Android源码解析之一 异步消息机制
1，handler的常规使用方式
/** * 测试Activity，主要用于测试异步消息（Handler）的使用方式 */public class MainActivity extends AppCompatActivity { public static final String TAG = MainActivity.class.getSimpleName(); private TextView texttitle = null; /** * 在主线程中定义Handler，并实现对应的handleMessage方法 */ public static Handler mHandler = new Handler() { @Override public void handleMessage(Message msg) { if (msg.what == 101) { Log.i(TAG, "接收到handler消息..."); } } }; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); texttitle = (TextView) findViewById(R.id.texttitle); texttitle.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { new Thread() { @Override public void run() { // 在子线程中发送异步消息 mHandler.sendEmptyMessage(101); } }.start(); } }); }}复制代码 根据上述代码可以看出，一般handler的使用方式都是在主线程中定义handler，然后在子线程中调用mHandler.sendEnptyMessage();方法，那么这里就有一个疑问了，我们可以在子线程中定义handler 吗？
2，如何在子线程中定义Handler ？
我们在子线程中定义Handler,看看测试结果：
/** * 定义texttitle的点击事件处理 */texttitle.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { /** * 定义在子线程中执行handler的创建 */ new Thread() { @Override public void run() { Handler mHandler = new Handler() { @Override public void handleMessage(Message msg) { if (msg.what == 101) { Log.i(TAG, "在子线程中定义Handler，并接收到消息。。。"); } } }; } }.start(); } });复制代码 可以看出在子线程中定义的Handler对象报错了，难道Handler对象的定义或者出事中能在主线程中？其实不是这样，错误信息中提示的很明显了，在初始化Handler对象之前需要调用Looper.prepare()方法。
/** * 定义texttitle的点击事件处理 */texttitle.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { new Thread() { @Override public void run() { Looper.prepare(); Handler mHandler = new Handler() { @Override public void handleMessage(Message msg) { if (msg.what == 101) { Log.i(TAG, "在子线程中定义Handler，并接收到消息。。。"); } } }; } }.start(); } }); 复制代码 再次点击按钮运行此段代码，程序就不会报错了，那么这说明初始化Handler对象的时候我们是需要调用Looper.prepare()方法的，那么主线程中为什么可以直接初始化Handler呢？
再次查阅一下源码发现，APP初始化的时候会执行ActivityThread 的main方法：
public static void More ...main(String[] args) { SamplingProfilerIntegration.start(); // CloseGuard defaults to true and can be quite spammy. We // disable it here, but selectively enable it later (via // StrictMode) on debug builds, but using DropBox, not logs. CloseGuard.setEnabled(false); // 初始化应用中需要使用的系统路径 Environment.initForCurrentUser(); // Set the reporter for event logging in libcore EventLogger.setReporter(new EventLoggingReporter()); //增加一个保存key的provider Security.addProvider(new AndroidKeyStoreProvider()); // Make sure TrustedCertificateStore looks in the right place for CA certificates //为应用设置当前用户的CA证书保存的位置 final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId()); TrustedCertificateStore.setDefaultUserDirectory(configDir); //设置进程的名称 Process.setArgV0(" "); Looper.prepareMainLooper(); //创建ActivityThread 对象 ActivityThread thread = new ActivityThread(); thread.attach(false); if (sMainThreadHandler == null) { sMainThreadHandler = thread.getHandler(); } if (false) { Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread")); } Looper.loop(); throw new RuntimeException("Main thread loop unexpectedly exited"); }复制代码 首先Looper.prepareMainLooper();是为主线程创建了Looper，然后thread.getHandler();是保存了主线程的Handler，最后Looper.loop();进入消息循环。
3, 查看Handler源码
① 查看Looper.prepare()方法
// sThreadLocal.get() will return null unless you've called prepare(). static final ThreadLocalsThreadLocal = new ThreadLocal ();/** 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)); } 复制代码
可以看到Looper中有一个ThreadLocal成员变量，熟悉JDK的同学应该知道，当使用ThreadLocal 维护变量时，ThreadLocal为每个使用该成员的线程提供独立的变量副本，所以每一个线程都可以独立地改变自己的副本，而不会影响其他线程所对应的副本。
由此可以看出在每个线程中Looper.prepare() 能且只能调用一次。
继续看Looper对象的构造方法，可以看到在其构造方法中初始化了一个MessageQueue对象：
