一 前言
对于Android开发者来说，在日常开发过程中不可避免会涉及到Android中的消息机制的场景应用，比如在子线程进行一些耗时操作，操作完成后需要在主线程更新UI，该场景想必大家都在实际开发过程中遇到过。另外，看过Android源码的开发者想必也知道，多处Android源码中也使用到消息机制进行通信，比如，Activity、Service的启动过程就都涉及到。因此，本文从源码层面分析Android中消息机制的工作原理。而要理解Android的消息机制的运行机制，需要从Looper , Handler , Message等工作原理进分析。下面先写一个使用示例，然后根据该示例进行源码跟踪。
二 示例展示
public class MainActivity extends Activity {
public static final String TAG = MainActivity.class.getSimpleName();
private Handler mainHandler;
private Handler threadHandler;
/**
* 在主线程中创建Handler，并实现对应的handleMessage方法
*/
public static Handler mainHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
Log.i(TAG, "主线程中接收到handler消息...")
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//子线程中创建handler
new Thread(new Runnable() {
@Override
public void run() {
threadHandler = new Handler();
}
}).start();
}
此时运行程序，你会发现，在子线程中创建的Handler是会导致程序崩溃的，提示的错误信息为 Can't create handler inside thread that has not called Looper.prepare() 。如何解决该问题，其实只需按提示信息所述，在当前线程中调用Looper.prepare()，即为当前线程创建了Looper。代码如下：
//子线程中创建handler
new Thread(new Runnable() {
@Override
public void run() {
Looper.prepare();
threadHandler = new Handler();
Looper.loop();
}
}).start();
另外，通过查看下Handler的源码，也可以弄清楚为什么不调用Looper.prepare()就crash。Handler的构造函数如下所示：
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，上述代码块中，调用了Looper.myLooper();语句，然后赋值给一个Looper对象mLooper ，如果mLooper实例为空，则会抛出一个运行时异常（Can’t create handler inside thread that has not called Looper.prepare()！）。
这里调用了Looper.myLooper()，那么我们接下来通过它来了解Looper的工作原理。
三 源码分析
接着上一小节分析，查看myLooper()方法的代码：
public static final Looper myLooper() {
return (Looper)sThreadLocal.get();
}
方法就是从sThreadLocal对象中取出Looper。如果sThreadLocal中有Looper存在就返回Looper，如果没有Looper存在自然就返回空了。因此，源码中肯定在某处给sThreadLocal设置Looper？显然是的，并且是在Looper.prepare()方法中进行设置，接着来看下它的源码：
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));
}
代码中首先判断sThreadLocal中是否已经存在Looper了，如果还没有则创建一个新的Looper设置进去。如果有也会抛出异常，即要求每个线程中最多有且只有一个Looper对象。其中，sThreadLocal是ThreadLocal的实例，后面会再专门介绍ThreadLocal，当前只要知道ThreadLocal的作用是要可以在每个线程中存储数据，并且不同线程中互不干扰。接着看Looper的构造函数源码：
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
在Looper构造函数创建了一个MessageQueue和获取当前Thread实例引用。MessageQueue是消息队列，即用于存储消息，以及以队列的形式对外提供入队及出队操作，后面再通过源码再分析。
通过以上子线程创建Handle的过程分析可知，子线程中需要先调用Looper.prepare()才能创建Handler对象。那么示例展示的主线程中mainHandler创建为何不崩溃呢？其原因是在程序启动的时候，系统已经帮我们自动调用了Looper.prepare()方法。我们可以查看ActivityThread中的main()方法，代码如下所示：
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，而这个方法又会再去调用Looper.prepare()方法，代码如下所示：
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，ActivityThread被创建时变会初始化Looper，从而不需要再手动去调用Looper.prepare()方法了。
此外，ActivityThread中的main()方法中最后调用了 Looper.loop()，它是Looper中一个重要方法，只有调用了loop后消息循环系统才会真正的起作用，它的源码实现如下：
