Android消息循环机制源码分析

概述

上一篇博客讲解了 Message、Handler、Message Queue、Looper之间的关系 本文将从源码角度深度讲解Android消息循环机制。

我们平时使用是这样的：

    //1. 主线程    Handler handler = new MyHandler();    //2. 非主线程    HandlerThread handlerThread = new HandlerThread("handlerThread");    handlerThread.start();    Handler handler = new Handler(handlerThread.getLooper());    //发送消息    handler.sendMessage(msg);    //接收消息    static class MyHandler extends Handler {        //对于非主线程处理消息需要传Looper，主线程有默认的sMainLooper        public MyHandler(Looper looper) {            super(looper);        }        @Override        public void handleMessage(Message msg) {            super.handleMessage(msg);        }    }

那么为什么初始化的时候，我们执行了1或2，后面只需要sendMessage就可处理任务了呢？先以非主线程为例进行介绍，handlerThread.start()的时候，实际上创建了一个用于消息循环的Looper和消息队列MessageQueue，同时启动了消息循环，并将这个循环传给Handler，这个循环会从MessageQueue中依次取任务出来执行。用户若要执行某项任务，只需要调用handler.sendMessage即可，这里做的事情是将消息添加到MessaeQueue中。对于主线程也类似，只是主线程sMainThread和sMainLooper不需要我们主动去创建，程序启动的时候Application就创建好了，我们只需要创建Handler即可。

我们这里提到了几个概念：

HandlerThread 支持消息循环的线程

Handler 消息处理器

Looper 消息循环对象

MessageQueue 消息队列

Message 消息体

对应关系是：一对多，即（一个）HandlerThread、Looper、MessageQueue －> （多个）Handler、Message

源码解析

1. Looper

（1）创建消息循环

prepare()用于创建Looper消息循环对象。Looper对象通过一个成员变量ThreadLocal进行保存。

（2）获取消息循环对象

myLooper()用于获取当前消息循环对象。Looper对象从成员变量ThreadLocal中获取。

（3）开始消息循环

loop()开始消息循环。循环过程如下：

每次从消息队列MessageQueue中取出一个Message

使用Message对应的Handler处理Message

已处理的Message加到本地消息池，循环复用

循环以上步骤，若没有消息表明消息队列停止，退出循环

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

1. Handler

（1）发送消息

Handler支持2种消息类型，即Runnable和Message。因此发送消息提供了post(Runnable r)和sendMessage(Message msg)两个方法。从下面源码可以看出Runnable赋值给了Message的callback，最终也是封装成Message对象对象。外部调用不统一使用Message，应该是兼容Java的线程任务，这种思想也可以借鉴到平常开发过程中。发送的消息都会入队到MessageQueue队列中。

（2）处理消息

Looper循环过程的时候，是通过dispatchMessage(Message msg)对消息进行处理。处理过程：先看是否是Runnable对象，如果是则调用handleCallback(msg)进行处理，最终调到Runnable.run()方法执行线程；如果不是Runnable对象，再看外部是否传入了Callback处理机制，若有则使用外部Callback进行处理；若既不是Runnable对象也没有外部Callback，则调用handleMessage(msg)，这个也是我们开发过程中最常覆写的方法了。

（3）移除消息

removeCallbacksAndMessages()，移除消息其实也是从MessageQueue中将Message对象移除掉。

public void handleMessage(Message msg) {}public void dispatchMessage(Message msg) {    if (msg.callback != null) {        handleCallback(msg);    } else {        if (mCallback != null) {            if (mCallback.handleMessage(msg)) {                return;            }        }        handleMessage(msg);    }}private static void handleCallback(Message message) {    message.callback.run();}public final Message obtainMessage(){    return Message.obtain(this);}public final boolean post(Runnable r){   return  sendMessageDelayed(getPostMessage(r), 0);}public final boolean sendMessage(Message msg){    return sendMessageDelayed(msg, 0);}private static Message getPostMessage(Runnable r) {    Message m = Message.obtain();    m.callback = r;    return m;}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);}public final void removeCallbacksAndMessages(Object token) {    mQueue.removeCallbacksAndMessages(this, token);}

