;try {method = MessageQueue.class.getDeclaredMethod("removeSyncBarrier", int.class);method.invoke(Looper.getMainLooper().getQueue(), token);} catch (NoSuchMethodException e) {e.printStackTrace();} catch (IllegalAccessException e) {e.printStackTrace();} catch (InvocationTargetException e) {e.printStackTrace();}
}

源码分析 --- 消息屏障

// 8.0之后该方法改为 @hide隐藏了，因此需要通过反射获取。
public int postSyncBarrier() {return postSyncBarrier(SystemClock.uptimeMillis());
}
//注意⚠️ 这里添加message的时候，target没有设置，默认为null — — 即消息屏障
private int postSyncBarrier(long when) {// Enqueue a new sync barrier token.// We don't need to wake the queue because the purpose of a barrier is to stall it.synchronized (this) {final int token = mNextBarrierToken++;final Message msg = Message.obtain();msg.markInUse();msg.when = when;msg.arg1 = token;Message prev = null;Message p = mMessages;// 从链表的头部开始，找出队列中最后一个message，或者找出queue 中间的延迟时间晚于当前message延迟时间的if (when != 0) {while (p != null && p.when <= when) {prev = p;p = p.next;}}if (prev != null) { // 如果queue队列前面有其他message，则将当前 message插入到这两个 message中间 msg.next = p;prev.next = msg;} else { // 如果前面没有 message，则将当前的 message设置为最头部的 message。msg.next = p;mMessages = msg;}// 返回一个 token，用于接下来的 移除工作。return token;}
}

步骤三、发送消息

public void removeSyncBarrier(int token) {// Remove a sync barrier token from the queue.// If the queue is no longer stalled by a barrier then wake it.synchronized (this) {Message prev = null;Message p = mMessages;while (p != null && (p.target != null || p.arg1 != token)) {prev = p;p = p.next;}if (p == null) {throw new IllegalStateException("The specified message queue synchronization "+ " barrier token has not been posted or has already been removed.");}final boolean needWake;if (prev != null) {prev.next = p.next;needWake = false;} else {mMessages = p.next;needWake = mMessages == null || mMessages.target != null;}p.recycleUnchecked();// If the loop is quitting then it is already awake.// We can assume mPtr != 0 when mQuitting is false.if (needWake && !mQuitting) {nativeWake(mPtr);}}
}