从源码的角度分析Android消息处理机制

我们都知道Android的UI线程是非安全的，如果要在子线程中更新UI界面，出现会报错”Only the original thread that created a view hierarchy can touch its views.“。我们通常的做法是在UI线程中创建一个Handler，然后在子线程中通过sendMessage来更新UI。为什么这样做就可以呢？今天我们就通过源码来解释下。

Android 的消息机制主要是通过Handler，Looper，Message，MessageQueue这几个类来实现的。Handler是发送消息（Message）和处理消息的。MessageQueue表示一个消息队列，负责消息入队和出队；Looper类用于创建消息循环。

下面我们通过创建一个Handler来开始我们今天的主题。

protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.test_handler_layout);

new Thread(){
public void run() {
Handler mHandler = new Handler();
};
}.start();

}

我们在onCreate方法中创建一个线程，并在该线程中new 了一个Handler。运行结果如下：



提示的错误信息为 Can’t create handler inside thread that has not called Looper.prepare() 。这句话是什么意思呢？就是说不能在子线程中创建Handler而不调用Looper.prepare()。我们进Handler的构造函数中看看为什么会报这样的错误。

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;
}

在第13行抛出了异常。是因为mLooper 为null导致的。我们去Looper.myLooper()方法中看看，

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

这个方法很简单，就是从sThreadLocal中取出一个Looper对象。那么我们可以猜到应该是在Looper.prepare()中为sThreadLocal设置了Looper对象。看下Looper.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));
}

可以看到，如果已经有一个Looper对象，你再调用prepare()方法会抛出异常，这表明一个线程只能有一个Looper对象。如果没有则new 一个Looper对象。

看到这里大家一定觉得很奇怪，为什么我平时写的更新UI界面的Handler时不需要调用Looper.prepare()呢？其实不然，那是因为在主线程ActivityThread中已经做好了。

public static void main(String[] args) {
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());

Security.addProvider(new AndroidKeyStoreProvider());

// 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"));
}

Looper.loop();

throw new RuntimeException("Main thread loop unexpectedly exited");
}

我们看第22行调用了Looper.prepareMainLooper()，再看一下prepareMainLooper具体做了哪些工作：

public static void prepareMainLooper() {
prepare();
setMainLooper(myLooper());
myLooper().mQueue.mQuitAllowed = false;
}

可以看到在第2行调用了prepare()，不难推测在该方法中会创建Looper对象。

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

这就保证了，我们在UI线程中创建Handler不需要再调用Looper.prepare()了。

下面我们看看Handler是怎么发生消息的。我们平时都类似这样写的：

private Handler mHandler = new Handler(){
public void handleMessage(Message msg) {
if(msg.what == 11){
bm = (Bitmap)msg.obj;
mImageView.setImageBitmap(bm);
}
};
};

@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.test_handler_layout);

new Thread(){
public void run() {
Message msg = Message.obtain();
msg.what = 11;
msg.obj = getBitMapFromURL(url);
mHandler.sendMessage(msg);
};
}.start();

在子线程中发送Message，在Handler中通过handleMessage来更新UI。现在我们从源码看看Handler是怎么发送消息，消息又是如何回到Handler中的。

Handler中有很多发送消息的方法，其中除了sendMessageAtFrontOfQueue()方法之外，其它的发送消息方法最终都会辗转调用到sendMessageAtTime()方法中，这个方法的源码如下所示：

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);
}

其中的两个参数分别是待发送的消息以及发送消息的时间。queue就是消息队列。如果 queue 不为空就进行消息的入队操作，enqueueMessage源码如下：

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

这个方法很简单，将msg.target表示未Handler,然后调用queue.enqueueMessage(msg, uptimeMillis)。源码如下：

final boolean enqueueMessage(Message msg, long when) {
if (msg.isInUse()) {
throw new AndroidRuntimeException(msg
+ " This message is already in use.");
}
if (msg.target == null && !mQuitAllowed) {
throw new RuntimeException("Main thread not allowed to quit");
}
final boolean needWake;
synchronized (this) {
if (mQuiting) {
RuntimeException e = new RuntimeException(
msg.target + " sending message to a Handler on a dead thread");
Log.w("MessageQueue", e.getMessage(), e);
return false;
} else if (msg.target == null) {
mQuiting = true;
}

msg.when = when;
Message p = mMessages;
if (p == null || when == 0 || when < p.when) {
msg.next = p;
mMessages = msg;
needWake = mBlocked; // new head, might need to wake up
} else {
Message prev = null;
while (p != null && p.when <= when) {
prev = p;
p = p.next;
}
msg.next = prev.next;
prev.next = msg;
needWake = false; // still waiting on head, no need to wake up
}
}
if (needWake) {
nativeWake(mPtr);
}
return true;
}

入队操作中使用mMessages表示当前待处理的消息，29-35行表示根据消息的处理时间Message.when来确定消息插入到队列中的具体位置，从而保证队首的消息是下一个要处理的。

入队讲完了，我们接着讲消息的出队。出队是通过Looper.loop()来循环实现的。源码如下：

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.recycle();
}
}

从代码的第13行就进入了一个死循环，通过queue.next()来出去出队的消息。我们进到MessageQueue.next()方法看看消息时怎么出队的：

final Message next() {
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;

for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(mPtr, nextPollTimeoutMillis);

synchronized (this) {
// Try to retrieve the next message.  Return if found.
final long now = SystemClock.uptimeMillis();
final Message msg = mMessages;
if (msg != null) {
final long when = msg.when;
if (now >= when) {
mBlocked = false;
mMessages = msg.next;
msg.next = null;
if (false) Log.v("MessageQueue", "Returning message: " + msg);
msg.markInUse();
return msg;
} else {
nextPollTimeoutMillis = (int) Math.min(when - now, Integer.MAX_VALUE);
}
} else {
nextPollTimeoutMillis = -1;
}

// If first time, then get the number of idlers to run.
if (pendingIdleHandlerCount < 0) {
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;
}
}

代码14-29解读：

首先 ， 定义一个Message 变量(msg)指向当前待处理的消息(mMessages)。

如果当前待处理的消息mMessages不为空，那么进入第17行，判断是否已到发送时间，如果时间到了，那么让待处理的消息(mMessages)指向下一个msg。

msg.next = null表示将当前的消息从消息队列中移除，因为msg.next = null表示该消息不再指向其他的消息。

最后返回待处理的消息。

接着看Looper.loop()中第27行代码，每当有一个消息出队，就将它传递到msg.target的dispatchMessage()方法中，那么msg.target指的是什么呢？msg.target就是Handler，在之前的入队操作中可以看出来（ msg.target = this）。

dispatchMessage源码如下：

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

第2行中的msg.callback其实是一个Runnable对象。一般我们调用Handler.post(runnable)时会赋值的。源码如下：

public final boolean post(Runnable r)
{
return  sendMessageDelayed(getPostMessage(r), 0);
}

看看getPostMessage(r)都做了什么？

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

第3行为m.callback 赋值为一个Runnable 对象。

回到dispatchMessage方法中。其中第5行的mCallback，是在Handler的内部接口CallBack中实现的。最后执行 handleMessage(msg);

源码：

public void handleMessage(Message msg) {
}

可以看出这个函数是空的，这就要求我们自己在创建Handler的时候来实现它。这就是为什么handleMessage()方法中可以获取到之前发送的消息。