源码分析Android Handler是如何实现线程间通信的
Handler作为Android消息通信的基础，它的使用是每一个开发者都必须掌握的。开发者从一开始就被告知必须在主线程中进行UI操作。但Handler是如何实现线程间通信的呢？本文将从源码中分析Handler的消息通信机制。
0x00 Handler使用
首先看看我们平时是如何使用的Handler的。先看看以下代码
//定义HandlerHandler mHandler = new Handler(){public void handleMessage(Message msg){switch(msg.what){case UPDATE_UI:updateUI(msg);break;}}};class MyThread extends Thread{public void run(){//do same work!...//send messageMessage msg = mHandler.obtainMessage(UPDATE_UI);mHandler.sendMessage(msg);}}private void updateUI(Message msg){//update UI}
在子线程中sendMessage(Message)发送消息，然后在Handler的handleMessage(Message)接收消息，执行更新UI操作。那么Handler是如何把消息从MyThread传递到MainThread中来呢？我们从sendMessage()开始慢慢揭开它的面纱。
0x01 sendMessage(Message)
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);}
我们发现调用sendMessage()方法最后都走到enqueueMessage()这个方法，一开始就把当前Handler实例赋给了Message.target的属性里面，后面可以知道这个target是用来执行处理函数回调的。
enqueueMessage方法是把Message信息放入到一个MessageQueue的队列中。顾名思义MessageQueue就是消息队列。从sendMessageAtTime()方法知道这个MessageQueue是Handler中的一个成员。它是在Handler的构造函数中通过Loopger对象来初始化的。
0x02 Handler构造函数
public Handler(Callback callback, boolean async) {if (FIND_POTENTIAL_LEAKS) {final Class<? extends Handler> klass = getClass();if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&(klass.getModifiers() & Modifier.STATIC) == 0) {Log.w(TAG, "The following Handler class should be static or leaks might occur: " +klass.getCanonicalName());}}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;}
这时候我们脑海知道创建Handler的时候，同时也创建了Looper实例和MessageQueue引用（MessageQueue对象其实是在Looper中构造的）。Looper是何物呢？简单地说就是消息循环，这个我们稍后会分析。
0x03 enqueueMessage(MessageQueue)
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;//这里把消息插入到队列中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;}
在MessageQueue中可以看到这个入列方法中有一个for循环就是把当前的需要处理Message放到队列的合适位置。因为需要处理的Message对象都有一个开始处理的时间when，这个队列是按照when排序的。
至此，Handler调用sendMessage()方法后就把Message消息通过enqueueMessage()插入MessageQueue队列中。
而这个MessageQueue是在Looper中维护的。
0x04 prepare()创建Looper
在0x02中我们知道创建Handler时就使用静态方法Looper.myLooper()得到当前线程的Looper对象。
/*** Return the Looper object associated with the current thread. Returns* null if the calling thread is not associated with a Looper.*/public static @Nullable Looper myLooper() {return sThreadLocal.get();}
sThreadLocal是一个ThreadLocal类型的静态变量。什么时候会把Looper对象放在sThreadLocal中呢？通过prepare()方法。
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));}
继续翻阅源码知道Looper在构造函数中创建MessageQueue对象
private Looper(boolean quitAllowed) {mQueue = new MessageQueue(quitAllowed);mThread = Thread.currentThread();}
调用prepare()方法将一个Looper对象放在了静态的ThreadLocal对象中。这个是一个与线程绑定的对象，且在内存中仅保存了一份引用。
使用ThreadLocal对象这一点非常巧妙，也非常重要，这是线程间通信的基础。即在线程中调用prepare()时就在该线程中绑定了Looper对象，而Looper对象中拥有MessageQueue引用。所以每个线程都有一个消息队列。
这样Handler、Looper、MessageQueue这几个类关系大概就可以画出来了。
0x05 启动循环loop()
/*** Run the message queue in this thread. Be sure to call* {@link #quit()} to end the loop.*/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();//这里执行消息队列循环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;if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {Trace.traceBegin(traceTag, msg.target.getTraceName(msg));}//执行处理消息的回调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();}}