1. MessageQueue

（1）消息入队

消息入队方法enqueueMessage(Message msg, long when)。其处理过程如下：

待入队的Message标记为InUse，when赋值

若消息链表mMessages为空为空，或待入队Message执行时间小于mMessage链表头，则待入队Message添加到链表头

若不符合以上条件，则轮询链表，根据when从低到高的顺序，插入链表合适位置

（2）消息轮询

next()依次从MessageQueue中取出Message

（3）移除消息

removeMessages()可以移除消息，做的事情实际上就是将消息从链表移除，同时将移除的消息添加到消息池，提供循环复用。

ean 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;        }        msg.markInUse();        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;}Message next() {    // Return here if the message loop has already quit and been disposed.    // This can happen if the application tries to restart a looper after quit    // which is not supported.    final long ptr = mPtr;    if (ptr == 0) {        return null;    }    int pendingIdleHandlerCount = -1; // -1 only during first iteration    int nextPollTimeoutMillis = 0;    for (;;) {        if (nextPollTimeoutMillis != 0) {            Binder.flushPendingCommands();        }        nativePollOnce(ptr, 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);                    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;    }}void removeMessages(Handler h, int what, Object object) {    if (h == null) {        return;    }    synchronized (this) {        Message p = mMessages;        // Remove all messages at front.        while (p != null && p.target == h && p.what == what               && (object == null || p.obj == object)) {            Message n = p.next;            mMessages = n;            p.recycleUnchecked();            p = n;        }        // Remove all messages after front.        while (p != null) {            Message n = p.next;            if (n != null) {                if (n.target == h && n.what == what                    && (object == null || n.obj == object)) {                    Message nn = n.next;                    n.recycleUnchecked();                    p.next = nn;                    continue;                }            }            p = n;        }    }}

1. Message

（1）消息创建

Message.obtain()创建消息。若消息池链表sPool不为空，则从sPool中获取第一个，flags标记为UnInUse，同时从sPool中移除，sPoolSize减1；若消息池链表sPool为空，则new Message()

（2）消息释放

recycle()将消息释放，从内部实现recycleUnchecked()可知，将flags标记为InUse，其他各种状态清零，同时将Message添加到sPool，且sPoolSize加1

n 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) {            Message m = sPool;            sPool = m.next;            m.next = null;            m.flags = 0; // clear in-use flag            sPoolSize--;            return m;        }    }    return new Message();}/** * Return a Message instance to the global pool. * <p> * You MUST NOT touch the Message after calling this function because it has * effectively been freed.  It is an error to recycle a message that is currently * enqueued or that is in the process of being delivered to a Handler. * </p> */public void recycle() {    if (isInUse()) {        if (gCheckRecycle) {            throw new IllegalStateException("This message cannot be recycled because it "                    + "is still in use.");        }        return;    }    recycleUnchecked();}/** * Recycles a Message that may be in-use. * Used internally by the MessageQueue and Looper when disposing of queued Messages. */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++;        }    }}

1. HandlerThread

由于Java中的Thread是没有消息循环机制的，run()方法执行完，线程则结束。HandlerThreadandlerThread通过使用Looper实现了消息循环，只要不主动调用HandlerThread或Looper的quit()方法，循环就是一直走下去。

andlerThread extends Thread {int mPriority;int mTid = -1;Looper mLooper;public HandlerThread(String name) {    super(name);    mPriority = Process.THREAD_PRIORITY_DEFAULT;}@Overridepublic void run() {    mTid = Process.myTid();    Looper.prepare();    synchronized (this) {        mLooper = Looper.myLooper();        notifyAll();    }    Process.setThreadPriority(mPriority);    onLooperPrepared();    Looper.loop();    mTid = -1;}public Looper getLooper() {    if (!isAlive()) {        return null;    }    // If the thread has been started, wait until the looper has been created.    synchronized (this) {        while (isAlive() && mLooper == null) {            try {                wait();            } catch (InterruptedException e) {            }        }    }    return mLooper;}public boolean quit() {    Looper looper = getLooper();    if (looper != null) {        looper.quit();        return true;    }    return false;}}

总结

1.关键类：HandlerThread、Handler、Looper、MessageQueue、Messaga
2.MessageQueue数据结构，链表。