Android MessageQueue源码分析

Android应用开发中离不开Handler，而Handler实际上最终是将Message交给MessageQueue。MessageQueue是Android消息机制的核心，熟悉MessageQueue能够帮助我们更清楚详细地理解Android的消息机制。这篇文章会介绍MessageQueue消息的插入(enqueueMessage)和读取(next)，native层的消息机制，以及IdleHandler和SyncBarrier的逻辑原理。源码是基于6.0。

MessageQueue的next与enqueueMessage方法

MessageQueue enqueueMessage

每次使用Handler发送一个Message的时候，最终会先调用MessageQueue的enqueueMessage方法将Message方法放入到MessageQueue里面。先看Handler的sendMessage方法，其他发送Message的内容也是一样的:

public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0); // 调用下面这个方法
}

public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis); // 调用下面方法
}

public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
MessageQueue queue = mQueue; //Handler中的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); // 下面方法
}

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis); //调用MessageQueue的enqueueMessage
}

最后会调用Handler的mQueue的enqueueMessage方法，而Handler的mQueue是从哪里来的呢？在Handler的构造函数中设置的，看默认的情况:

public Handler() {
this(null, false);
}

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;
}

无参Handler构造函数对应的是当前调用无参Handler构造函数线程的Looper，Looper是一个ThreadLocal变量，也就是说但是每个线程独有的，每个线程调用了Looper.prepare方法后，就会给当前线程设置一个Looper:

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));
}

Looper里面包含了一个MessageQueue, 在Handler的构造函数中，会将当前关联的Looper的MessageQueue赋值给Handler的成员变量mQueue，enqueueMessage的时候就是调用该mQueue的enqueueMessage。关于Handler与Looper可以理解为每个Handler会关联一个Looper，每个线程最多只有一个Looper。Looper创建的时候会创建一个MessageQueue，而发送消息的时候，Handler就会通过调用mQueue.enqueueMessage方法将Message放入它关联的Looper的MessageQueue里面。介绍了Handler与Looper，然后继续看看MessageQueue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;
}

整个enqueueMessage方法的过程就是先持有MessageQueue.this锁，然后将Message放入队列中，放入队列的过程是：

如果队列为空，或者当前处理的时间点为0（when的数值，when表示Message将要执行的时间点），或者当前Message需要处理的时间点先于队列中的首节点，那么就将Message放入队列首部，否则进行第2步。

遍历队列中Message，找到when比当前Message的when大的Message，将Message插入到该Message之前，如果没找到则将Message插入到队列最后。

判断是否需要唤醒，一般是当前队列为空的情况下，next那边会进入睡眠，需要enqueue这边唤醒next函数。后面会详细介绍

执行完后，会释放持有的MessageQueue.this的锁。这样整个enqueueMessage方法算是完了，然后看看读取Message的MessageQueue的next方法。

MessageQueue的next方法

MessageQueue的next方法是从哪里调用的呢？先看一个线程对Looper的标准用法是:

class LoopThread extends Thread{

public Handler mHandler;

public void run(){

Looper.prepare();

mHandler = new Handler() {
public void handleMessage(Message msg) {
// process incoming messages here
}
};

Looper.loop();

}

}

prepare方法我们前面已经看过了，就是初始化ThreadLocal变量Looper。loop()方法就是循环读取MessageQueue中Message，然后处理每一个Message。我们看看Looper.loop方法源码:

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 此处就是next方法调用的地方
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();
}
}

整个loop函数大概的过程就是先调用MessageQueue.next方法获取一个Message，然后调用Message的target的dispatchMessage方法来处理Message，Message的target就是发送这个Message的Handler。处理的过程是先看Message的callback有没有实现，如果有，则使用调用callback的run方法，如果没有则看Handler的callback是否为空，如果非空，则使用handler的callback的handleMessage方法来处理Message，如果为空，则调用Handler的handleMessage方法处理。

我们主要看next，从注释来看，next方法可能会阻塞，先看next方法的源码：

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;  //mPrt是native层的MessageQueue的指针
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); // jni函数

