Android(安卓)的Handler、Looper和MessageQueue的关系和实现原理

最近比较闲，看了一些关于android消息机制的书籍和文摘，写个文档总结下。

一、基本概念

Message：消息，其中包含了消息ID，消息处理对象以及处理的数据等，由MessageQueue统一列队，终由Handler处理。

Handler：处理者，负责Message的发送及处理。使用Handler时，需要实现handleMessage(Message msg)方法来对特定的Message进行处理，例如更新UI等。

MessageQueue：消息队列，用来存放Handler发送过来的消息，将Message以链表的方式串联起来的，等待Looper的抽取。

Looper：消息泵，不断地从MessageQueue中抽取Message执行。因此，一个MessageQueue需要一个Looper。

Thread：线程，负责调度整个消息循环，即消息循环的执行场所。

二、Handler、looper和消息队列的关系

2.1、从一个简单的程序说起

代码段1：

public class MainActivity extends Activity{    private TextView mText;   @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        mText = (TextView)findViewById(R.id.myText);    }    public void click(View view)    {        new Thread(new Runnable() {            @Override            public void run() {                // 耗时的操作......                // 设置UI                mText.setText("padmatek");            }        }).start();    }}

布局文件：

<?xml version="1.0" encoding="utf-8"?>

运行并点击“测试”按钮，app崩溃，报错如下；原因是因为android中UI控件不是线程安全的，所以我们不能在工作线程（非UI）中访问UI控件

但是如果我们确实有在工作线程运行的过程中去处理UI的需求该怎么办呢？

我们将代码段1稍作修改：

代码段2：

public class MainActivity extends Activity{    private TextView mText;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        mText = (TextView)findViewById(R.id.myText);    }    private final static int MSG_SET_TEXT = 0;    private Handler handler = new Handler(){        public void handleMessage(Message msg)        {            switch (msg.what)            {                case MSG_SET_TEXT:                    mText.setText("padmatek");                    break;                default:                    break;            }        }    };    public void click(View view)    {        new Thread(new Runnable() {            @Override            public void run() {                // 耗时的操作......                // 设置UI                handler.sendEmptyMessage(MSG_SET_TEXT);            }        }).start();    }}

在该代码段中，我们创建了一个handler,并将按钮处理函数的设置Text的语句改为了使用handler发送一个消息；

此时我们再运行并点击按钮，发现文本设置成功。

那么问题来了:

问题1:handler.sendEmptyMessage做了什么？

2.2、解析Handler的消息传递（问题1）

首先我们进入sendEmptyMessage，并依次找到被调用的函数，代码如下:

    public final boolean sendEmptyMessage(int what)    {        return sendEmptyMessageDelayed(what, 0);    }

    public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {        Message msg = Message.obtain();        msg.what = what;        return sendMessageDelayed(msg, delayMillis);    }

    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);    }

从函数sendMessageAtTime里面蹦出来个mQueue

且当queue == null时，会弹出以“Looper”作为Tag的异常信息；

如果queue != null时，通过queue.enqueueMessage将消息交给了消息队列。

上面我们解答了问题1,此时我们又有以下问题:

2、Looper是什么东西，有什么作用？

3、这个消息队列是怎么来的？

2.3、Handler与Looper的关系

Handler有两个主要的构造函数，在这里贴上默认构造函数对应的代码:

public Handler() {        this(null, false);    }

其中this对应的实现如下:

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;    }

从以上代码可以看出，消息队列mQueue 来自于mLooper（问题3），而 mLooper 又来自于Looper.myLooper()；

Looper.myLooper()的实现如下:

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

看到这里，是不是有点“拔出萝卜带出泥”的感觉......

