Android基于源码分析Handler的消息机制

Handler介绍

    handler是Android系统中提供的一种异步回调机制，通过使用handler，我们可以完成一些耗时的任务，然后通知相关的页面做出相应的改变。在我们工作的场合中，大部分开发者会认为handler就是用来更新UI的，这样说没错，然而这只是从我们开发的场景去看待handler，其实Android底层在进行跨进程通信中（ActivityManagerService或WindowManagerService），也同样大量的通过使用handler来执行相关的操作。好，废话不多说，今天，我主要是从源码的角度来讲解handler的运行机制（基于Android4.4源码）。

Message、Handler、MessageQueue、Looper和 Thread的介绍

    Message：熟称消息，这其中包含了消息ID，消息处理对象以及处理的数据，是一个消息的最小单元。由MessageQueue统一管理，最终由handler处理。

    Handler：处理者，负责message的发送和处理。

    MessageQueue：消息队列，用来存放handler发送过来的消息，并将message以单链表的形式串联保存起来，并等待looper抽取。

    Looper：消息循环者，不断的从MessageQueue中拉取message来执行，一个Looper对应一个MessageQueue，但是可以对应多个handler。

    Thread：线程，负责调度整个消息循环，也就是消息循环执行的场所。

Android消息机制原理基于源码介绍

    介绍了这些基本的概念后，下面我们言归正传：通常，咱们在使用handler的时候先会声明并创建一个handler，并重写handleMessage，方法如下所示：

private Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
switch (msg.what) {
case 0:
break;
case 1:
break;
case 2:
break;
default:
break;
}
}
};

在使用handler发送消息的时候代码如下：

Message msg = Message.obtain();
msg.what = 0;
msg.obj = mData;
mHandler.sendMessage(msg);

代码里面的msg.what = 0和msg.obj = mData这里只是举例子哈，是这个意思大家能理解就行。当我们按住ctrl+鼠标左击sendMessage(msg)这个方法的时候，就会顺利得进入到今天要讲的主题啦：

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

代码在handler类的501行（下面的我就都以我的源码行数为准了，再重申一下我看的是Android4.4的源码），这个类没什么，我们直接往下看：

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

在handler类的561行，这个方法里面就是赋值了一个long行的参数，没什么，我们接着看sendMessageDelayed(Message msg, long delayMillis)这个方法：

public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;

f557
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);
}

在handler类的587行，这里此方法将mQueue赋值给了局部变量queue，下面的也什么难理解的，那，这个mQueue哪来的呢，我们找找：

final MessageQueue mQueue;
final Looper mLooper;
final Callback mCallback;
final boolean mAsynchronous;
IMessenger mMessenger;

从handler类的736行开始，声明了这个几个成员变量，但这只是声明，没有赋值，好，咱们接着看：

/**
* Default constructor associates this handler with the {@link Looper} for the
* current thread.
*
* If this thread does not have a looper, this handler won't be able to receive messages
* so an exception is thrown.
*/
public Handler() {
this(null, false);
}

/**
* Constructor associates this handler with the {@link Looper} for the
* current thread and takes a callback interface in which you can handle
* messages.
*
* If this thread does not have a looper, this handler won't be able to receive messages
* so an exception is thrown.
*
* @param callback The callback interface in which to handle messages, or null.
*/
public Handler(Callback callback) {
this(callback, false);
}

/**
* Use the provided {@link Looper} instead of the default one.
*
* @param looper The looper, must not be null.
*/
public Handler(Looper looper) {
this(looper, null, false);
}

/**
* Use the provided {@link Looper} instead of the default one and take a callback
* interface in which to handle messages.
*
* @param looper The looper, must not be null.
* @param callback The callback interface in which to handle messages, or null.
*/
public Handler(Looper looper, Callback callback) {
this(looper, callback, false);
}

/**
* Use the {@link Looper} for the current thread
* and set whether the handler should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with represent to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @hide
*/
public Handler(boolean async) {
this(null, async);
}

/**
* Use the {@link Looper} for the current thread with the specified callback interface
* and set whether the handler should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with represent to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param callback The callback interface in which to handle messages, or null.
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @hide
*/
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handler类的106行，我特意复制了一堆的方法，这里也不再熬述Java语法，我们只管看到188行的public Handler(Callback callback, boolean async)这个方法，大意就是我们创建handler后进入到构造方法，然后来到这个方法里面，这里面就有我们需要找的mQueue，我们看到，mQueue = mLooper.mQueue，这行，原来mQueue是从Looper这个类里面得到的，mLooper = Looper.myLooper()，至于为什么直接就从Looper.myLooper()得到呢：

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());

Process.setArgV0("<pre-initialized>");