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) { //target 正常情况下都不会为null，在postBarrier会出现target为null的Message
// 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; // 等待时间无限长
}

// 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++) { //运行idle
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;
}
}

整个next函数的主要是执行步骤是:

step1: 初始化操作，如果mPtr为null，则直接返回null，设置nextPollTimeoutMillis为0，进入下一步。

step2: 调用nativePollOnce, nativePollOnce有两个参数,第一个为mPtr表示native层MessageQueue的指针，nextPollTimeoutMillis表示超时返回时间，调用这个nativePollOnce会等待wake，如果超过nextPollTimeoutMillis时间，则不管有没有被唤醒都会返回。-1表示一直等待，0表示立刻返回。下一小节单独介绍这个函数。

step3: 获取队列的头Message(msg)，如果头Message的target为null，则查找一个异步Message来进行下一步处理。当队列中添加了同步Barrier的时候target会为null。

step4: 判断上一步获取的msg是否为null，为null说明当前队列中没有msg，设置等待时间nextPollTimeoutMillis为-1。实际上是等待enqueueMessage的nativeWake来唤醒，执行step4。如果非null，则下一步

step5: 判断msg的执行时间(when)是否比当前时间(now)的大，如果小，则将msg从队列中移除，并且返回msg，结束。如果大则设置等待时间nextPollTimeoutMillis为(int) Math.min(msg.when - now, Integer.MAX_VALUE)，执行时间与当前时间的差与MAX_VALUE的较小值。执行下一步

step6: 判断是否MessageQueue是否已经取消，如果取消的话则返回null，否则下一步

step7: 运行idle Handle，idle表示当前有空闲时间的时候执行，而运行到这一步的时候，表示消息队列处理已经是出于空闲时间了（队列中没有Message，或者头部Message的执行时间(when)在当前时间之后）。如果没有idle，则继续step2，如果有则执行idleHandler的queueIdle方法，我们可以自己添加IdleHandler到MessageQueue里面（addIdleHandler方法），执行完后，回到step2。

需要说的时候，我们平常只是使用Message，但是实际上IdleHandler如果使用的好，应该会达到意想不到的效果，它表示MessageQueue有空闲时间的时候执行一下。然后介绍一下nativePollOnce与nativeWake方法

native层机制

nativePollOnce与nativeWake是两个jni方法，这两个方法jni实现方法在frameworks/base/core/jni/android_os_MessageQueue.cpp。这个是MessageQueue的native层内容。native层的NativeMessageQueue初始化是在nativeInit方法：

static jlong android_os_MessageQueue_nativeInit(JNIEnv* env, jclass clazz) {
NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue();
if (!nativeMessageQueue) {
jniThrowRuntimeException(env, "Unable to allocate native queue");
return 0;
}

nativeMessageQueue->incStrong(env);
return reinterpret_cast<jlong>(nativeMessageQueue);
}

对应的java层方法是nativeInit，在MessageQueue构造函数的时候调用：

MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}

而NativeMessageQueue的构造函数是:

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。Looper的源码在system/core/libutils/Looper.cpp。Looper通过epoll_create创建了一个mEpollFd作为epoll的fd，并且创建了一个mWakeEventFd，用来监听java层的wake，同时可以通过Looper的addFd方法来添加新的fd监听。

nativePollOnce

nativePollOnce是每次调用next方法获取消息的时候调用的:

static void android_os_MessageQueue_nativePollOnce(JNIEnv* env, jobject obj,
jlong ptr, jint timeoutMillis) {
NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
nativeMessageQueue->pollOnce(env, obj, timeoutMillis);
}

void NativeMessageQueue::pollOnce(JNIEnv* env, jobject pollObj, int timeoutMillis) {
mPollEnv = env;
mPollObj = pollObj;
mLooper->pollOnce(timeoutMillis);
mPollObj = NULL;
mPollEnv = NULL;

if (mExceptionObj) {
env->Throw(mExceptionObj);
env->DeleteLocalRef(mExceptionObj);
mExceptionObj = NULL;
}
}

