Android Handler、Looper、MessageQueue以及Message源码分析

一、APP应用程序入口

APP应用程序入口为ActivityThread类中的main方法（主线程）。主要代码如下：

public static void main(String[] args) { Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain"); SamplingProfilerIntegration.start(); Process.setArgV0("<pre-initialized>"); Looper.prepareMainLooper(); ActivityThread thread = new ActivityThread(); thread.attach(false); if (sMainThreadHandler == null) { sMainThreadHandler = thread.getHandler(); } Looper.loop(); throw new RuntimeException("Main thread loop unexpectedly exited"); }

其中可以看到初始化UI主线程的Looper对象，通过Looper.prepareMainLooper()方法，在本方法中，调用了，prepare（boolean quitAllowed），该标识会传递给其对应的消息队列MessageQueue，通过代码可以看出，UI主线程对应的消息循环队列是不允许退出的。quitAllowed默认传入了false- prepare(false); 如下：

public static void prepareMainLooper() { prepare(false); synchronized (Looper.class) { if (sMainLooper != null) { throw new IllegalStateException("The main Looper has already been prepared."); } sMainLooper = myLooper(); } } private static void prepare(boolean quitAllowed) { if (sThreadLocal.get() != null) { throw new RuntimeException("Only one Looper may be created per thread"); } sThreadLocal.set(new Looper(quitAllowed)); } //Looper构造器 private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }

二、Handler

Handler的构造方法可以分为2类： （1）传入指定线程looper对象； （2）不传入looper对象，默认为UI主线程的looper对象。

（1）传入指定线程looper对象的构造方法 public Handler(Looper looper, Callback callback, boolean async) { mLooper = looper; mQueue = looper.mQueue; mCallback = callback; mAsynchronous = async; } （2）不传入looper对象，默认为UI主线程的looper对象。 public Handler(Callback callback, boolean async) { ... mLooper = Looper.myLooper(); if (mLooper == null) { throw new RuntimeException( "Can't create handler inside thread that has not called Looper.prepare()"); } mQueue = mLooper.mQueue; mCallback = callback; mAsynchronous = async; }

在Handler初始化时，会初始化一个Looper对象和Looper中对应的消息队列mLooper.mQueue对象。 mAsynchronous 的作用：为True时，发送消息为异步消息。

如下为Handler中的消息分发方法dispatchMessage。也就是如果在初始化Handler时，传入Callback，优先将消息传递给Callback，否则，传递给大家最为熟悉的Handler的handleMessage(msg)方法。

public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } }

Handler初始化后，接着去看看Looper中的实现。

二、Looper

（1）Looper初始化

Looper对外提供了两个静态的公共方法 其中在工作线程调用Looper.prepare()方法，其对应的MessageQueue消息队列是可以退出的。

源代码如下：

//（1） prepare()用于在工作线程中调用 public static void prepare() { prepare(true); } private static void prepare(boolean quitAllowed) { if (sThreadLocal.get() != null) { throw new RuntimeException("Only one Looper may be created per thread"); } sThreadLocal.set(new Looper(quitAllowed)); } //（2）prepareMainLooper()用于在UI主线程中调用 public static void prepareMainLooper() { prepare(false); synchronized (Looper.class) { if (sMainLooper != null) { throw new IllegalStateException("The main Looper has already been prepared."); } sMainLooper = myLooper(); } } //Looper构造器 private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }

（2）Looper的loop()方法

从源码中可以看出，loop()方法中通过死循环for，将消息队列MessageQueue中的消息取出，然后调用 msg.target.dispatchMessage(msg);方法将消息分发传递到Handler的dispatchMessage方法。 从而可以在Handler的callback或者handleMessage()中处理改消息。 在loop（）方法的最后，调用了msg.recycleUnchecked();处理完的消息msg，会被置空初始化，并加入到消息缓冲池中。Message中缓冲池的最大为50。

