一、简介

Android的消息机制主要是指Handler的运行机制，那么什么是Handler的运行机制那？通俗的来讲就是，使用Handler将子线程的Message放入主线程的Messagequeue中，在主线程使用。

二、学习内容

学习Android的消息机制，我们需要先了解如下内容。

消息的表示：Message

消息队列：MessageQueue

消息循环，用于循环取出消息进行处理：Looper

消息处理，消息循环从消息队列中取出消息后要对消息进行处理：Handler

平常我们接触的大多是Handler和Message，今天就让我们来深入的了解一下他们。

三、代码详解

一般而言我们都是这样使用Handler的

xxHandler.sendEmptyMessage(xxx);

当然还有其他表示方法，但我们深入到源代码中，会发现，他们最终都调用了一个方法

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，但这依然不是结束，我们可以看到最后一句enqueueMessage(queue, msg, uptimeMillis);按字面意思来说插入一条消息，那么疑问来了，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; }

进入源代码，我们发现，我们需要了解一个新类Messagequeue。

虽然我们一般把他叫做消息队列，但是通过研究，我们发下，它实际上是一种单链表的数据结构，而我们对它的操作主要是插入和读取。

看代码33-44，学过数据结构，我们可以轻松的看出，这是一个单链表的插入末尾的操作。

这样就明白了，我们send方法实质就是向Messagequeue中插入这么一条消息，那么另一个问题随之而来，我们该如何处理这条消息。

处理消息我们离不开一个重要的，Looper。那么它在消息机制中又有什么样的作用那？

Looper扮演着消息循环的角色，具体而言它会不停的从MessageQueue中查看是否有新消息如果有新消息就会立刻处理，否则就已知阻塞在那里，现在让我们来看一下他的代码实现。

首先是构造方法

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

可以发现，它将当前线程对象保存了起来。我们继续

Looper在新线程创建过程中有两个重要的方法looper.prepare() looper.loop

new Thread(){ public void run(){ Looper.prepare(); Handler handler = new Handler()； Looper.loop(); }}.start();

我们先来看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，我们可以看到这里面又有一个ThreadLocal类，我们在这简单了解一下，他的特性，set()，get()方法。

首先ThreadLocal是一个线程内部的数据存储类，通过它可以在指定的线程中存储数据，数据存储后，只有在制定线程中可以获取存储的数据，对于其他线程而言则无法获取到数据。简单的来说。套用一个列子：

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();

上面的代码运行后，我们会发现，每一个线程的值都是不同的，即使他们访问的是同意个ThreadLocal对象。

那么我们接下来会在之后分析源码，为什么他会不一样。现在我们跳回prepare()方法那一步，loop()方法源码贴上

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，获得这个线程的Looper，若没有抛出异常。再获得新建的Messagequeue，在这里我们有必要补充一下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; } }

从24-30我们可以看到，他遍历了整个queue找到msg，若是msg为null，我们可以看到50，他把nextPollTimeoutMillis = -1;实际上是等待enqueueMessage的nativeWake来唤醒。较深的源码涉及了native层代码，有兴趣可以研究一下。简单来说next()方法，在有消息是会返回这条消息，若没有，则阻塞在这里。

我们回到loop()方法27msg.target.dispatchMessage(msg);我们看代码

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

msg.target实际上就是发送这条消息的Handler，我们可以看到它将msg交给dispatchMessage(),最后调用了我们熟悉的方法handleMessage(msg);

三、总结

到目前为止，我们了解了android的消息机制流程，但它实际上还涉及了深层的native层方法.

以上就是本文的全部内容，希望本文的内容对大家的学习或者工作能带来一定的帮助，同时也希望多多支持！