这个方法的native层方法最终会调用Looper的pollOnce:

int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) {
int result = 0;
for (;;) {
while (mResponseIndex < mResponses.size()) {
const Response& response = mResponses.itemAt(mResponseIndex++);
int ident = response.request.ident;
if (ident >= 0) {
int fd = response.request.fd;
int events = response.events;
void* data = response.request.data;
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ pollOnce - returning signalled identifier %d: "
"fd=%d, events=0x%x, data=%p",
this, ident, fd, events, data);
#endif
if (outFd != NULL) *outFd = fd;
if (outEvents != NULL) *outEvents = events;
if (outData != NULL) *outData = data;
return ident;
}
}

if (result != 0) {
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ pollOnce - returning result %d", this, result);
#endif
if (outFd != NULL) *outFd = 0;
if (outEvents != NULL) *outEvents = 0;
if (outData != NULL) *outData = NULL;
return result;
}

result = pollInner(timeoutMillis);
}
}

int Looper::pollInner(int timeoutMillis) {
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ pollOnce - waiting: timeoutMillis=%d", this, timeoutMillis);
#endif

// Adjust the timeout based on when the next message is due.
if (timeoutMillis != 0 && mNextMessageUptime != LLONG_MAX) {
nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);
int messageTimeoutMillis = toMillisecondTimeoutDelay(now, mNextMessageUptime);
if (messageTimeoutMillis >= 0
&& (timeoutMillis < 0 || messageTimeoutMillis < timeoutMillis)) {
timeoutMillis = messageTimeoutMillis;
}
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ pollOnce - next message in %" PRId64 "ns, adjusted timeout: timeoutMillis=%d",
this, mNextMessageUptime - now, timeoutMillis);
#endif
}

// Poll.
int result = POLL_WAKE;
mResponses.clear();
mResponseIndex = 0;

// We are about to idle.
mPolling = true;

struct epoll_event eventItems[EPOLL_MAX_EVENTS];
int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);

// No longer idling.
mPolling = false;

// Acquire lock.
mLock.lock();

// Rebuild epoll set if needed.
if (mEpollRebuildRequired) {
mEpollRebuildRequired = false;
rebuildEpollLocked();
goto Done;
}

// Check for poll error.
if (eventCount < 0) {
if (errno == EINTR) {
goto Done;
}
ALOGW("Poll failed with an unexpected error, errno=%d", errno);
result = POLL_ERROR;
goto Done;
}

// Check for poll timeout.
if (eventCount == 0) {
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ pollOnce - timeout", this);
#endif
result = POLL_TIMEOUT;
goto Done;
}

// Handle all events.
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ pollOnce - handling events from %d fds", this, eventCount);
#endif

for (int i = 0; i < eventCount; i++) {
int fd = eventItems[i].data.fd;
uint32_t epollEvents = eventItems[i].events;
if (fd == mWakeEventFd) {
if (epollEvents & EPOLLIN) {
awoken();
} else {
ALOGW("Ignoring unexpected epoll events 0x%x on wake event fd.", epollEvents);
}
} else {
ssize_t requestIndex = mRequests.indexOfKey(fd);
if (requestIndex >= 0) {
int events = 0;
if (epollEvents & EPOLLIN) events |= EVENT_INPUT;
if (epollEvents & EPOLLOUT) events |= EVENT_OUTPUT;
if (epollEvents & EPOLLERR) events |= EVENT_ERROR;
if (epollEvents & EPOLLHUP) events |= EVENT_HANGUP;
pushResponse(events, mRequests.valueAt(requestIndex));
} else {
ALOGW("Ignoring unexpected epoll events 0x%x on fd %d that is "
"no longer registered.", epollEvents, fd);
}
}
}
Done: ;

