一 前言

對于Android開發(fā)者來說，在日常開發(fā)過程中不可避免會(huì)涉及到Android中的消息機(jī)制的場景應(yīng)用咧纠，比如在子線程進(jìn)行一些耗時(shí)操作瞬内，操作完成后需要在主線程更新UI，該場景想必大家都在實(shí)際開發(fā)過程中遇到過碘勉。另外，看過Android源碼的開發(fā)者想必也知道桩卵，多處Android源碼中也使用到消息機(jī)制進(jìn)行通信验靡，比如，Activity雏节、Service的啟動(dòng)過程就都涉及到胜嗓。因此，本文從源碼層面分析Android中消息機(jī)制的工作原理钩乍。而要理解Android的消息機(jī)制的運(yùn)行機(jī)制辞州，需要從Looper , Handler , Message等工作原理進(jìn)分析。下面先寫一個(gè)使用示例寥粹，然后根據(jù)該示例進(jìn)行源碼跟蹤变过。

二 示例展示

public class MainActivity extends Activity {
        public static final String TAG = MainActivity.class.getSimpleName();
    private Handler mainHandler;
    private Handler threadHandler;
     /**
     * 在主線程中創(chuàng)建Handler，并實(shí)現(xiàn)對應(yīng)的handleMessage方法
     */
    public static Handler mainHandler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
             Log.i(TAG, "主線程中接收到handler消息...")
        }
    };
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
                //子線程中創(chuàng)建handler
        new Thread(new Runnable() {
            @Override
            public void run() {
                threadHandler = new Handler();
            }
        }).start();
    }

此時(shí)運(yùn)行程序涝涤，你會(huì)發(fā)現(xiàn)媚狰，在子線程中創(chuàng)建的Handler是會(huì)導(dǎo)致程序崩潰的，提示的錯(cuò)誤信息為 Can't create handler inside thread that has not called Looper.prepare() 阔拳。如何解決該問題崭孤，其實(shí)只需按提示信息所述，在當(dāng)前線程中調(diào)用Looper.prepare()，即為當(dāng)前線程創(chuàng)建了Looper辨宠。代碼如下：

//子線程中創(chuàng)建handler
        new Thread(new Runnable() {
            @Override
            public void run() {
                                Looper.prepare();
                threadHandler = new Handler();
                                Looper.loop();
            }
        }).start();

另外遗锣，通過查看下Handler的源碼，也可以弄清楚為什么不調(diào)用Looper.prepare()就crash彭羹。Handler的構(gòu)造函數(shù)如下所示：

 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;
    }

可知黄伊，上述代碼塊中泪酱，調(diào)用了Looper.myLooper();語句派殷，然后賦值給一個(gè)Looper對象mLooper ，如果mLooper實(shí)例為空墓阀，則會(huì)拋出一個(gè)運(yùn)行時(shí)異常（Can’t create handler inside thread that has not called Looper.prepare()Ｕ毕А）。
這里調(diào)用了Looper.myLooper()斯撮，那么我們接下來通過它來了解Looper的工作原理经伙。

三 源碼分析

接著上一小節(jié)分析，查看myLooper()方法的代碼：

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

方法就是從sThreadLocal對象中取出Looper勿锅。如果sThreadLocal中有Looper存在就返回Looper帕膜，如果沒有Looper存在自然就返回空了。因此溢十，源碼中肯定在某處給sThreadLocal設(shè)置Looper垮刹？顯然是的，并且是在Looper.prepare()方法中進(jìn)行設(shè)置张弛，接著來看下它的源碼：

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));
    }

代碼中首先判斷sThreadLocal中是否已經(jīng)存在Looper了荒典，如果還沒有則創(chuàng)建一個(gè)新的Looper設(shè)置進(jìn)去。如果有也會(huì)拋出異常吞鸭，即要求每個(gè)線程中最多有且只有一個(gè)Looper對象寺董。其中，sThreadLocal是ThreadLocal的實(shí)例刻剥，后面會(huì)再專門介紹ThreadLocal遮咖，當(dāng)前只要知道ThreadLocal的作用是要可以在每個(gè)線程中存儲數(shù)據(jù)，并且不同線程中互不干擾造虏。接著看Looper的構(gòu)造函數(shù)源碼：

