Messsage、MessageQueue、Looper、Handler的工作原理就像工厂的生产线，Looper就像发送机，MessageQueue相当于传送带，Handler相当于工人，Message相当于待处理的产品。
如下图：
主线程looper创建过程：
#ActivityThread
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
// 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);
Environment.initForCurrentUser();
// Set the reporter for event logging in libcore
EventLogger.setReporter(new EventLoggingReporter());
// Make sure TrustedCertificateStore looks in the right place for CA certificates
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
Process.setArgV0("<pre-initialized>");
//1.创建消息循环Looper，就是UI线程的消息队列
Looper.prepareMainLooper();
//启动ActivityThread，这里最终会启动应用程序。
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
/// M: ANR Debug Mechanism
mAnrAppManager.setMessageLogger(Looper.myLooper());
//2.执行消息循环
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}执行ActivityThread.main方法后，应用程序就启动了，UI线程的消息循环也在Looper.loop函数中启动。此后，Looper会一直从消息队列中取出消息，然后处理消息。用户或者系统通过Handler不断往消息队列中添加消息，这些消息不断被取出、处理、回收，使得应用迅速运转起来。
#Handler
/**
* Use the {@link Looper} for the current thread with the specified callback interface
* and set whether the handler should be asynchronous.
*（使用{@link Looper}作为当前线程和指定的回调接口，并设置处理程序是否应该是异步的。）
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*(默认情况下，处理程序是同步的，除非使用此构造函数来生成严格异步的处理程序。)(这个与障碍阻塞机制有关吗)
* Asynchronous messages represent interrupts or events that do not require global ordering
* with respect to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*(异步消息表示不需要关于同步消息的全局排序的中断或事件。 异步消息不受{@link MessageQueue #enqueueSyncBarrier（long）}引入的同步障碍的影响。)
* @param callback The callback interface in which to handle messages, or null.
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @hide
*/
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(); //获取Looper
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue; //获取消息队列
mCallback = callback;
mAsynchronous = async;
}空参的构造函数主要用于主线程，因为主线程的Looper在ActivityThread的main方法中就被创建了。
从Handler默认 的构造函数中可以看到，Handler会在内部通过Looper.myLooper()来获取Looper对象，并且与之关联，最重要的就是获取到Looper持有的消息队列mQueue。
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/（返回与当前线程关联的Looper对象。 如果调用线程未与Looper关联，则返回null。）
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
/**
* Initialize the current thread as a looper, marking it as an
* application's main looper. The main looper for your application
* is created by the Android environment, so you should never need
* to call this function yourself. See also: {@link #prepare()}
*/（将当前线程初始化为looper，将其标记为应用程序的主循环。 应用程序的主要循环器是由Android环境创建的，因此您永远不需要自己调用此函数。）
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}/** 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()}.
*/(将当前线程初始化为looper。这使您有机会创建handlers，然后在实际启动looper之前关联此循环器。)
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));
}
是在Looper的prepareMainLooper()方法中，在这个方法中又调用了 prepare(false)方法，在 prepare(false)方法中创建looper并设置到ThreadLocal中，这样，队列就与线程关联上了。
再回到Handler中来，Looper属于某个线程，消息队列存储在Looper，因此，消息队列就通过Looper与特定线程关联上。而Handler又与Looper、消息队列关联，因此，Handler最终就和线程、线程的消息队列关联上了，通过该Handler发送的消息最好就会被执行在这个线程上。
创建了Looper之后，会调用Looper的loop函数，在这个函数中会不断从消息队列中取出、处理消息，具体源码如下：
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/(在此线程中运行消息队列。 务必调用{@link #quit（）}来结束循环。)
public static void loop() {
//1.获取与当前线程关联的looper
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
//2.获取与当前线程相关联的消息队列
final MessageQueue queue = me.mQueue;//获取消息队列
// 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 (;;) {//死循环，即消息循环
//3.获取消息(might block)
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 slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
final long traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
final long end;
try {
msg.target.dispatchMessage(msg);//处理消息
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (slowDispatchThresholdMs > 0) {
final long time = end - start;
if (time > slowDispatchThresholdMs) {
Slog.w(TAG, "Dispatch took " + time + "ms on "
+ Thread.currentThread().getName() + ", h=" +
msg.target + " cb=" + msg.callback + " msg=" + msg.what);
}
}
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);
}
//5.回收消息，也就是我们分析享元模式中提到的将Message添加到消息池的操作
msg.recycleUnchecked();
}
}#MessageQueue
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();
}
// 1.处理Native层的事件
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
//2.Java层消息队列
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) {
//msg这个消息有延迟，因此做了一个延迟处理
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;
}
}
next函数的基本思路是从消息队列中依次取出消息，如果这个消息到了执行时间，那么将这条消息返回给Looper，并且将消息列表的指针后移。这个消息队列链表结构与Message中的消息池结构一致。也是通过Message的next字段 将多个Message对象 串连在一起。但是在从消息队列获取消息之前，还有一个nativePollOnce函数调用，第一个参数是mPtr，第二个参数是超时时间。