public static void loop() { final Looper me = myLooper(); if (me == null) { throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread."); } final MessageQueue queue = me.mQueue; // Make sure the identity of this thread is that of the local process, // and keep track of what that identity token actually is. Binder.clearCallingIdentity(); final long ident = Binder.clearCallingIdentity(); for (;;) { Message msg = queue.next(); // might block if (msg == null) { // No message indicates that the message queue is quitting. return; } // This must be in a local variable, in case a UI event sets the logger final Printer logging = me.mLogging; if (logging != null) { logging.println(">>>>> Dispatching to " + msg.target + " " + msg.callback + ": " + msg.what); } final long traceTag = me.mTraceTag; if (traceTag != 0 && Trace.isTagEnabled(traceTag)) { Trace.traceBegin(traceTag, msg.target.getTraceName(msg)); } try { msg.target.dispatchMessage(msg); } finally { if (traceTag != 0) { Trace.traceEnd(traceTag); } } if (logging != null) { logging.println("<<<<< Finished to " + msg.target + " " + msg.callback); } // Make sure that during the course of dispatching the // identity of the thread wasn't corrupted. final long newIdent = Binder.clearCallingIdentity(); if (ident != newIdent) { Log.wtf(TAG, "Thread identity changed from 0x" + Long.toHexString(ident) + " to 0x" + Long.toHexString(newIdent) + " while dispatching to " + msg.target.getClass().getName() + " " + msg.callback + " what=" + msg.what); } msg.recycleUnchecked(); } } //消息缓冲池的最大容量为50 private static final int MAX_POOL_SIZE = 50; //Message中的recycleUnchecked（）方法 void recycleUnchecked() { // Mark the message as in use while it remains in the recycled object pool. // Clear out all other details. flags = FLAG_IN_USE; what = 0; arg1 = 0; arg2 = 0; obj = null; replyTo = null; sendingUid = -1; when = 0; target = null; callback = null; data = null; synchronized (sPoolSync) { if (sPoolSize < MAX_POOL_SIZE) { next = sPool; sPool = this; sPoolSize++; } } }

三、MessageQueue

1. enqueueMessage(Message msg, long when)方法 —— Handler中发送消息的方法最终都是调用消息队列中的该方法，把消息加入到消息队列中。

boolean enqueueMessage(Message msg, long when) { 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) { if (mQuitting) { IllegalStateException e = new IllegalStateException( msg.target + " sending message to a Handler on a dead thread"); Log.w(TAG, e.getMessage(), e); msg.recycle(); return false; } msg.markInUse(); msg.when = when; Message p = mMessages; boolean needWake; if (p == null || when == 0 || when < p.when) { // New head, wake up the event queue if blocked. msg.next = p; mMessages = msg; 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. needWake = mBlocked && p.target == null && msg.isAsynchronous(); Message prev; for (;;) { prev = p; p = p.next; if (p == null || when < p.when) { break; } if (needWake && p.isAsynchronous()) { needWake = false; } } msg.next = p; // invariant: p == prev.next prev.next = msg; } // We can assume mPtr != 0 because mQuitting is false. if (needWake) { nativeWake(mPtr); } } return true; }

2 . Message next() {}方法，返回一个msg，该方法是上面提到的在Looper中的loop（）方法中调用。

3. IdleHander内部接口——可通过addIdleHandler/removeIdleHandler方法，添加或移除回调。

如果消息队列没有消息，也就是在UI主线程或者工作线程空闲时候，会循环调用mIdleHandlers集合中的IdleHander对象，执行queueIdle()方法。如果queueIdle()返回false，则就会从改集合中移除，也就是只执行一次。适合做延迟加载。

