一、簡介
Android的消息機制主要是指Handler的運行機制，那么什么是Handler的運行機制那？通俗的來講就是，使用Handler將子線程的Message放入主線程的Messagequeue中，在主線程使用。
二、學習內容
學習Android的消息機制，我們需要先了解如下內容。
平常我們接觸的大多是Handler和Message，今天就讓我們來深入的了解一下他們。
三、代碼詳解
一般而言我們都是這樣使用Handler的
xxHandler.sendEmptyMessage(xxx);
當然還有其他表示方法，但我們深入到源代碼中，會發現，他們最終都調用了一個方法
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); }sendMessageAtTime()方法，但這依然不是結束，我們可以看到最后一句enqueueMessage(queue, msg, uptimeMillis);按字面意思來說插入一條消息，那么疑問來了，消息插入了哪里。
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; }進入源代碼，我們發現，我們需要了解一個新類Messagequeue。
雖然我們一般把他叫做消息隊列，但是通過研究，我們發下，它實際上是一種單鏈表的數據結構，而我們對它的操作主要是插入和讀取。
看代碼33-44，學過數據結構，我們可以輕松的看出，這是一個單鏈表的插入末尾的操作。
這樣就明白了，我們send方法實質就是向Messagequeue中插入這么一條消息，那么另一個問題隨之而來，我們該如何處理這條消息。
處理消息我們離不開一個重要的，Looper。那么它在消息機制中又有什么樣的作用那？
Looper扮演著消息循環的角色，具體而言它會不停的從MessageQueue中查看是否有新消息如果有新消息就會立刻處理，否則就已知阻塞在那里，現在讓我們來看一下他的代碼實現。
首先是構造方法
private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }可以發現，它將當前線程對象保存了起來。我們繼續
Looper在新線程創建過程中有兩個重要的方法looper.prepare() looper.loop
new Thread(){ public void run(){ Looper.prepare(); Handler handler = new Handler()； Looper.loop(); }}.start();我們先來看prepare()方法
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)); }咦，我們可以看到這里面又有一個ThreadLocal類，我們在這簡單了解一下，他的特性，set()，get()方法。
首先ThreadLocal是一個線程內部的數據存儲類，通過它可以在指定的線程中存儲數據，數據存儲后，只有在制定線程中可以獲取存儲的數據，對于其他線程而言則無法獲取到數據。簡單的來說。套用一個列子：
private ThreadLocal<Boolean> mBooleanThreadLocal = new ThreadLocal<Boolean>();//mBooleanThreadLocal.set(true);Log.d(TAH,"Threadmain"+mBooleanThreadLocal.get());new Thread("Thread#1"){ public void run(){ mBooleanThreadLocal.set(false); Log.d(TAH,"Thread#1"+mBooleanThreadLocal.get()); }； }.start();new Thread("Thread#2"){ public void run(){ Log.d(TAH,"Thread#2"+mBooleanThreadLocal.get()); }； }.start();上面的代碼運行后，我們會發現，每一個線程的值都是不同的，即使他們訪問的是同意個ThreadLocal對象。
那么我們接下來會在之后分析源碼，為什么他會不一樣。現在我們跳回prepare()方法那一步，loop()方法源碼貼上
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(); } }首先loop()方法，獲得這個線程的Looper，若沒有拋出異常。再獲得新建的Messagequeue，在這里我們有必要補充一下Messagequeue的next()方法。
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; } }從24-30我們可以看到，他遍歷了整個queue找到msg，若是msg為null，我們可以看到50，他把nextPollTimeoutMillis = -1;實際上是等待enqueueMessage的nativeWake來喚醒。較深的源碼涉及了native層代碼，有興趣可以研究一下。簡單來說next()方法，在有消息是會返回這條消息，若沒有，則阻塞在這里。
我們回到loop()方法27msg.target.dispatchMessage(msg);我們看代碼
public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } }msg.target實際上就是發送這條消息的Handler，我們可以看到它將msg交給dispatchMessage(),最后調用了我們熟悉的方法handleMessage(msg);
三、總結
到目前為止，我們了解了android的消息機制流程，但它實際上還涉及了深層的native層方法.
以上就是本文的全部內容，希望本文的內容對大家的學習或者工作能帶來一定的幫助，同時也希望多多支持武林網！
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