// Invoke pending message callbacks.
mNextMessageUptime = LLONG_MAX;
while (mMessageEnvelopes.size() != 0) {
nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);
const MessageEnvelope& messageEnvelope = mMessageEnvelopes.itemAt(0);
if (messageEnvelope.uptime <= now) {
// Remove the envelope from the list.
// We keep a strong reference to the handler until the call to handleMessage
// finishes.  Then we drop it so that the handler can be deleted *before*
// we reacquire our lock.
{ // obtain handler
sp<MessageHandler> handler = messageEnvelope.handler;
Message message = messageEnvelope.message;
mMessageEnvelopes.removeAt(0);
mSendingMessage = true;
mLock.unlock();

#if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS
ALOGD("%p ~ pollOnce - sending message: handler=%p, what=%d",
this, handler.get(), message.what);
#endif
handler->handleMessage(message);
} // release handler

mLock.lock();
mSendingMessage = false;
result = POLL_CALLBACK;
} else {
// The last message left at the head of the queue determines the next wakeup time.
mNextMessageUptime = messageEnvelope.uptime;
break;
}
}

// Release lock.
mLock.unlock();

// Invoke all response callbacks.
for (size_t i = 0; i < mResponses.size(); i++) {
Response& response = mResponses.editItemAt(i);
if (response.request.ident == POLL_CALLBACK) {
int fd = response.request.fd;
int events = response.events;
void* data = response.request.data;
#if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS
ALOGD("%p ~ pollOnce - invoking fd event callback %p: fd=%d, events=0x%x, data=%p",
this, response.request.callback.get(), fd, events, data);
#endif
// Invoke the callback.  Note that the file descriptor may be closed by
// the callback (and potentially even reused) before the function returns so
// we need to be a little careful when removing the file descriptor afterwards.
int callbackResult = response.request.callback->handleEvent(fd, events, data);
if (callbackResult == 0) {
removeFd(fd, response.request.seq);
}

// Clear the callback reference in the response structure promptly because we
// will not clear the response vector itself until the next poll.
response.request.callback.clear();
result = POLL_CALLBACK;
}
}
return result;
}

这个方法超长，但实际上Looper的pollOnce方法主要有5步：

调用epoll_wait方法等待所监听的fd的写入，其方法原型如下:

int epoll_wait(int epfd, struct epoll_event * events, intmaxevents, int timeout)

调用的方法参数为:

int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);

eventItems里面就包含了mWakeEvent和通过addFd添加fd时加入的Event。该方法会阻塞，当timeoutMillis(对应java层的nextPollTimeoutMillis)到了时间，该方法会返回，或者eventItems有事件来了，该方法会返回。返回之后就是干下一件事

判断有没有event，因为可能是timeoutMillis到了返回的，如果没有直接进行4.

读取eventItems的内容，如果eventItem的fd是mWakeEventFd，则调用awoken方法，读取Looper.wake写入的内容，如果是其他的fd，则使用pushResponse来读取，并且将内容放入Response当中。

处理NativeMessageQueue的消息，这些消息是native层的消息

处理pushResponse写入的内容。

里面主要是干了三件事处理wakeEventFd的输入内容，其他fd的输入内容，以及NativeMessageQueue里面的Message。

nativeWake

实际上最后就是调用了Looper的wake方法:

//android_os_MessageQueue.cpp

static void android_os_MessageQueue_nativeWake(JNIEnv* env, jclass clazz, jlong ptr) {
NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
nativeMessageQueue->wake();
}

void NativeMessageQueue::wake() {
mLooper->wake();
}

//Looper.cpp

void Looper::wake() {
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ wake", this);
#endif

uint64_t inc = 1;
ssize_t nWrite = TEMP_FAILURE_RETRY(write(mWakeEventFd, &inc, sizeof(uint64_t)));
if (nWrite != sizeof(uint64_t)) {
if (errno != EAGAIN) {
ALOGW("Could not write wake signal, errno=%d", errno);
}
}
}

这样native层的消息队列就算是完了。

SyncBarrier

我们在next方法里面看到有这么一段代码

if (msg != null && msg.target == null) { //target 正常情况下都不会为null，在postBarrier会出现target为null的Message
// Stalled by a barrier.  Find the next asynchronous message in the queue.
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}