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

在Looper構(gòu)造函數(shù)創(chuàng)建了一個(gè)MessageQueue和獲取當(dāng)前Thread實(shí)例引用盯滚。MessageQueue是消息隊(duì)列，即用于存儲消息酗电，以及以隊(duì)列的形式對外提供入隊(duì)及出隊(duì)操作魄藕，后面再通過源碼再分析。
通過以上子線程創(chuàng)建Handle的過程分析可知撵术，子線程中需要先調(diào)用Looper.prepare()才能創(chuàng)建Handler對象背率。那么示例展示的主線程中mainHandler創(chuàng)建為何不崩潰呢？其原因是在程序啟動(dòng)的時(shí)候，系統(tǒng)已經(jīng)幫我們自動(dòng)調(diào)用了Looper.prepare()方法寝姿。我們可以查看ActivityThread中的main()方法交排，代碼如下所示：

public static void main(String[] args) {
    SamplingProfilerIntegration.start();
    CloseGuard.setEnabled(false);
    Environment.initForCurrentUser();
    EventLogger.setReporter(new EventLoggingReporter());
    Process.setArgV0("<pre-initialized>");
    Looper.prepareMainLooper();
    ActivityThread thread = new ActivityThread();
    thread.attach(false);
    if (sMainThreadHandler == null) {
        sMainThreadHandler = thread.getHandler();
    }
    AsyncTask.init();
    if (false) {
        Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread"));
    }
    Looper.loop();
    throw new RuntimeException("Main thread loop unexpectedly exited");
}

用了Looper.prepareMainLooper()方法，而這個(gè)方法又會(huì)再去調(diào)用Looper.prepare()方法饵筑，代碼如下所示：

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

可知埃篓，ActivityThread被創(chuàng)建時(shí)變會(huì)初始化Looper，從而不需要再手動(dòng)去調(diào)用Looper.prepare()方法了根资。
此外架专，ActivityThread中的main()方法中最后調(diào)用了 Looper.loop()，它是Looper中一個(gè)重要方法玄帕，只有調(diào)用了loop后消息循環(huán)系統(tǒng)才會(huì)真正的起作用部脚，它的源碼實(shí)現(xiàn)如下：

/**
     * 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) {
            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();
        }
    }

這個(gè)方法進(jìn)入了一個(gè)死循環(huán)，然后不斷地調(diào)用的MessageQueue的next()方法裤纹，MessageQueue的next()方法即是出隊(duì)方法委刘，源碼如下：

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;
        }
    }

方法中判斷當(dāng)前MessageQueue中如果存在mMessages(即待處理消息)，就將這個(gè)消息出隊(duì)鹰椒，然后讓下一條消息成為mMessages锡移，否則就進(jìn)入一個(gè)阻塞狀態(tài)，一直等到有新的消息入隊(duì)漆际。
回到Looper類的loop()方法淆珊，如果next()方法返回的msg等于null，就退出該循環(huán)灿椅，否則每當(dāng)有一個(gè)消息出隊(duì)套蒂，接著就將它傳遞到msg.target的dispatchMessage()方法中，那這里msg.target又是什么呢茫蛹？其實(shí)就是Handler啦操刀，也就是回調(diào)到Handler進(jìn)行處理消息，那么如何處理呢婴洼？在介紹由Handler的dispatchMessage()方法進(jìn)行處理消息之前骨坑，我們首先得知道如何向MessageQueue中插入消息，即Handler如何發(fā)送消息柬采？示例代碼如下：

new Thread() {
              @Override
              public void run() {
                // 在子線程中發(fā)送異步消息
              Message message = new Message();
              message.arg1 = 1;
              mainHandler.sendMessage(message);
             }
     }.start();

