Handler和Message是Android开发者常用的两个API，我一直对于它的内部实现比较好奇，所以用空闲的时间，阅读了一下他们的源码。
相关的Java Class：
- android.os.Message
- android.os.MessageQueue
- android.os.Looper
- android.os.Handler
相关的C++ Class：
- android.NativeMessageQueue
- android.Looper
- android.LooperCallback
- android.SimpleLooperCallback
- android.Message
- android.MessageHandler
首先，来看看这些类之间的关系：
首先，让我们从相对简单的java实现开始看起：
Message
Message类可以说是最简单的，主要提供了一些成员，用以保存消息数据。
public int what;//用以表示消息类别
public int arg1;//消息数据
public int arg2;//消息数据
public Object obj;//消息数据
/*package*/ long when;//消息应该被处理的时间
/*package*/ Bundle data;//消息数据
/*package*/ Handler target;//处理这个消息的handler
/*package*/ Runnable callback;//回调函数
// sometimes we store linked lists of these things
/*package*/ Message next;//形成链表，保存Message实例
值得一提的是，Android提供了一个简单，但是有用的消息池，对于Message这种使用频繁的类型，可以有效的减少内存申请和释放的次数，提高性能。
private static final Object sPoolSync = new Object();
private static Message sPool;
private static int sPoolSize = 0;
private static final int MAX_POOL_SIZE = 50;
/**
* Return a new Message instance from the global pool. Allows us to
* avoid allocating new objects in many cases.
*/
public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {//消息池不为空，则从消息池中获取实例
Message m = sPool;
sPool = m.next;
m.next = null;
sPoolSize--;
return m;
}
}
return new Message();
}
/**
* Return a Message instance to the global pool. You MUST NOT touch
* the Message after calling this function -- it has effectively been
* freed.
*/
public void recycle() {
clearForRecycle();
synchronized (sPoolSync) {
if (sPoolSize < MAX_POOL_SIZE) {//消息池大小未满，则放入消息池
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
/*package*/ void clearForRecycle() {
flags = 0;
what = 0;
arg1 = 0;
arg2 = 0;
obj = null;
replyTo = null;
when = 0;
target = null;
callback = null;
data = null;
}
小结：
- Message的核心在于它的数据域，Handler根据这些内容来识别和处理消息
- 应该使用Message.obtain（或者Handler.obtainMessage)函数获取message实例
Handler
首先看看构造函数：
public interface Callback {
public boolean handleMessage(Message msg);
}
public Handler() {
this(null, false);
}
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; //使用Callback可以拦截Handler处理消息，之后会在dispatchMessage函数中，大展身手
mAsynchronous = async;//设置handler的消息为异步消息，暂时先无视这个变量
}
Handler的构造函数最主要的就是初始化成员变量：mLooper和mQueue。 这边需要注意的一个问题是：Looper.myLooper（）不能返回null，否则抛出RuntimeExeception。稍后详解Looper.myLooper();函数在何种情况下会抛出异常。
Handler.obtainMessage系列的函数都会调用Message类中对应的静态方法，从消息池中获取一个可用的消息实例。典型实现如下：
public final Message obtainMessage()
{
return Message.obtain(this);
}
Handler.post系列和send系列函数最终都会调用enqueueMessage函数，把message入列，不同之处在于post系列函数会以Runable参数构建一个Message实例。
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;//一会我们会看到callback非空的message和callback为空的mesage在处理时的差异
return m;
}
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
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);
}
//最终都会调用这个函数，把message入列
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);//Handler的mAsynchronous属性，决定了msg是否为asynchronous，稍后在MessageQueue.next函数中，可以看到asynchronous对于消息处理的影响 }
return queue.enqueueMessage(msg, uptimeMillis);
}
除了这些之外，Handler还提供了hasMessage系列和removeMessages系列函数用以管理Handler对应的MessageQueue中的消息。
接下来主角登场，Handler.dispatchMessage:
private static void handleCallback(Message message) {
message.callback.run();
}
/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {//message的callback不为null，则执行
handleCallback(msg);
} else {
if (mCallback != null) {//如果Hanlder的mCallback成员不为null，则调用
if (mCallback.handleMessage(msg)) {//如果handleMessage返回值为true，则拦截消息
return;
}
}
handleMessage(msg);//处理消息
}
}
注释应该比较清楚，不多说。 小结：
- Handler类最为核心的函数是enqueueMessage和dispatcherMessage,前者把待处理的消息放入MessageQueue，而Looper调用后者来处理从MessageQueue获取的消息。
- callback不为null（通过post系列函数添加到消息队列中）的message无法被拦截，而callback为null的函数可以被Handler的mCallback拦截
Looper
同样从构造函数看起：
好了，Looper的实例是构造出来，但是如何获取构造出来的实例呢？
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);//每个Looper有一个MessageQueue
mRun = true;
mThread = Thread.currentThread();
}
** Initialize the current thread as a looper.
* This gives you a chance to create handlers that then reference
* this looper, before actually starting the loop. Be sure to call
* {@link #loop()} after calling this method, and end it by calling
* {@link #quit()}.