2.4、浅尝sThreadLocal

既然到了这里，我们再来看一下sThreadLocal是个什么玩意，其定义如下:

static final ThreadLocal sThreadLocal = new ThreadLocal();private static Looper sMainLooper;

而其get函数的定义如下:

public T get() {        // Optimized for the fast path.        Thread currentThread = Thread.currentThread();        Values values = values(currentThread);        if (values != null) {            Object[] table = values.table;            int index = hash & values.mask;            if (this.reference == table[index]) {                return (T) table[index + 1];            }        } else {            values = initializeValues(currentThread);        }        return (T) values.getAfterMiss(this);    }

由于sThreadLocal为类静态变量，说明他是全局有效的，说明我们的所有的looper应该都保存在 sThreadLocal中，所以get的实现原理大概就是通过线程句柄来获取对应的looper；

好了，sThreadLocal的探讨就先到这里，我们还是回到原来的地方；

2.4、Handler、looper和消息队列的关系

通过2.3我们大概知道了Handler和looper的关系，而且也知道消息队列来自于looper，为了肯定这一点，我们查看了源码，如下:

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

到现在为止，我们说了这么多，是不是有点小晕了，反正我是有点，那么放下脚步小结一下：

一句话:handler通过looper获取消息队列

由于主线程中默认有looper，而子线程没有，我们接下来在子线程更加详细的说明3者的关系

三、Looper的原理（问题2）

3.1、looper.prepare的作用

修改代码片段1的click函数如下:

public void click(View view)    {        new Thread(new Runnable() {            @Override            public void run() {                // 耗时的操作......                // 设置UI                handler.sendEmptyMessage(MSG_SET_TEXT);                Handler handlerSub = new Handler();            }        }).start();    }

只是添加了一行代码，运行并点击按钮，会报错，错误信息如下:

意思就是，没有调用Looper.prepare(),这个是干神马的呢？看源码如下:

/** 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);    }

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));    }

通过源码知道，prepare创建了looper，并且将其保存在sThreadLocal中，通过注释理解到创建完成后要调用loop函数；

注意:主线程创建Handler并不需要手动创建looper，是因为主线程在初始化的main函数中调用Looper.prepareMainLooper()生成了looper；

3.2、Looper.loop的作用

贴源代码:

 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();        }    }

由以上源代码可知，loop方法是一个死循环，不断地从消息队列中取消息，将取出的消息交给msg.target的

dispatchMessage处理，其中msg.target就是我们创建的handler. 如果消息队列返回为null，则跳出loop，终止获取消息.

那么问题来了

问题4：以上源代码可知、不发送消息，loop调用后是不是循环就立马终止呢？

我们先来看看dispatchMessage的源代码:

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

如果msg带的callback和Handler的们Callback都为空，则调用handleMessage来处理消息，也就是代码片段1handleMessage函数；

3.3、正常创建消息循环

说了这么多，那么我们就看如何在工作线程中创建消息循环吧

public void click(View view)    {        new Thread(new Runnable() {            @Override            public void run() {                // 耗时的操作......                // 设置UI                handler.sendEmptyMessage(MSG_SET_TEXT);                Looper.prepare();                Handler handlerSub = new Handler();                Looper.loop();            }        }).start();    }

这样子，就可以在子线程中通handler来发送和处理消息了，自己去实现吧。

四、消息队列机制

从2.2的最好一个片段的代码，我们知道，handler最终将消息放入到了消息，队列中，那么消息队列又是怎样存储消息的呢？

4.1、消息存储机制

MessageQueue的enqueueMessage源码如下:

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(TAG, 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;    }

从源码可知，消息按照其delay的时间从小到大保存在一个链表中，并且消息保存时线程安全的；

4.2、消息获取机制

从3.2中的loop函数，我们可以得知，消息队列调用类next函数来获取消息，MessageQueue的next源码如下:

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 (DEBUG) Log.v(TAG, "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(TAG, "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;        }    }

由此可知，当mQuitting为false时，不管有没有消息，for循环都不会结束，如果消息队列中有消息，则返回消息，如果没有一直循环，不返回；

那么3.2节中的问题4的答案就出来了，当没有消息，且没有调用Looper.quit时，消息队列的next函数会一直阻塞，那么loop函数也会阻塞；

参考的资料：

1、http://blog.csdn.net/liuhe688/article/details/6407225

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