之前写过一篇文章，概述了Android应用程序消息处理机制。本文在此文基础上，在源码级别上展开进行概述

简单用例

Handler的使用方法如下所示：

Handler myHandler = new Handler() {            public void handleMessage(Message msg) {                  switch (msg.what) {                       ...                 }          }        };class myThread implements Runnable {             public void run() {                 while (!Thread.currentThread().isInterrupted()) {                                               Message message = Message.obtain();                       message.what = TestHandler.GUIUPDATEIDENTIFIER;                    TestHandler.this.myHandler.sendMessage(message);                       message.recycle();                    try {                            Thread.sleep(100);                        } catch (InterruptedException e) {                            Thread.currentThread().interrupt();                       }                  }             }        }

或者:

mHandler=new Handler();mHandler.post(new Runnable(){    void run(){       ...     }});

又或者：

class LooperThread extends Thread {    public Handler mHandler;    public void run() {        Looper.prepare();        mHandler = new Handler() {            public void handleMessage(Message msg) {                // process incoming messages here            }        };        Looper.loop();    }}

源码解析

首先看其构造函数：

new Handler()...public Handler() {    this(null, false);}...public Handler(Looper looper, Callback callback) {    this(looper, callback, false);}...public Handler(Callback callback, boolean async) {    if (FIND_POTENTIAL_LEAKS) { // 默认为false，若为true则会检测当前handler是否是静态类        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());        }    }    // 1. 获得当前线程的looper    mLooper = Looper.myLooper();    if (mLooper == null) {        throw new RuntimeException(            "Can't create handler inside thread that has not called Looper.prepare()");    }    //2. 获得looper上的message queue    mQueue = mLooper.mQueue;    mCallback = callback;    mAsynchronous = async;}

由此引入了两个关键对象Looper和MessageQueue。

先看 mLooper = Looper.myLooper(); 这一句发生了什么：

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

可以看到，该方法返回一个sThreadLocal对象中保存的Looper。关于ThreadLocal类，请参考这里，本文不展开。
如果尚未在当前线程上运行过Looper.prepare()的话，myLooper会返回null。接下来看看Looper.prepare()的实现：

public static void prepare() {    prepare(true);}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));}

可以看到该方法只是简单地新建了一个Looper对象，并将其保存在sThreadLocal中。接下来看一下Looper的构造函数。

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

调用完Looper.prepare()后，需调用Looper.loop()才能使消息循环运作起来，其源码如下所示：

public static void loop() {    final Looper me = myLooper(); //1. 取出looper对象    if (me == null) {        throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");    }    final MessageQueue queue = me.mQueue; //2. 取出looper绑定的message queue    // 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();    // loop time.    long tm = 0;    ...    for (;;) {        Message msg = queue.next(); // 3. 堵塞式在message queue中取数据        if (msg == null) {            // No message indicates that the message queue is quitting.            return;        }        ...        msg.target.dispatchMessage(msg); 4. 分发message到指定的target handler        ...        // Make sure that during the course of dispatching the        // identity of the thread wasn't corrupted.        final long newIdent = Binder.clearCallingIdentity();        if (ident != newIdent) {            ...        }        msg.recycleUnchecked(); // 5. 回收message对象        ...    }}

可以简单地将Looper.loop()理解成一个不断检测message queue是否有数据，若有即取出并执行回调的死循环。接下来看一下Message类：

public final class Message implements Parcelable {    public int what;    public int arg1;    public int arg2;    public Object obj;    ...    /*package*/ int flags;    /*package*/ long when;    /*package*/ Bundle data;    /*package*/ Handler target;    /*package*/ Runnable callback;    /*package*/ Message next;    private static final Object sPoolSync = new Object();    private static Message sPool;    private static int sPoolSize = 0;}

what、arg1、arg2这些属性本文不作介绍，我们把目光集中在next、sPoolSync、sPool、sPoolSize这四个静态属性上。

当我们调用Message.obtain()时，返回了一个Message对象。Message对象使用完毕后，调用recycle()方法将其回收。其中obtain方法的代码如下所示：

public static Message obtain() {    synchronized (sPoolSync) {        if (sPool != null) {            Message m = sPool;            sPool = m.next;            m.next = null;            m.flags = 0; // clear in-use flag            sPoolSize--;            return m;        }    }    return new Message();}