*/
public static void prepare() {
prepare(true);//后台线程的looper都允许退出
}
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");//每个线程只能有一个Looper
}
sThreadLocal.set(new Looper(quitAllowed));//把实例保存到TLS（Thread Local Save），仅有每个线程访问自己的Looper
}
/**
* 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);//主线程的lopper不可以退出
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
因为是私有的构造函数，所以理论上来说只能通过prepare和prepareMainLooper两个函数来实例化Looper，但是google的注释也说的很清楚：prepareMainLooper（）应该由系统调用（有兴趣的同学可以去看看AtivityThread类的main函数），所以，应用开发者可以使用的只剩下prepare函数。 好了，Looper的实例是构造出来，但是如何获取构造出来的实例呢？
/** Returns the application's main looper, which lives in the main thread of the application.
*/
public static Looper getMainLooper() {
synchronized (Looper.class) {
return sMainLooper;
}
}
/**
* 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();
}
现在，我们应该知道如何防止Handler实例化的时候，抛出RuntimeException：在守护线程中实例化Handler之前，需要先调用Looper.perpare函数来构造Looper实例。
然后，重头戏来了：
/**
* Quits the looper.
*
* Causes the {@link #loop} method to terminate as soon as possible.
*/
public void quit() {
mQueue.quit();
}
/**
* 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) {//调用looper之前，需要先调用perpare，否则您懂的...
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();//不太明白这个函数，但不是重点可以无视
final long ident = Binder.clearCallingIdentity();
for (;;) {
Message msg = queue.next(); // might block 获取一个下一个消息，如果当前没有要处理的消息，则block，之后我们会看到这个API的实现
if (msg == null) {//调用了MessgeQueu的quit函数后，MessageQueue.next会返回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) {//借助logging我们可以打印Looper中处理的消息
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
msg.target.dispatchMessage(msg);//调用handler处理消息
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();//选择性无视
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();//回收消息到消息池
}
}
Looper.loop()函数是Looper类的核心函数，主要循环进行两个操作：- 从MessageQueue中获取一个消息，当前没有消息需要处理时，则block
- 调用message的Handler（target）处理消息
基本上，我们可以把Looper理解为一个死循环，Looper开始work以后，线程就进入了以消息为驱动的工作模型。
小结：
- 每个线程最多可以有一个Looper。
- 每个Looper有且仅有一个MessageQueue
- 每个Handler关联一个MessageQueue，由该MessageQueue关联的Looper执行（调用Hanlder.dispatchMessage)
- 每个MessageQueue可以关联任意多个Handler
- Looper API的调用顺序：Looper.prepare >> Looper.loop >> Looper.quit
- Looper的核心函数是Looper.loop，一般loop不会返回，直到线程退出，所以需要线程完成某个work时，请发送消息给Message（或者说Handler）
MessageQueue
MessageQueue类是唯一包含native函数的类，我们先大致看一下，稍后C++的部分在详细解释:
private native void nativeInit(); //初始化
private native void nativeDestroy(); //销毁
private native void nativePollOnce(int ptr, int timeoutMillis); //等待timeoutMillis指定的时间
private native void nativeWake(int ptr);//唤醒nativePollOnce的等待
Message mMessages;//数据域mMessages的类型虽然是Message，但是因为Message.next数据域的原因，其实mMessage是链表的第一个元素
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
nativeInit();//初始化nativeMessageQueue
}
对应的，在销毁的时候： @Override
protected void finalize() throws Throwable {
try {
nativeDestroy();//销毁nativeMessageQueue
} finally {
super.finalize();
}
}
此外，MessageQueue提供了一组函数（e.g. hasMessage, removeMessage)来查询和移除待处理的消息，我们在前面的Handler类上看到的对应函数的实现就是调用这组函数。
接下来，看看enqueueMessage函数，Handler函数就是调用这个函数把message放到MessageQueue中：
final boolean enqueueMessage(Message msg, long when) {
if (msg.isInUse()) {//检查msg是否在使用中，一会我们可以看到MessageQueue.next()在返回前通过Message.makeInUse函数设置msg为使用状态，而我们之前看到过Looper.loop中通过调用调用Message.recycle()，把Message重置为未使用的状态。
throw new AndroidRuntimeException(msg + " This message is already in use.");
}
if (msg.target == null) {//msg必须知道由那个Handler负责处理它
throw new AndroidRuntimeException("Message must have a target.");
}
boolean needWake;
synchronized (this) {
if (mQuiting) {//如果已经调用MessageQueue.quit，那么不再接收新的Message
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;
if (p == null || when == 0 || when < p.when) {//插到列表头
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;//当前MessageQueue处于block状态，所以需要唤醒
} 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();//当且仅当MessageQueue因为Sync Barrier而block，并且msg为异步消息时，唤醒。 关于msg.isAsyncChronous(),请回去看看Handler.enqueueMessage函数和构造函数
Message prev;
for (;;) {// 根据when的大小顺序，插入到合适的位置
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {//如果在插入位置以前，发现异步消息，则不需要唤醒
needWake = false;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
}
if (needWake) {
nativeWake(mPtr);//唤醒nativeMessageQueue
}
return true;
}
final void quit() {
if (!mQuitAllowed) {//UI线程的Looper消息队列不可退出
throw new RuntimeException("Main thread not allowed to quit.");
}
synchronized (this) {
if (mQuiting) {
return;
}
mQuiting = true;
}
nativeWake(mPtr);//唤醒nativeMessageQueue
}
关于sync barrier，再补充点解释： sync barrier是起到了一个阻塞器的作用，它可以阻塞when>它（即执行时间比它晚）的同步消息的执行，但不影响异步消息。sync barrier的特征是targe为null，所以它只能被remove，无法被执行。MessageQueue提供了下面两个函数来控制MessageQueue中的sync barrier（如何觉得sync barrier和异步消息难以理解的话，选择性无视就好，因为它们不妨碍我们理解Android消息机制的原理）： final int enqueueSyncBarrier(long when) {
// Enqueue a new sync barrier token.