private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }复制代码 综上小结①：Looper.prepare()方法初始话了一个Looper对象并关联在一个MessageQueue对象，并且一个线程中只有一个Looper对象，只有一个MessageQueue对象。
② 查看Handler对象的构造方法
public Handler(Callback callback, boolean async) { if (FIND_POTENTIAL_LEAKS) { final Class 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; }复制代码 可以看出在Handler中的构造方法中，主要初始化了一下变量，并判断Handler对象的初始化不应在内部类，静态类，匿名类中，并且保存了当前线程中的Looper对象。
综上小结② ：Looper.prepare()方法初始话了一个Looper对象并关联在一个MessageQueue对象，并且一个线程中只有一个Looper对象，只有一个MessageQueue对象。而Handler的构造方法则在Handler内部维护了当前线程的Looper对象
③查看handler.setMesssage(msg)方法 一般情况下，我们发送异步消息的时候会这样调用：
mHandler.sendMessage(new Message());
通过不断的跟进源代码，其最后会调用：
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); } 复制代码 原来msg.target就是Handler对象本身；而这里的queue对象就是我们的Handler内部维护的Looper对象关联的MessageQueue对象。查看messagequeue对象的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; } 复制代码 可以看到这里MessageQueue并没有使用列表将所有的Message保存起来，而是使用Message.next保存下一个Message,从而按照时间将所有的Message排序。
④ 查看Looper.loop()方法
/** * 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 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(); } } 复制代码 可以看到Looper.loop()方法里起了一个死循环，不断的判断MessageQueue中的消息是否为空，如果为空则直接return掉，然后执行queue.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; } } 复制代码 可以看到其大概的实现逻辑就是Message的出栈操作，里面可能对线程，并发控制做了一些限制等。获取到栈顶的Message对象之后开始执行：
msg.target.dispatchMessage(msg);
那么 msg.target 是什么呢？ 痛过追踪可以知道就是我们定义的Handler对象，然后我们查看一下Handler类的dispatchMessage 方法：
/** * 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); } } 复制代码 可以看到，如果我们设置了callback(Runnable对象)的话，则会直接调用handlerCallback方法：
private static void handleCallback(Message message) { message.callback.run(); } 复制代码 即，如果我们在初始化Handler的时候设置callback （Runnable）对象，则直接调用run方法。 比如我们经常写的runOnUiThread 方法：
runOnUiThread(new Runnable() { @Override public void run() { } }); 复制代码 看起内部实现：
public final void runOnUiThread(Runnable action) { if (Thread.currentThread() != mUiThread) { mHandler.post(action); } else { action.run(); } }复制代码 而如果msg.callback为空的话，会直接调用我们的mCallback.handleMessage(msg)，即handler的handlerMessage方法。由于Handler对象是在主线程中创建的，所以handler的handlerMessage方法的执行也会在主线程中。
总结
- 主线程中定义Handler,直接执行：
Handler mHandler = new Handler() { @Override public void handleMessage(Message msg) { super.handleMessage(msg); }};复制代码 而如果想要在子线程中定义Hanlder,则标准写法为：
// 初始化该线程Looper，MessageQueue，执行且只能执行一次 Looper.prepare(); // 初始化Handler对象，内部关联Looper对象 Handler mHandler = new Handler() { @Override public void handleMessage(Message msg) { super.handleMessage(msg); } }; // 启动消息队列出栈死循环 Looper.loop(); 复制代码 -
一个线程中只存在一个Looper对象，只存在一个MessageQueue对象，可以存在N个Handler对象，Handler对象内部关联了本线程中唯一的Looper对象，Looper对象内部关联着唯一的一个MessageQueue对象。
-
MessageQueue消息队列不是通过列表保存消息列表的，而且通过Message对象next属性关联下一个Message 从而实现列表的功能，同时所有的消息都是按时间来排序的。
-
Android中两个子线程相互交互同样可以通过Handler的异步消息机制实现，可以在线程A中定义Handler对象，而在线程B中获取handler的引用并调用sendMessage方法。
-
activity内部默认存在一个handler的成员变量，Android中的一些其他的异步消息机制实现方法：
Handler的post方法：
mHandler.post(new Runnable() { @Override public void run() { } }); 复制代码 查看其内部实现：
public final boolean post(Runnable r) { return sendMessageDelayed(getPostMessage(r), 0); } 复制代码 可以发现其内部调用就是sendMessage系列方法。
view的post方法：
public boolean post(Runnable action) { final AttachInfo attachInfo = mAttachInfo; if (attachInfo != null) { return attachInfo.mHandler.post(action); } // Assume that post will succeed later ViewRootImpl.getRunQueue().post(action); return true; }复制代码 可以发现其调用的就是activity中默认保存的handler对象的post方法。
activity的runOnUiThread方法：
public final void runOnUiThread(Runnable action) { if (Thread.currentThread() != mUiThread) { mHandler.post(action); } else { action.run(); } }复制代码 判断当前线程是否是UI线程，如果不是，则调用handler的post方法，否则直接执行run方法。