可以看到，obtain方法被调用时，首先检测sPool对象是否为空，若否则将其当做新的message对象返回，并“指向"message对象的next属性，sPoolSize自减。可以看出message对象通过next属性串成了一个链表，sPool为“头指针”。再来看看recycle方法的实现：

public void recycle() {    if (isInUse()) {        if (gCheckRecycle) {            throw new IllegalStateException("This message cannot be recycled because it is still in use.");        }        return;    }    recycleUnchecked();}void recycleUnchecked() {    // Mark the message as in use while it remains in the recycled object pool.    // Clear out all other details.    flags = FLAG_IN_USE;    what = 0;    arg1 = 0;    arg2 = 0;    obj = null;    replyTo = null;    sendingUid = -1;    when = 0;    target = null;    callback = null;    data = null;    synchronized (sPoolSync) {        if (sPoolSize < MAX_POOL_SIZE) {            next = sPool;            sPool = this;            sPoolSize++;        }    }}

如果message对象不是处于正在被使用的状态，则会被回收。其属性全部恢复到原始状态后，放在了链表的头部。sPool对象“指向”它，sPoolSize自增。

综上可以看出，通过obtain和recycle方法可以重用message对象。通过操作next、sPoolSync、sPool、sPoolSize这四个属性，实现了一个类似栈的对象池。

msg.target为handler类型，即向handler成员的dispatchMessage方法传入msg参数，其实现如下所示：

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

这里可以看到回调了各种接口。

到目前为止，我们知道了如何处理在消息队列里面的msg对象，但仍不知道msg对象是如何放到消息队列里面的。通常来说，我们通过Handler的sendMessage(msg)方法来发送消息，其源码如下所示：

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

可知sendMessage最终会调用queue.enqueueMessage(msg, uptimeMillis)将msg对象保存至message queue中，uptimeMillis表示msg执行回调的时刻。我们来看一下MessageQueue类的enqueueMessage方法：

boolean enqueueMessage(Message msg, long when) {    if (msg.target == null) {        throw new IllegalArgumentException("Message must have a target.");    }    if (msg.isInUse()) {        throw new IllegalStateException(msg + " This message is already in use.");    }    synchronized (this) {        if (mQuitting) {            IllegalStateException e = new IllegalStateException(                    msg.target + " sending message to a Handler on a dead thread");            Log.w("MessageQueue", e.getMessage(), e);            msg.recycle();            return false;        }        // 1.设置当前msg的状态        msg.markInUse();        msg.when = when;        Message p = mMessages;        boolean needWake;        // 2.检测当前头指针是否为空（队列为空）或者没有设置when 或者设置的when比头指针的when要前        if (p == null || when == 0 || when < p.when) {            // 3. 插入队列头部，并且唤醒线程处理msg            msg.next = p;            mMessages = msg;            needWake = mBlocked;        } else {                        //4. 几种情况要唤醒线程处理消息：1）队列是堵塞的 2)barrier，头部结点无target 3）当前msg是堵塞的            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;                }            }            // 5. 将当前msg插入第一个比其when值大的结点前。            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;}

结合注释，我们可以了解到msg push到queue中时，queue的状态的变化和处理队列的逻辑。

前文中Looper对象的loop方法中：

for (;;) {    ...    Message msg = queue.next(); // 3. 堵塞式在message queue中取数据    if (msg == null) {        // No message indicates that the message queue is quitting.        return;    }    ...    msg.target.dispatchMessage(msg); 4. 分发message到指定的target handler        ...}