mPtr存储了Native层的消息队列对象，也就是说Native层还有一个MessageQueue类型。mPtr的初始化是在MessageQueue的构造函数中，如下：
private native static long nativeInit();
//消息队列构造
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}mPtr的值是在nativeInit() 函数中返回。
#android_os_MessageQueue.cpp
static jlong android_os_MessageQueue_nativeInit(JNIEnv* env, jclass clazz) {
//构造NativeMessageQueue
NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue();
if (!nativeMessageQueue) {
jniThrowRuntimeException(env, "Unable to allocate native queue");
return 0;
}
nativeMessageQueue->incStrong(env);
//2.将NativeMessageQueue对象转换为一个整型变量
return reinterpret_cast<jlong>(nativeMessageQueue);
}
我们看到，在nativeInit函数中构造一个NativeMessageQueue 对象，然后将该对象转换为一个整型值，并且返回给Java层，而当Java层需要与Native层的MessageQueue通信时，只要把这个int值传递给Native层，然后Native层通过reinterpret_cast将传递进来的int转换为NativeMessageQueue 指针即可得到这个NativeMessageQueue 对象指针。
下面看一下NativeMessageQueue类的构造函数
NativeMessageQueue::NativeMessageQueue() :
mPollEnv(NULL), mPollObj(NULL), mExceptionObj(NULL) {
mLooper = Looper::getForThread();
if (mLooper == NULL) {
mLooper = new Looper(false);
Looper::setForThread(mLooper);
}
}代码很简单，创建了一个Native层的Looper，然后将这个Looper设置给了当前线程。也就是说java层的MessageQueue和Looper在Native层也都有，但是功能并不是一一对应的。
下面看一下NativeMessageQueue类的构造函数
NativeMessageQueue::NativeMessageQueue() :
mPollEnv(NULL), mPollObj(NULL), mExceptionObj(NULL) {
mLooper = Looper::getForThread();
if (mLooper == NULL) {
mLooper = new Looper(false);
Looper::setForThread(mLooper);
}
}创建了一个Native层的Looper，然后将这个Looper设置给了当前线程。也就是说java层的MessageQueue和Looper在Native层也都有，但是功能并不是一一对应的。
Looper::Looper(bool allowNonCallbacks) :
mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false),
mPolling(false), mEpollFd(-1), mEpollRebuildRequired(false),
mNextRequestSeq(0), mResponseIndex(0), mNextMessageUptime(LLONG_MAX) {
//创建管道， 管道读写端
mWakeEventFd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);
LOG_ALWAYS_FATAL_IF(mWakeEventFd < 0, "Could not make wake event fd: %s",
strerror(errno));
AutoMutex _l(mLock);
rebuildEpollLocked();
}
void Looper::rebuildEpollLocked() {
// Close old epoll instance if we have one.
if (mEpollFd >= 0) {
#if DEBUG_CALLBACKS
ALOGD("%p ~ rebuildEpollLocked - rebuilding epoll set", this);
#endif
close(mEpollFd);
}
// Allocate the new epoll instance and register the wake pipe.
//2.创建epoll文件描述符
mEpollFd = epoll_create(EPOLL_SIZE_HINT);
LOG_ALWAYS_FATAL_IF(mEpollFd < 0, "Could not create epoll instance: %s", strerror(errno));
struct epoll_event eventItem;
memset(& eventItem, 0, sizeof(epoll_event)); // zero out unused members of data field union
//设置事件类型和文件描述符
eventItem.events = EPOLLIN;
eventItem.data.fd = mWakeEventFd;
//监听事件
int result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeEventFd, & eventItem);
LOG_ALWAYS_FATAL_IF(result != 0, "Could not add wake event fd to epoll instance: %s",
strerror(errno));
for (size_t i = 0; i < mRequests.size(); i++) {
const Request& request = mRequests.valueAt(i);
struct epoll_event eventItem;
request.initEventItem(&eventItem);
int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, request.fd, & eventItem);
if (epollResult < 0) {
ALOGE("Error adding epoll events for fd %d while rebuilding epoll set: %s",
request.fd, strerror(errno));
}
}
}sp<Looper> Looper::getForThread() {
int result = pthread_once(& gTLSOnce, initTLSKey);
LOG_ALWAYS_FATAL_IF(result != 0, "pthread_once failed");
return (Looper*)pthread_getspecific(gTLSKey);
}
void Looper::setForThread(const sp<Looper>& looper) {
sp<Looper> old = getForThread(); // also has side-effect of initializing TLS
if (looper != NULL) {
looper->incStrong((void*)threadDestructor);
}
pthread_setspecific(gTLSKey, looper.get());
if (old != NULL) {
old->decStrong((void*)threadDestructor);
}
}首先创建一个管道(pipe)，管道本质上是一个文件，一个管道中包含两个文件描述符，分别对应读和写。一般的使用方式是一个线程通过读文件描述符来读取管道内容，当管道没有内容时，这个线程就会进入等待状态；而另一个线程通过写文件描述符来向管道中写入内容，写入内容的时候，如果另一端正有线程正在等待管道中的内容，那么这个线程就会被唤醒。
这个等待和唤醒的操作是通过linux系统epoll机制完成的。要使用Linux系统的epoll操作，首先要通过epoll_create来创建一个epoll专用的文件描述符。最后通过epoll_ctl函数设置监听的事件类型为EPOLLIN。
此时Native层的MessageQueue和Looper就构建完毕了，在底层也通过和epoll建立来了一套消息机制。Native层构建完毕后，则会返回到Java层Looper的构造函数，因此，Java层的Looper和MessageQueue也构建完毕。
总结一下：
1.首先构造Java层的Looper对象，Java层looper对象又会在构造 函数中创建Java层的MessageQueue对象。
2.Java层的MessageQueue的构造函数中调用nativeInit函数初始化Native层的NativeMessageQueue，
NativeMessageQueue的构造函数又会建立Native层的looper，并且通过管道和epoll建立一套消息机制。
3.Native层构建完毕后，将NativeMessageQueue对象转化为一个整型值存储到Java层的MessageQueue的mPtr中。
4.启动Java层的消息循环，不断读取、处理消息。
这个初始化过程都是在ActivityThread的main函数中完成的，因此，main函数运行之后，UI线程的消息循环就启动了，消息循环不断从消息队列中读取、处理消息，使得系统运转起来。
参考《Android源码设计模式》