Looper.prepareMainLooper();

ActivityThread thread = new ActivityThread();
thread.attach(false);

if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}

AsyncTask.init();

if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}

Looper.loop();

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

这段代码是ActivityThread里面的main()，也就是咱们在启动Activity的时候会执行到这个方法里头，多的也不看，这里面有Looper.prepareMainLooper()这行代码，好，我们看看Looper中的这段代码：

public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}

/**
* Return the Looper object associated with the current thread.  Returns
* null if the calling thread is not associated with a Looper.
*/
public static Looper myLooper() {
return sThreadLocal.get();
}

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

prepareMainLooper()在Looper类的88页，myLooper()在Looper类的161页。实质上主线程里面就直接创建好的looper，Looper中有个构造函数，构造函数里面创建了MessageQueue对象，所以一个Looper对象对应一个MessageQueue对象来源与此。这也是为什么我们在UI线程中直接用handler不需要创建Looper的原因。上面AcrivityThread中有个Looper.loop()方法，接着看Looper类的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();

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();
}
}

在looper类的110行。看重点，里面有个for (;;)的循环，这里暂时不对里面的逻辑分析，从这里就知道了looper创建好了之后，就一直不停的执行了。接着回到handler类的587行

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);
}

mQueue知道怎么来的了，Looper也知道怎么来的了，然后再查看enqueueMessage(queue, msg, uptimeMillis)方法吧：

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

在handler类的621行。这里有个很重要的细节，就是msg.target = this，也就是说把当前handler对象传递给了msg.target，我们看看msg.target是什么：

/*package*/ Handler target;

这是Message类第89行的代码，这里是把msg的成员变量target赋值为当前Message坐在的Handler。接着看return queue.enqueueMessage(msg, uptimeMillis)这行，这里就到了MessageQueue类里面了：

boolean enqueueMessage(Message msg, long when) {
if (msg.isInUse()) {
throw new AndroidRuntimeException(msg + " This message is already in use.");
}
if (msg.target == null) {
throw new AndroidRuntimeException("Message must have a target.");
}

synchronized (this) {
if (mQuitting) {
RuntimeException e = new RuntimeException(
msg.target + " sending message to a Handler on a dead thread");
Log.w("MessageQueue", e.getMessage(), e);
return false;
}

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;
}

也是看重点，这个方法下面也有个无限循环，就是说不停的往当前的MessageQueue中插入message，这里采用的是单链表结构将message对象插入到消息队列：prev = p;p = p.next;msg.next = p;prev.next = msg;这几行代码对于有过数据结构知识的同学来说应该不难理解，仔细琢磨下不难。说到这里的时候，我们回顾下上面所说的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();

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();
}
}

再贴一遍吧，这里来详细分析下：Looper me = myLooper()，final MessageQueue queue = me.mQueue这两行代码，前面说了looper创建的同时也创建了MessageQueue对象，即一个Looper中对应有一个MessageQueue对象。这里直接获得到Looper中的MessageQueue所以在Looper的loop()方法中这个looper通过MessageQueue不断的拉取消息队列中的消息。执行Message msg = queue.next()这行代码，我们再看看MessageQueue中：

Message next() {
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}

// We can assume mPtr != 0 because the loop is obviously still running.
// The looper will not call this method after the loop quits.
nativePollOnce(mPtr, 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);
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 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;
}
}

这是MessageQueue类的128行，大意就是从单链表中不断的获取message，然后返回这个message。到这里，实质上就是说Looper在主线程里面创建好了后，开启无限循环模式，不停得从MessageQueue中拉取message，当有handler发送消息时，就通过MessageQueue中的 enqueueMessage(Message msg, long when)此方法以单链表的形式存储了message，这个时候无限循环的looper中的 Looper.loop()方法中Message
msg = queue.next()的msg取到之后就不为空 ，然后执行到loop()中的这一行：

msg.target.dispatchMessage(msg);

前面说了target就是handler对象，所以当looper从MessageQueue中获取到一个message后，在通过message的handler去把这个message分发处理：

/**
* 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);
}
}

handler类的第93页，上面的if和else我们不用管，那是基于handleCallback处理的，这里我们没有创建相关Callback类的话，就直接看下面的handleMessage(msg)这行：

/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}

在handler类的第84页，从源码中可以得出，这个类实际上需要在我们自己创建handler的类中实现，还记得博客开头咱们自己new的handler么，里面的确有这个重写的方法。相当于在loop()中获取到MessageQueue中的message后，msg.target.dispatchMessage(msg)其实分发执行到handler的handleMessage()方法里面，而这个方法在应用自己创建的页面实现，所以整个handler的运行机制到这就分析完了。