可以看出，message queue的next方法被调用时，可能会发生堵塞。我们来看一看message queue的next方法：

Message next() {    // 1. 判断当前loop是否已经使用过，下文会解释这个mPtr    final long ptr = mPtr;    if (ptr == 0) {        return null;    }    int pendingIdleHandlerCount = -1; // -1 only during first iteration    int nextPollTimeoutMillis = 0;        // 2. 进入死循环，直到获取到合法的msg对象为止。    for (;;) {        if (nextPollTimeoutMillis != 0) {            Binder.flushPendingCommands(); // 这个是什么？        }                // 3. 进入等待，nextPollTimeoutMillis为等待超时值        nativePollOnce(ptr, nextPollTimeoutMillis);        synchronized (this) {            // 4. 获取下一个msg            final long now = SystemClock.uptimeMillis();            Message prevMsg = null;            Message msg = mMessages;            if (msg != null && msg.target == null) {                                // 当前节点为barrier，所以要找到第一个asynchronous节点                do {                    prevMsg = msg;                    msg = msg.next;                } while (msg != null && !msg.isAsynchronous());            }            if (msg != null) {                if (now < msg.when) {                                        // 当前队列里最早的节点比当前时间还要晚，所以要进入堵塞状态，超时值为nextPollTimeoutMillis                    nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);                } else {                    // 删除当前节点，并返回                    mBlocked = false;                    if (prevMsg != null) {                        prevMsg.next = msg.next;                    } else {                        mMessages = msg.next;                    }                    msg.next = null;                    if (false) Log.v("MessageQueue", "Returning message: " + msg);                    return msg;                }            } else {                // 头结点指向null                nextPollTimeoutMillis = -1;            }            // Process the quit message now that all pending messages have been handled.            if (mQuitting) {                dispose();                return null;            }            // 5. 如果当前状态为idle（就绪），则进入idle handle的代码块            //    进入idle的情况有：队列为空；队列头元素blocking;            if (pendingIdleHandlerCount < 0                    && (mMessages == null || now < mMessages.when)) {                pendingIdleHandlerCount = mIdleHandlers.size();            }            if (pendingIdleHandlerCount <= 0) {                // 6. 本轮唤醒（next被调用）时没处理任何东西，故再次进入等待。                mBlocked = true;                continue;            }            if (mPendingIdleHandlers == null) {                mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];            }            mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);        }        // 在一次next调用中，这个代码块只会执行一次        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 {                // 如果返回true，则idler被保留，下次next的idle时会被调用。                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;    }}

代码执行流程见注释。其中IdleHandler是一个接口：

public static interface IdleHandler {    boolean queueIdle();}

IdleHandler提供了一个在MessageQueue进入idle时的一个hook point。更多时与barrier机制一起使用，使message queue遇到barrier时产生一个回调。

总结

前面涉及到的几个主要的类Handler、Looper、MessageQueue和Message的关系如下所述：

Handler负责将Looper绑定到线程，初始化Looper和提供对外API。
Looper负责消息循环和操作MessageQueue对象。
MessageQueue实现了一个堵塞队列。
Message是一次业务中所有参数的载体。

框架图如下所示：

            +------------------+            |      Handler     |            +----+--------^----+                 |        |           send  |        |  dispatch                 |        |                 v        |                +----- <---+                |          |                |  Looper  |                |          |                |          |                +---> -----+                  |      ^           enqueue|      | next                  |      |         +--------v------+----------+         |       MessageQueue       |         +--------+------^----------+                  |      |  nativePollOnce  |      |   nativeWake                  |      |+-----------------v------+---------------------+                Lower Layer

最后，留意到MessageQueue中有4个native方法：

// 初始化和销毁private native static long nativeInit();private native static void nativeDestroy(long ptr);// 等待和唤醒private native static void nativePollOnce(long ptr, int timeoutMillis);private native static void nativeWake(long ptr);// 判断native层的状态private native static boolean nativeIsIdling(long ptr);

将会在后续文章中进行介绍。

[Android] Handler源码解析 (Java层)

简单用例

源码解析

总结

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