参考资料：gityuan的知乎回答，http://gityuan.com，weibo.com/gityuan(微博)

1.基础知识

1.android系统不能在子线程中更新UI；因为系统的UI控件不是线程安全的，如果使用加锁使其线程安全，则会让UI控件变得复杂和低效，并可能阻塞某些进程的执行。

2.ThreadLocal类用来提供线程内部的局部变量。这些变量在多线程环境下访问(通过get或set方法访问)时能保证各个线程里的变量相对独立于其他线程内的变量，ThreadLocal实例通常来说都是private static类型。

3.一个线程中只有一个Looper实例和一个MessageQueue实例，可以有多个handler对象。

2.Looper笔记

一般先调用Looper.prepare，再调用Looper.loop()方法

Looper.prepare()方法中，根据sThreadLocal.get()是否为空，为空时才调用sThreadLocal.set(new Looper(quitAllowed))，不为空则抛出异常，从而保证了一个线程中只有一个Looper。

private static void prepare(booleanquitAllowed) { if (sThreadLocal.get() != null) { throw new RuntimeException("Only oneLooper may be created per thread"); } sThreadLocal.set(new Looper(quitAllowed)); }

Looper的构造函数中新建了MessageQueue对象，每个线程中都有一个MessageQueue对象：

private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }

通过myLooper()方法可获得当前线程的looper实例：

public static Looper myLooper() { return sThreadLocal.get(); }

Looper中有static对象sMainLooper，可通过static方法getMainLooper()来获得主线程的looper实例。

3.Handler笔记

Handler的构造函数，把handler和looper关联起来:

public Handler(Callback callback, boolean async) { //省略一部分 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; } handler的dispatchMessage()方法中分发message：

public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } } handler.handleMessage()，一般需重写该方法来处理消息

post(Runnable r)方法也是通过sendMessage实现的，post()会将该runable放入message，并把message的callback指向该runnable，并通过handleCallback最终运行该可执行代码：

private static Message getPostMessage(Runnable r) { Message m = Message.obtain(); m.callback = r; return m; } private static void handleCallback(Message message) { message.callback.run(); }

4.MessageQueue笔记

MessageQueue中的mMessages指向队头的message，对于一个新的message，会在enqueueMessage()方法中，根据msg.when等信息，将该message插入到适当的位置：

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("MessageQueue", 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; }MessageQueue的next()方法会进入一个死循环，返回一下个要处理的message。

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 (false) Log.v("MessageQueue", "Returning message: " + msg); 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("MessageQueue", "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; } }

5.发送message的过程

handler.sendMessage()——hanler.enqueueMessage()——queue.enqueueMessage()

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { //把将要发送的message.target指向自己 msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }

6.处理message的过程

Looper.loop()——queue.next()——msg.target.dispatchMessage()

Looper.loop()代码如下：

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 Printer logging = me.mLogging; if (logging != null) { logging.println(">>>>> Dispatching to " + msg.target + " " + msg.callback + ": " + msg.what); } msg.target.dispatchMessage(msg); 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(); } }

7.主线程的消息机制

ActivityThread实际上并非线程，并没有真正继承Thread类，只是往往运行在主线程，承载ActivityThread的主线程是由Zygote fork而创建的进程。

ActivityThread的main()代码如下：

public static void main(String[] args) { Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain"); SamplingProfilerIntegration.start(); // CloseGuard defaults to true and can be quite spammy. We // disable it here, but selectively enable it later (via // StrictMode) on debug builds, but using DropBox, not logs. CloseGuard.setEnabled(false); Environment.initForCurrentUser(); // Set the reporter for event logging in libcore EventLogger.setReporter(new EventLoggingReporter()); AndroidKeyStoreProvider.install(); // Make sure TrustedCertificateStore looks in the right place for CA certificates final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId()); TrustedCertificateStore.setDefaultUserDirectory(configDir); Process.setArgV0("<pre-initialized>"); Looper.prepareMainLooper(); ActivityThread thread = new ActivityThread(); thread.attach(false); if (sMainThreadHandler == null) { sMainThreadHandler = thread.getHandler(); } if (false) { Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread")); } // End of event ActivityThreadMain. Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); Looper.loop(); throw new RuntimeException("Main thread loop unexpectedly exited"); } 其中：

main()方法中调用了prepareMainLooper()，所以主线程中可以直接新建handler，而不用先调用prepare()方法；此外，ActivityThread中有一个内部类Activity.H(继承自handler)的对象mH，用来处理Activity生命周期相关的message。

thread.attach(false)会创建一个Binder线程（具体是指ApplicationThread，Binder的服务端，用于接收系统服务ApplicationManagerService发送来的事件），该Binder线程通过Activity.H将Message发送给主线程。

Looper.loop()方法会循环读取主线程的MessageQueue中的message，当MessageQueue中没有message时，message.next()方法会发生阻塞(通过nativePollOnce()方法)，此时主线程会释放CPU资源进入休眠状态，直到下个消息到达或者有事务发生，通过往pipe管道写端写入数据来唤醒主线程工作。

当有新的事件发生时，系统进程会通过binder方式调用ApplicationThreadd的对应方法(比如启动服务时调用scheduleCreateService()方法)，在该方法内调用了sendMessage()方法，通过mH对象sendMessage()向主线程的消息队列中发送了消息，进而唤醒了主线程(MessageQueue的enqueueMessage()方法会调用nativeWake(mPtr)方法进行唤醒)。

public final void scheduleCreateService(IBinder token, ServiceInfo info, CompatibilityInfo compatInfo, int processState) { updateProcessState(processState, false); CreateServiceData s = new CreateServiceData(); s.token = token; s.info = info; s.compatInfo = compatInfo; sendMessage(H.CREATE_SERVICE, s); } private void sendMessage(int what, Object obj, int arg1, int arg2, boolean async) { if (DEBUG_MESSAGES) Slog.v( TAG, "SCHEDULE " + what + " " + mH.codeToString(what) + ": " + arg1 + " / " + obj); Message msg = Message.obtain(); msg.what = what; msg.obj = obj; msg.arg1 = arg1; msg.arg2 = arg2; if (async) { msg.setAsynchronous(true); } mH.sendMessage(msg); }