Android Handler、Looper、MessageQueue以及Message源码分析

一、APP应用程序入口

APP应用程序入口为ActivityThread类中的main方法（主线程）。主要代码如下：

    public static void main(String[] args) {
        Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
        SamplingProfilerIntegration.start();

        Process.setArgV0("");

        Looper.prepareMainLooper();
        ActivityThread thread = new ActivityThread();
        thread.attach(false);
        if (sMainThreadHandler == null) {
            sMainThreadHandler = thread.getHandler();
        }
        Looper.loop();
        throw new RuntimeException("Main thread loop unexpectedly exited");
    }

其中可以看到初始化UI主线程的Looper对象，通过Looper.prepareMainLooper()方法，在本方法中，调用了，prepare（boolean quitAllowed），该标识会传递给其对应的消息队列MessageQueue，通过代码可以看出，UI主线程对应的消息循环队列是不允许退出的。quitAllowed默认传入了false- prepare(false);
如下：

   public static void prepareMainLooper() {
        prepare(false);
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }

    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));
    }

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

二、Handler

Handler的构造方法可以分为2类：
（1）传入指定线程looper对象；
（2）不传入looper对象，默认为UI主线程的looper对象。

（1）传入指定线程looper对象的构造方法
    public Handler(Looper looper, Callback callback, boolean async) {
        mLooper = looper;
        mQueue = looper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

（2）不传入looper对象，默认为UI主线程的looper对象。
public Handler(Callback callback, boolean async) {
        ...
        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;
    }

在Handler初始化时，会初始化一个Looper对象和Looper中对应的消息队列mLooper.mQueue对象。
mAsynchronous 的作用：为True时，发送消息为异步消息。

如下为Handler中的消息分发方法dispatchMessage。也就是如果在初始化Handler时，传入Callback，优先将消息传递给Callback，否则，传递给大家最为熟悉的Handler的handleMessage(msg)方法。

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

Handler初始化后，接着去看看Looper中的实现。

二、Looper

（1）Looper初始化

Looper对外提供了两个静态的公共方法
其中在工作线程调用Looper.prepare()方法，其对应的MessageQueue消息队列是可以退出的。

源代码如下：

//（1） prepare()用于在工作线程中调用
    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));
    }

//（2）prepareMainLooper()用于在UI主线程中调用
    public static void prepareMainLooper() {
        prepare(false);
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }
    //Looper构造器
    private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }

（2）Looper的loop()方法

从源码中可以看出，loop()方法中通过死循环for，将消息队列MessageQueue中的消息取出，然后调用
msg.target.dispatchMessage(msg);方法将消息分发传递到Handler的dispatchMessage方法。
从而可以在Handler的callback或者handleMessage()中处理改消息。
在loop（）方法的最后，调用了msg.recycleUnchecked();处理完的消息msg，会被置空初始化，并加入到消息缓冲池中。Message中缓冲池的最大为50。

   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
            final Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            final long traceTag = me.mTraceTag;
            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }
            try {
                msg.target.dispatchMessage(msg);
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }

            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();
        }
    }

//消息缓冲池的最大容量为50
private static final int MAX_POOL_SIZE = 50;

//Message中的recycleUnchecked（）方法
   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++;
            }
        }
    }

三、MessageQueue

1. enqueueMessage(Message msg, long when)方法 —— Handler中发送消息的方法最终都是调用消息队列中的该方法，把消息加入到消息队列中。

    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;
    }

2 . Message next() {}方法，返回一个msg，该方法是上面提到的在Looper中的loop（）方法中调用。

3. IdleHander内部接口——可通过addIdleHandler/removeIdleHandler方法，添加或移除回调。

如果消息队列没有消息，也就是在UI主线程或者工作线程空闲时候，会循环调用mIdleHandlers集合中的IdleHander对象，执行queueIdle()方法。如果queueIdle()返回false，则就会从改集合中移除，也就是只执行一次。适合做延迟加载。

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;
        }
    }

四、Message

1. 创建Handler发送所需的消息Msg——通过Message.obtain（）以及重载方法

在Message中定义了消息缓存的最大容量MAX_POOL_SIZE 为50。sPool的赋值是通过recycleUnchecked()方法中。之前提到过该方法是在Looper.loop方法的最后才调用。也就是处理完第一个消息的时候，就开始把消息加入到sPool这个缓存对象的next中。下次调用msg.obtain()方法就会从缓存中取出。

private static Message sPool;
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) {
                Message m = sPool;
                sPool = m.next;
                m.next = null;
                m.flags = 0; // clear in-use flag
                sPoolSize--;
                return m;
            }
        }
        return new Message();
    }

    /**
     * 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++;
            }
        }
    }

---

至此对Handler、Looper、MessageQueue和Message的部分源码分析完毕。

收获要点总结：

1. App程序入口——ActivityThread中main()方法，并对UI主线程的Looper进行初始化。
2. Handler创建——Handler初始化不指定Looper对象默认为UI主线程对应的Looper，消息发送底层是通过Looper对象中的mQueue对象，把消息加入到消息队列。通过loop()方法循环取出处理。
3. Looper——UI主线程的消息队列是不允许停止退出的。Looper.prepare（）方法调用会和所在线程进行绑定，保存到ThreadLocal中。一一对应的关系。
4. MessageQueue——消息队列中有next（）方法取出消息和enqueueMessage（）方法把消息加入到队列中。还有一个重要的内部接口IdleHandler，在消息队列消息为空时执行。
5. Message——消息，内部做了缓冲的机制，缓冲容量为50。