loop()方法中有一个无限循环，不停地读取调用MessageQueue的next()方法。当next()没有返回时就阻塞在这里。当获取到MessageQueue中的消息时，就执行了处理消息的回调函数msg.target.dispatchMessage(msg)。
前面0x01分析我们知道msg.target是在Handler中的enqueueMessage()进行赋值，即它指向当前的Handler实例。
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {msg.target = this;if (mAsynchronous) {msg.setAsynchronous(true);}return queue.enqueueMessage(msg, uptimeMillis);}
执行msg.target.dispatchMessage(msg)后便走到了以下流程
/*** Handle system messages here.*/public void dispatchMessage(Message msg) {if (msg.callback != null) {handleCallback(msg);} else {if (mCallback != null) {if (mCallback.handleMessage(msg)) {return;}}handleMessage(msg);}}
这里就是回调handleMessage(msg)函数处理消息的地方。Handler负责将Message入列，Looper则负责循环从MessageQueue中取出需要处理的Message并交由Handler来处理。
0x06 启动主线程的消息循环
我们知道通过静态方法Looper.prepare()创建了绑定当前线程的Looper对象，而通过loop()启动一个循环不停地读取队列中Message。但是Android系统是什么时候启动了主线程的消息循环呢？
要理解这一点就必须进入Android应用程序的入口ActivityThread的main方法。
public static void main(String[] args) {...Looper.prepareMainLooper();...Looper.loop();throw new RuntimeException("Main thread loop unexpectedly exited");}
可以看出main方法中先后执行了Looper.prepareMainLooper()方法和Looper.loop()方法。正常情况下main方法不会退出，只有loop()方法发生异常后将会抛出RuntimeException。
0x07 Looper.prepareMainLooper()
/*** Initialize the current thread as a looper, marking it as an* application's main looper. The main looper for your application* is created by the Android environment, so you should never need* to call this function yourself. See also: {@link #prepare()}*/public static void prepareMainLooper() {prepare(false);synchronized (Looper.class) {if (sMainLooper != null) {throw new IllegalStateException("The main Looper has already been prepared.");}sMainLooper = myLooper();}}
prepareMainLooper()方法其实是调用了prepare()方法。
当我们启动应用时系统就调用了prepareMainLooper()并在主线程中绑定了一个Looper对象。
这时候我们回过来看看一开始的Handler使用方式。在主线程中我们创建了Handler对象，在Handler构造函数中初始化了Looper（即获取到了绑定在主线程中的Looper对象）。当在子线程MyThread中通过mHandler.sendMessage(msg)方法发送一个消息时就把Message放在与主线程绑定的MessageQueue中。这样在子线程中使用Handler就实现了消息的通信。
可以简单的使用以下类图表示，每个线程都由一个Handler，每个Handler都是与当前所在线程的Looper绑定。
0x08 主线程是否会阻塞
在0x06中知道在ActivityThead的main方法中启动了一个死循环。那主线程是不是就一直阻塞在这里呢？其实不然。可以看到ActivityThread类里面有一个自定义的Handler对象mH，在这里对象中handleMessage()回调中定义了Activity的各种交互如管理Activity生命周期，启动service，显示window等，都是通过Handler进行处理的。同时可以看出只有当应用退出EXIT_APPLICATION之后才回调用Looper.quit()停止消息循环。
public void handleMessage(Message msg) {...switch (msg.what) {case LAUNCH_ACTIVITY: {...handleLaunchActivity(r, null, "LAUNCH_ACTIVITY");Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);} break;...case PAUSE_ACTIVITY: {...handlePauseActivity((IBinder) args.arg1, false,(args.argi1 & USER_LEAVING) != 0, args.argi2,(args.argi1 & DONT_REPORT) != 0, args.argi3);...} break;...case SHOW_WINDOW:...handleWindowVisibility((IBinder)msg.obj, true);...break;...case EXIT_APPLICATION:if (mInitialApplication != null) {mInitialApplication.onTerminate();}Looper.myLooper().quit();break;...}...}
0x09 总结
当创建Handler时将通过ThreadLocal在当前线程绑定一个Looper对象，而Looper持有MessageQueue对象。执行Handler.sendMessage(Message)方法将一个待处理的Message插入到MessageQueue中，这时候通过Looper.loop()方法获取到队列中Message，然后再交由Handler.handleMessage(Message)来处理。
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