Message next() { // Return here if the message loop has already quit and been disposed. // This can happen if the application tries to restart a looper after quit // which is not supported. final long ptr = mPtr; if (ptr == 0) { return null; } int pendingIdleHandlerCount = -1; // -1 only during first iteration int nextPollTimeoutMillis = 0; for (;;) { if (nextPollTimeoutMillis != 0) { Binder.flushPendingCommands(); } 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; if (msg != null && msg.target == null) { // Stalled by a barrier. Find the next asynchronous message in the queue. 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. nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE); } else { // Got a message. mBlocked = false; if (prevMsg != null) { prevMsg.next = msg.next; } else { mMessages = msg.next; } msg.next = null; if (DEBUG) Log.v(TAG, "Returning message: " + msg); msg.markInUse(); return msg; } } else { // No more messages. nextPollTimeoutMillis = -1; } // Process the quit message now that all pending messages have been handled. if (mQuitting) { dispose(); return null; } // If first time idle, then get the number of idlers to run. // Idle handles only run if the queue is empty or if the first message // in the queue (possibly a barrier) is due to be handled in the future. if (pendingIdleHandlerCount < 0 && (mMessages == null || now < mMessages.when)) { pendingIdleHandlerCount = mIdleHandlers.size(); } if (pendingIdleHandlerCount <= 0) { // No idle handlers to run. Loop and wait some more. mBlocked = true; continue; } if (mPendingIdleHandlers == null) { mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)]; } mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers); } // Run the idle handlers. // We only ever reach this code block during the first iteration. for (int i = 0; i < pendingIdleHandlerCount; i++) { final IdleHandler idler = mPendingIdleHandlers[i]; mPendingIdleHandlers[i] = null; // release the reference to the handler boolean keep = false; try { keep = idler.queueIdle(); } catch (Throwable t) { Log.wtf(TAG, "IdleHandler threw exception", t); } if (!keep) { synchronized (this) { mIdleHandlers.remove(idler); } } } // Reset the idle handler count to 0 so we do not run them again. pendingIdleHandlerCount = 0; // While calling an idle handler, a new message could have been delivered // so go back and look again for a pending message without waiting. nextPollTimeoutMillis = 0; } }

四、Message

1. 创建Handler发送所需的消息Msg——通过Message.obtain（）以及重载方法

在Message中定义了消息缓存的最大容量MAX_POOL_SIZE 为50。sPool的赋值是通过recycleUnchecked()方法中。之前提到过该方法是在Looper.loop方法的最后才调用。也就是处理完第一个消息的时候，就开始把消息加入到sPool这个缓存对象的next中。下次调用msg.obtain()方法就会从缓存中取出。

private static Message sPool; private static final int MAX_POOL_SIZE = 50; /** * Return a new Message instance from the global pool. Allows us to * avoid allocating new objects in many cases. */ public static Message obtain() { synchronized (sPoolSync) { if (sPool != null) { Message m = sPool; sPool = m.next; m.next = null; m.flags = 0; // clear in-use flag sPoolSize--; return m; } } return new Message(); } /** * Recycles a Message that may be in-use. * Used internally by the MessageQueue and Looper when disposing of queued Messages. */ void recycleUnchecked() { // Mark the message as in use while it remains in the recycled object pool. // Clear out all other details. flags = FLAG_IN_USE; what = 0; arg1 = 0; arg2 = 0; obj = null; replyTo = null; sendingUid = -1; when = 0; target = null; callback = null; data = null; synchronized (sPoolSync) { if (sPoolSize < MAX_POOL_SIZE) { next = sPool; sPool = this; sPoolSize++; } } }

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至此对Handler、Looper、MessageQueue和Message的部分源码分析完毕。

收获要点总结：

App程序入口——ActivityThread中main()方法，并对UI主线程的Looper进行初始化。Handler创建——Handler初始化不指定Looper对象默认为UI主线程对应的Looper，消息发送底层是通过Looper对象中的mQueue对象，把消息加入到消息队列。通过loop()方法循环取出处理。Looper——UI主线程的消息队列是不允许停止退出的。Looper.prepare（）方法调用会和所在线程进行绑定，保存到ThreadLocal中。一一对应的关系。MessageQueue——消息队列中有next（）方法取出消息和enqueueMessage（）方法把消息加入到队列中。还有一个重要的内部接口IdleHandler，在消息队列消息为空时执行。Message——消息，内部做了缓冲的机制，缓冲容量为50。