/**
* 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();
}
}
这个方法进入了一个死循环，然后不断地调用的MessageQueue的next()方法，MessageQueue的next()方法即是出队方法，源码如下：
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;
}
}
方法中判断当前MessageQueue中如果存在mMessages(即待处理消息)，就将这个消息出队，然后让下一条消息成为mMessages，否则就进入一个阻塞状态，一直等到有新的消息入队。
回到Looper类的loop()方法，如果next()方法返回的msg等于null，就退出该循环，否则每当有一个消息出队，接着就将它传递到msg.target的dispatchMessage()方法中，那这里msg.target又是什么呢？其实就是Handler啦，也就是回调到Handler进行处理消息，那么如何处理呢？在介绍由Handler的dispatchMessage()方法进行处理消息之前，我们首先得知道如何向MessageQueue中插入消息，即Handler如何发送消息？示例代码如下：
new Thread() {
@Override
public void run() {
// 在子线程中发送异步消息
Message message = new Message();
message.arg1 = 1;
mainHandler.sendMessage(message);
}
}.start();
示例中通过sendMessage()方法发送消息，并且为什么最后又可以在Handler的handleMessage()方法中重新得到这条Message？接着就通过发送消息这个源码流程来分析其原因。Handler中提供了很多个发送消息的方法，除了sendMessageAtFrontOfQueue()方法之外，其它的发送消息方法最终都会辗转调用到sendMessageAtTime()方法中，分别看下这两个方法的源码，sendMessageAtTime源码如下
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方法源码
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);
}
对比上面两个方法，表面上说Handler的sendMessageAtFrontOfQueue方法和其他发送方法不同，其实实质是相同的，仅仅是sendMessageAtFrontOfQueue方法是sendMessageAtTime方法的一个特例而已，即sendMessageAtTime最后一个参数uptimeMillis传递0。
因此，只要分析sendMessageAtTime这个方法，sendMessageAtTime(Message msg, long uptimeMillis)方法有两个参数：
msg：是我们发送的Message对象，
uptimeMillis：表示发送消息的时间，uptimeMillis的值等于从系统开机到当前时间的毫秒数再加上延迟时间。
最后，该方法返回一个enqueueMessage方法的返回值，那么enqueueMessage()方法毫无疑问就是入队的方法了，我们来看下这个方法的源码：
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对象本身；queue就是上一步传过来的mQueue，而mQueue是在Handler实例化时构造函数中实例化的MessageQueue实例。在Handler的构造函数中可以看见mQueue = mLooper.mQueue;接着就调用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;
}
可以看到这里MessageQueue根据时间将所有的Message排序，然后使用链表的形式将所有的Message保存起来。
介绍完Handle发送消息后，把消息插入到MessageQueue消息队列后，那么回到之前loop方法中不断的从MessageQueue中通过next()取消息，然后交由msg.target的dispatchMessage()方法处理。由之前分析可知msg.target指的是Handle对象，那么看下Handle中dispatchMessage()方法的源码：
/**
* 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方法中的逻辑比较简单，具体就是如果msg.callback不为空，则调用handleCallback()方法处理，如果mCallback不为空，则调用mCallback的handleMessage()方法，否则直接调用Handler的handleMessage()方法，并将消息对象作为参数传递过去。这样整个异步消息流程就串起来了。
四 总结
1.主线程中可以直接定义Handler，但如果想要在子线程中定义Handler，刚在创建Handler前需要调用Looper.prepare();，子线程中的标准的写法形式为：
//子线程中创建handler
new Thread(new Runnable() {
@Override
public void run() {
Looper.prepare();
threadHandler = new Handler();
Looper.loop();
}
}).start();
2.一个线程中只存在一个Looper对象，只存在一个MessageQueue对象，但可以可以有多个Handler对象，即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。
3.MessageQueue消息队列是按照时间排序以链表的形式进行消息存储的。
