最浅显讲解Android消息机制

前言：以《安卓开发艺术探索》和自己经验为基础，介绍Android消息机制。Android消息机制主要指Handler机制，同时Handler需要底层的MessageQueue和Looper支撑。Looper会对消息队列进行无限循环，一有消息就会处理。同时在Looper中还有一个重要角色，那就是ThreadLocal，注意他并不是线程，他可以在不同线程中互不干扰地存储数据。还要注意，线程是默认没有Looper的，所以需要自己创建，但是在MainActivity中的ActivityThread会默认初始化Looper，所以主线程默认可以使用Handler。

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目录

最浅显讲解Android消息机制

一、Android消息机制概述

二、Handler工作原理

三、ThreadLocal工作原理

四、消息队列的工作原理

五、Looper工作原理

六、Handler工作原理

七、主线程消息机制

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一、Android消息机制概述

Handler、MessageQueue、Looper其实是一个整体，只不过我们经常使用的是Handler而已，那么为什么Android要提供线程切换的功能呢，或者说为什么不能在子线程访问UI呢，这从ViewRootImpl的线程检查可以看出：

    void checkThread() {if (mThread != Thread.currentThread()) {throw new CalledFromWrongThreadException("Only the original thread that created a view hierarchy can touch its views.");}}

从源码可以看出，如果在子线程访问UI会抛出异常，但是安卓要求主线程又不能进行耗时操作，否则会抛出ANR异常，所以安卓提供了Handler机制，让我们可以在线程间进行切换。

有一个疑问？为什么安卓限制不能在子线程访问UI呢，原因是安卓UI空间是线程不安全的，当并发访问时，会出现错误。可是为什么安卓不加上锁进行控制呢？首先，加锁会让UI访问逻辑变得复杂；其次降低了访问效率。所以安卓工程师决定采用消息机制来进行子线程和UI线程的转换。

二、Handler工作原理

Handler创建时会使用当前线程的Looper，如果没有就会报错。

Handler创建完成后，此使Handler、MessageQueue、Looper就可以协同工作了。Handler可以通过post或者send方法将一个Runnable送到Handler内部的Looper处理，当send方法被调用时，MessageQueue的enqueueMessage方法会把消息放入消息队列中，然后Looper循环发现有新消息，最终handleMessage方法就会被调用。因为Looper在创建Handler的线程，所以Hnadler中的逻辑就被切换到所在线程处理了。

三、ThreadLocal工作原理

ThreadLocal是一个线程存储数据类，通过它可以在指定线程存储数据，且只有本线程可以访问该数据，其他线程无法访问。在安卓源码中Looper、ActivityThread以及AMS中都用到了ThreadLocal。在开发过程中，当我们需要一个对象贯穿整个线程时，就可以使用ThreadLocal，比如，当我们需要一个监听器对象对整个线程可见时，我们传统会采用1）静态变量或者作为2）函数参数进行传递，对于第一种情况，如果有N个线程就需要创建N个静态监听器，对于第二种情况，如果函数调用栈很深时，很容易导致Bug的出现。

例子：

public class ThreadLocalDemo {public static void main(String[] args) {final ThreadLocal<Boolean> threadLocal = new ThreadLocal<>();threadLocal.set(true);System.out.println("MainThread:"+threadLocal.get());new Thread(new Runnable() {@Overridepublic void run() {threadLocal.set(false);System.out.println("Thread2:"+threadLocal.get());}}, "Thread2").start();new Thread(new Runnable() {@Overridepublic void run() {System.out.println("Thread3:"+threadLocal.get());}}).start();}
}

从代码可以看出，虽然不同线程访问的是同一个ThreadLocal对象，但是他们通过ThreadLocal获取的值却是不一样的，这就是其神奇之处。其实现原理就是ThreadLocal内部会从各自线程中取出一个数组，然后根据索引得到相应的value值，当然不同线程的数组是不同的。

    public void set(T value) {Thread t = Thread.currentThread();ThreadLocalMap map = getMap(t);if (map != null)map.set(this, value);elsecreateMap(t, value);}

    public T get() {Thread t = Thread.currentThread();ThreadLocalMap map = getMap(t);if (map != null) {ThreadLocalMap.Entry e = map.getEntry(this);if (e != null) {@SuppressWarnings("unchecked")T result = (T)e.value;return result;}}return setInitialValue();}

四、消息队列的工作原理

消息队列在安卓中主要是指MessageQueue，主要包含插入和读取两个操作，其方法分别是enqueueMessage和next。

首先来看插入操作：主要是链表的插入操作

  boolean enqueueMessage(Message msg, long when) {...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 {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.nextprev.next = msg;}// We can assume mPtr != 0 because mQuitting is false.if (needWake) {nativeWake(mPtr);}}return true;}

再来看看next操作，主要是链表的删除操作： 

    Message next() {...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 (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);}...}}

五、Looper工作原理

Looper会不停地从消息队列中检查是否有新消息，如果有就会立即处理，如果没有就会阻塞。

首先看看他的构造器：在构造器中创建消息队列对象

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

我们知道Handler使用的前提是要有Looper，否则会报错，那么如何创建Looper呢，通过Looper.prepare()来创建，通过Looper.loop()来开启循环。

new Thread(new Runnable() {@Overridepublic void run() {Looper.prepare();Handler handler = new Handler();Looper.loop();}}).start();

