Android消息机制不完全解析（上）

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

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：

    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

然后，我们再从构造函数看起：

        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。

Message

Handler

Looper

MessageQueue

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