示例中通過sendMessage()方法發(fā)送消息欢唾，并且為什么最后又可以在Handler的handleMessage()方法中重新得到這條Message？接著就通過發(fā)送消息這個(gè)源碼流程來分析其原因粉捻。Handler中提供了很多個(gè)發(fā)送消息的方法礁遣，除了sendMessageAtFrontOfQueue()方法之外，其它的發(fā)送消息方法最終都會(huì)輾轉(zhuǎn)調(diào)用到sendMessageAtTime()方法中肩刃，分別看下這兩個(gè)方法的源碼祟霍，sendMessageAtTime源碼如下

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);
    }

sendMessageAtFrontOfQueue方法源碼

public final boolean sendMessageAtFrontOfQueue(Message msg) {
        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, 0);
    }

對比上面兩個(gè)方法杏头，表面上說Handler的sendMessageAtFrontOfQueue方法和其他發(fā)送方法不同，其實(shí)實(shí)質(zhì)是相同的沸呐，僅僅是sendMessageAtFrontOfQueue方法是sendMessageAtTime方法的一個(gè)特例而已醇王，即sendMessageAtTime最后一個(gè)參數(shù)uptimeMillis傳遞0。
因此崭添，只要分析sendMessageAtTime這個(gè)方法寓娩，sendMessageAtTime(Message msg, long uptimeMillis)方法有兩個(gè)參數(shù)：
msg：是我們發(fā)送的Message對象，
uptimeMillis：表示發(fā)送消息的時(shí)間呼渣，uptimeMillis的值等于從系統(tǒng)開機(jī)到當(dāng)前時(shí)間的毫秒數(shù)再加上延遲時(shí)間棘伴。

最后，該方法返回一個(gè)enqueueMessage方法的返回值徙邻，那么enqueueMessage()方法毫無疑問就是入隊(duì)的方法了排嫌，我們來看下這個(gè)方法的源碼：

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

方法中msg.target就是Handler對象本身畸裳；queue就是上一步傳過來的mQueue缰犁，而mQueue是在Handler實(shí)例化時(shí)構(gòu)造函數(shù)中實(shí)例化的MessageQueue實(shí)例。在Handler的構(gòu)造函數(shù)中可以看見mQueue = mLooper.mQueue;接著就調(diào)用MessageQueue中的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(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;
    }

可以看到這里MessageQueue根據(jù)時(shí)間將所有的Message排序怖糊，然后使用鏈表的形式將所有的Message保存起來帅容。
介紹完Handle發(fā)送消息后，把消息插入到MessageQueue消息隊(duì)列后伍伤，那么回到之前l(fā)oop方法中不斷的從MessageQueue中通過next()取消息并徘，然后交由msg.target的dispatchMessage()方法處理。由之前分析可知msg.target指的是Handle對象扰魂，那么看下Handle中dispatchMessage()方法的源碼：

/**
     * Handle system messages here.
     */
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

dispatchMessage方法中的邏輯比較簡單麦乞，具體就是如果msg.callback不為空，則調(diào)用handleCallback()方法處理劝评，如果mCallback不為空姐直，則調(diào)用mCallback的handleMessage()方法，否則直接調(diào)用Handler的handleMessage()方法蒋畜，并將消息對象作為參數(shù)傳遞過去声畏。這樣整個(gè)異步消息流程就串起來了。

四 總結(jié)

1.主線程中可以直接定義Handler姻成，但如果想要在子線程中定義Handler插龄，剛在創(chuàng)建Handler前需要調(diào)用Looper.prepare();，子線程中的標(biāo)準(zhǔn)的寫法形式為：

//子線程中創(chuàng)建handler
        new Thread(new Runnable() {
            @Override
            public void run() {
                Looper.prepare();
                threadHandler = new Handler();
                Looper.loop();
            }
        }).start();

2.一個(gè)線程中只存在一個(gè)Looper對象科展，只存在一個(gè)MessageQueue對象均牢，但可以可以有多個(gè)Handler對象，即Handler對象內(nèi)部關(guān)聯(lián)了本線程中唯一的Looper對象才睹，Looper對象內(nèi)部關(guān)聯(lián)著唯一的一個(gè)MessageQueue對象徘跪。
3.MessageQueue消息隊(duì)列是按照時(shí)間排序以鏈表的形式進(jìn)行消息存儲的见秤。