Looper除了prepare()方法，还提供了prepareMainLooper()方法，这个方法主要是给主线程创建Looper使用的，他就是封装了prepare方法，该Looper还提供了getMainLooper方法，可以在任何地方得到主线程的Looper。Looper也是可以退出的，使用quit和quitSafely。

Looper的loop方法:

    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();// Allow overriding a threshold with a system prop. e.g.// adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'final int thresholdOverride =SystemProperties.getInt("log.looper."+ Process.myUid() + "."+ Thread.currentThread().getName()+ ".slow", 0);boolean slowDeliveryDetected = false;for (;;) {Message msg = queue.next(); // might blockif (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 loggerfinal Printer logging = me.mLogging;if (logging != null) {logging.println(">>>>> Dispatching to " + msg.target + " " +msg.callback + ": " + msg.what);}final long traceTag = me.mTraceTag;long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;if (thresholdOverride > 0) {slowDispatchThresholdMs = thresholdOverride;slowDeliveryThresholdMs = thresholdOverride;}final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);final boolean needStartTime = logSlowDelivery || logSlowDispatch;final boolean needEndTime = logSlowDispatch;if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {Trace.traceBegin(traceTag, msg.target.getTraceName(msg));}final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;final long dispatchEnd;try {msg.target.dispatchMessage(msg);dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;} finally {if (traceTag != 0) {Trace.traceEnd(traceTag);}}if (logSlowDelivery) {if (slowDeliveryDetected) {if ((dispatchStart - msg.when) <= 10) {Slog.w(TAG, "Drained");slowDeliveryDetected = false;}} else {if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",msg)) {// Once we write a slow delivery log, suppress until the queue drains.slowDeliveryDetected = true;}}}if (logSlowDispatch) {showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", 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();}}

从源码可以看出：

1. 唯一跳出循环的条件是next方法返回null，即消息队列中没有消息了；
2. 当Looper的quit方法被调用，队列被标记为退出状态，他的next方法就会返回null；
3. 总之Looper必须退出，否则将一直阻塞在那里；
4. Looper使用msg.target.dispatchMessage(msg)来处理消息，而该方法又在创建Handler的线程中，所以代码逻辑就被切换到指定线程中去了。

六、Handler工作原理

Handler主要包含消息的发送和接收。以sendMessage方法为例:

    public final boolean sendMessage(Message msg){return sendMessageDelayed(msg, 0);}

    public final boolean sendMessageDelayed(Message msg, long delayMillis){if (delayMillis < 0) {delayMillis = 0;}return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);}

    public boolean sendMessageAtTime(Message msg, long uptimeMillis) {MessageQueue queue = mQueue;if (queue == null) {RuntimeException e = new RuntimeException(this + " sendMessageAtTime() called with no mQueue");Log.w("Looper", e.getMessage(), e);return false;}return enqueueMessage(queue, msg, uptimeMillis);}

    private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {msg.target = this;if (mAsynchronous) {msg.setAsynchronous(true);}return queue.enqueueMessage(msg, uptimeMillis);}

可以发现，Handler发送消息就是向消息队列插入一条消息，然后消息队列的next方法会把消息返回给Looper处理，最终Handler的handleMessage方法被调用。

七、主线程消息机制

主线程的入口为main方法，在main方法中，Looper.prepareMainLooper来创建主线程的Looper以及MessageQueue，并通过Looper.loop方法来开启循环。

    public static void main(String[] args) {Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");// 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 libcoreEventLogger.setReporter(new EventLoggingReporter());// Make sure TrustedCertificateStore looks in the right place for CA certificatesfinal File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());TrustedCertificateStore.setDefaultUserDirectory(configDir);Process.setArgV0("<pre-initialized>");Looper.prepareMainLooper();// Find the value for {@link #PROC_START_SEQ_IDENT} if provided on the command line.// It will be in the format "seq=114"long startSeq = 0;if (args != null) {for (int i = args.length - 1; i >= 0; --i) {if (args[i] != null && args[i].startsWith(PROC_START_SEQ_IDENT)) {startSeq = Long.parseLong(args[i].substring(PROC_START_SEQ_IDENT.length()));}}}ActivityThread thread = new ActivityThread();thread.attach(false, startSeq);if (sMainThreadHandler == null) {sMainThreadHandler = thread.getHandler();}if (false) {Looper.myLooper().setMessageLogging(newLogPrinter(Log.DEBUG, "ActivityThread"));}// End of event ActivityThreadMain.Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);Looper.loop();throw new RuntimeException("Main thread loop unexpectedly exited");}

主线程的消息循环开始以后，ActivityThread还需要一个Handler来和消息进行交互，这个Handler就是ActivityThread.H。

ActivityThread通过ApplicationThread和AMS进行进程间通信，AMS以进程间通信的方式完成ActivityThread的请求后回调Application中的Binder方法，然后ApplicationThread会向H发送消息，H收到消息后会将Application中的逻辑切换到ActivityThread中去执行，这就是主线程的消息循环模型。