// We don't need to wake the queue because the purpose of a barrier is to stall it.
synchronized (this) {
final int token = mNextBarrierToken++;
final Message msg = Message.obtain();
msg.arg1 = token;
Message prev = null;
Message p = mMessages;
if (when != 0) {
while (p != null && p.when <= when) {
prev = p;
p = p.next;
}
}
if (prev != null) { // invariant: p == prev.next
msg.next = p;
prev.next = msg;
} else {
msg.next = p;
mMessages = msg;
}
return token;
}
}
final void removeSyncBarrier(int token) {
// Remove a sync barrier token from the queue.
// If the queue is no longer stalled by a barrier then wake it.
final boolean needWake;
synchronized (this) {
Message prev = null;
Message p = mMessages;
while (p != null && (p.target != null || p.arg1 != token)) {
prev = p;
p = p.next;
}
if (p == null) {
throw new IllegalStateException("The specified message queue synchronization "
+ " barrier token has not been posted or has already been removed.");
}
if (prev != null) {
prev.next = p.next;
needWake = false;
} else {
mMessages = p.next;
needWake = mMessages == null || mMessages.target != null;//其实我觉得这边应该是needWake = mMessages != null && mMessages.target != null
}
p.recycle();
}
if (needWake) {
nativeWake(mPtr);//有需要的话，唤醒nativeMessageQueue
}
}
重头戏又来了：
final Message next() {
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();//不太理解，选择性无视
}
nativePollOnce(mPtr, nextPollTimeoutMillis);//等待nativeMessageQueue返回，最多等待nextPollTimeoutMillis毫秒
synchronized (this) {
if (mQuiting) {//如果要退出，则返回null
return null;
}
// 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) {//下一个消息为sync barrier
// Stalled by a barrier. Find the next asynchronous message in the queue.
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());//因为存在sync barrier，仅有异步消息可以执行，所以寻在最近的异步消息
}
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);//消息还没到执行的时间，所以我们继续等待msg.when - now毫秒
} else {
// Got a message.
mBlocked = false;//开始处理消息了，所以不再是blocked状态
if (prevMsg != null) {
prevMsg.next = msg.next;//从链表中间移除message
} else {
mMessages = msg.next;//从链表头移除message
}
msg.next = null;
if (false) Log.v("MessageQueue", "Returning message: " + msg);
msg.markInUse();//标记msg正在使用
return msg;//返回到Looper.loop函数
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;//没有消息可以处理，所以无限制的等待
}
// 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)) {// 目前无消息可以处理，可以执行IdleHandler
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;//Looper.looper调用一次MessageQueue.next(),只允许调用一轮IdleHandler
// 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;//因为执行IdleHandler的过程中，可能有新的消息到来，所以把等待时间设置为0
}
}
为了方便大家理解Message的工作原理，先简单描述nativeWake，和natePollonce的作用：- nativePollOnce(mPtr, nextPollTimeoutMillis);暂时无视mPtr参数，阻塞等待nextPollTimeoutMillis毫秒的时间返回，与Object.wait(long timeout)相似
- nativeWake(mPtr)；暂时无视mPtr参数，唤醒等待的nativePollOnce函数返回的线程，从这个角度解释nativePollOnce函数应该是最多等待nextPollTimeoutMillis毫秒
小结：
- MessageQueue作为一个容器，保存了所有待执行的消息。
- MessageQueue中的Message包含三种类型：普通的同步消息，Sync barrier（target = null），异步消息（isAsynchronous（） = true）。
- MessageQueue的核心函数为enqueueMessage和next,前者用于向容器内添加Message，而Looper通过后者从MessageQueue中获取消息，并实现无消息情况下的等待。
- MessageQueue把Android消息机制的Java实现和C++实现联系起来。
本来我是想一口气把java实现和C++实现都写完的，但是，无奈最近工作和个人事务都比较多，稍后为大家奉上C++实现的解析。