Android消息机制分析

转载请注明出处：/article/8521415.html

对Android开发有一定了解的人都知道，Android应用程序是消息驱动的。Android应用程序的消息机制是围绕消息队列来实现的，具体的，主要通过Handler、MessageQueue、Looper三个类来实现。其中Handler用来发送和处理消息；MessageQueue表示一个消息队列，负责入队和出队消息；Looper类用于创建消息循环。本文结合源码来介绍Android的消息机制包括：消息循环的创建、消息的发送和处理。

每个Android应用程序都一个主线程ActivityThread，每个主线程都有一个消息循环。先看一下ActivityThread的main方法：

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");
}

可以看到第22行调用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;
}

可以看到在第调用prepare，然后调用setMainLooper设置主线程的Looper。不难推测，prepare中会创建Looper，先贴上代码：

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

prepare()里面先检查是否已经创建了Looper，如果没有，则创建一个并保存到sThreadLocal中，反之，如果已经创建了一个，则会抛出异常。通过这里，我们也可以发现，prepare只能调用一次，每个线程只有一个Looper。那这里已经创建了Looper，那么第36行的Looper.loop()又是做什么的呢？继续看源码：

public static void loop() {
Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
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();

while (true) {
Message msg = queue.next(); // might block
if (msg != null) {
if (msg.target == null) {
// No target is a magic identifier for the quit message.
return;
}

long wallStart = 0;
long threadStart = 0;

// 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);
wallStart = SystemClock.currentTimeMicro();
threadStart = SystemClock.currentThreadTimeMicro();
}

msg.target.dispatchMessage(msg);

if (logging != null) {
long wallTime = SystemClock.currentTimeMicro() - wallStart;
long threadTime = SystemClock.currentThreadTimeMicro() - threadStart;

logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
if (logging instanceof Profiler) {
((Profiler) logging).profile(msg, wallStart, wallTime,
threadStart, threadTime);
}
}

// 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();
}
}
}

可以看到，在loop中会进入一个死循环，每次循环的开始调用MessageQueu.next()方法从消息队列中取出一个Message，然后调用 msg.target.dispatchMessage(msg)交给message的接受者处理。这里的msg.target就是message的接受者，其实就是一个Handler。如果消息队列为空，那么MessageQueue .next()会阻塞，直到有新的消息达到。到这里，就可以明白调用Looper.loop后，线程就进入了消息循环等待message到来并交给message对应的Handler来处理消息。正常情况下，一旦进入消息循环就不会退出，除非MessageMessage.target=null，见第16行代码。

到此为止，我们就知道如果要给线程创建一个消息循环，可以按如下方式来定义：

class LooperThread extends Thread {

public void run() {
Looper.prepare();

......

Looper.loop();
}
}

上面提到在Looper中会通过msg.target.dispatchMessage(msg)将msg分派到对应的Handler来出来，这里看一下dispatchMessage的实现：

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

在dispatchMessage中先检查在Message中是否定义了消息的callback，如果有，优先调用Message的callback来处理消息；反之，如果Handler的mCallBack不为null，则调用mCallBack的handleMessage来处理，当MCallBack也为null的时候，才调用Handdler的handleMessage来处理。这里可以看出,Message有三种方式来实现它的处理逻辑：

1)在Message.callback中实现，这里的callback是一个Runnable对象；

2)在Handler的内部接口CallBack中实现；

3)重写Handler的handleMessage来实现。

阅读Handler的源码，可以发现当调用handler.post(Runnable)的时候，就是通过第1种方式来实现：

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

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

通常我们会通过第3种方式来定义自己的Handler，如在主线程：

Handler mHandler=new Handler(){

@Override
public void handleMessage(Message msg) {
// TODO Auto-generated method stub
super.handleMessage(msg);
......//你的处理逻辑
}
};

这里看一下Handler的构造函数（还有其他几种构造函数）：

public Handler() {
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 = null;
}

这里可以看到，创建Handler的时候，会在保存当前线程的Looper及Looper对应的MessageQueue。这样发送消息时Handler就能将Message发送到当前线程的消息循环中。当然，我们也可以指定Looper，这样就可以给另一个线程发送消息，如在另一个线程中给主线程发送消息。这里也可以看出，定义Handler之前一定要先调用Looper.prepare()来创建一个Looper,主线程中会在ActivityThread的main方法中创建，不需要我们手动调用。但是在子线程中，一定要先调用Looper.prepare()才能创建 Handler,否则会抛出一个异常，如第78行所示。

Handler常见的用法之一就是在子线程中更新UI，我们可以通过handler.post(Runnable)让runnable在主线程中执行。其中Activity的runOnUIThread就是利用handler来实现的，view.post(Runnable)也是相似的实现。

public final void runOnUiThread(Runnable action) {
if (Thread.currentThread() != mUiThread) {
mHandler.post(action);
} else {
action.run();
}
}

当调用handler.sendMessage()发送消息时，最终会调用Handler.sendMessageAtTime,源码如下：

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

在这里可以看出，发送消息的时候，将Message添加到队列中，并且会将Message的接受者设置为发送Message的handler，如代码第6行所示。前面有提到，在loop中会调用MessageQueue.next()来取消息。这里先看一下消息的入队操作：

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;
//Log.d("MessageQueue", "Enqueing: " + msg);
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;
}

看代码的第29到35行代码，这里会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。在这里也可以发现，主线程的消息循环是不能退出的，如第6行所示。

前面有提到MessageQueue.next()可能会阻塞。

这里一起看一下消息的出队操作：

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;
}
}

在第3行，定义变量nextPollTimeoutMills,用来描述当消息队列中没有新的消息时，当前线程需要睡眠多长时间。当nextPollTimeoutMills等于-1时，表示无限制等待，直到有新的消息到达。在没有消息到达的时候会调用nativePollOnce来检查是否有新消息需要处理，nativePollOnce是一个native函数，主要是通过Linux中的poll函数来实现的。

当队列不为空时，在第17行先判断当前时间是否大于等于message的处理时间，如果是，则返回该message，反之，计算需要等待的时间，并更新nextPollTimeoutMills。

如果队列为空，则nextPollTimeoutMills的值设置为-1。在进入睡眠之前，先调用注册到消息队列中的空闲消息处理器IdleHandler对象的queueIdle函数，以便有机会在线程空闲时执行一些操作。可以通过MessageQueue的addIdleHandler和removeIdleHandler来注册和注销一个空闲消息处理器。

到此为止，已经分析完消息循环的创建，发送消息，以及消息的处理。