什么时候msg.target会为null呢？有sync barrier消息的时候，实际上msg.target为null表示sync barrier(同步消息屏障)。MessageQueue有一个postSyncBarrier方法:

public int postSyncBarrier() {
return postSyncBarrier(SystemClock.uptimeMillis());
}

private int postSyncBarrier(long when) {
// Enqueue a new sync barrier token.
// We don't need to wake the queue because the purpose of a barrier is to stall it.
synchronized (this) {
final int token = mNextBarrierToken++;
final Message msg = Message.obtain();
msg.markInUse();
msg.when = when;
msg.arg1 = token;

Message prev = null;
Message p = mMessages;
if (when != 0) {
while (p != null && p.when <= when) {
prev = p;
p = p.next;
}
}
if (prev != null) { // invariant: p == prev.next
msg.next = p;
prev.next = msg;
} else {
msg.next = p;
mMessages = msg;
}
return token;
}
}

对应有removeSyncBarrier方法:

public void removeSyncBarrier(int token) {
// Remove a sync barrier token from the queue.
// If the queue is no longer stalled by a barrier then wake it.
synchronized (this) {
Message prev = null;
Message p = mMessages;
while (p != null && (p.target != null || p.arg1 != token)) {
prev = p;
p = p.next;
}
if (p == null) {
throw new IllegalStateException("The specified message queue synchronization "
+ " barrier token has not been posted or has already been removed.");
}
final boolean needWake;
if (prev != null) {
prev.next = p.next;
needWake = false;
} else {
mMessages = p.next;
needWake = mMessages == null || mMessages.target != null;
}
p.recycleUnchecked();

// If the loop is quitting then it is already awake.
// We can assume mPtr != 0 when mQuitting is false.
if (needWake && !mQuitting) {
nativeWake(mPtr);  // 需要唤醒，因为队首元素是SyncBarrier，队列中有消息但是没有异步消息的时候，next方法同样会阻塞等待。
}
}
}

看next方法的源码，每次消息队列中有barrier的时候，next会寻找队列中的异步消息来处理。如果没有异步消息，设置nextPollTimeoutMillis = -1，进入阻塞等待新消息的到来。异步消息主要是系统发送的，而系统中的异步消息主要有触摸事件，按键事件的消息。系统中调用postSyncBarrier和removeSyncBarrier主要实在ViewRootImpl的scheduleTraversals和unscheduleTraversals，以及doTraversals方法中。从源码可以猜到每次调用postSyncBarrier后都会调用removeSyncBarrier，不然同步消息就没法执行了（看next源码理解这一点）。可以看一下scheduleTraversal方法：

//ViewRootImpl.java

void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
if (!mUnbufferedInputDispatch) {
scheduleConsumeBatchedInput();
}
notifyRendererOfFramePending();
pokeDrawLockIfNeeded();
}
}

实际上MessageQueue的源码一直在变化的，2.3才加入了native层的Message，在4.0.1还没有SyncBarrier，4.1才开始加入SyncBarrier的，而且MessageQueue没有postSyncBarrier方法，只有enqueueSyncBarrier方法，Looper里面有个postSyncBarrier方法。

SyncBarrier的意义

前面介绍了一下每个版本的特点，我想介绍一种SyncBarrier的意义，我们介绍了使用SyncBarrier主要是ViewRootImpl中的scheduleTraversal的时候，那是跟UI事件相关的，像派发消息会通过发送Message发到主线程：

public void dispatchInputEvent(InputEvent event, InputEventReceiver receiver) {
SomeArgs args = SomeArgs.obtain();
args.arg1 = event;
args.arg2 = receiver;
Message msg = mHandler.obtainMessage(MSG_DISPATCH_INPUT_EVENT, args);
msg.setAsynchronous(true);
mHandler.sendMessage(msg);
}

注意它这里就是使用的异步Message，使用了

msg.setAsyncronous(true)

。 而SyncBarrier有什么用处呢？我们刚刚介绍的时候，当消息队列的第一个Message的target的时候，表示它是一个SyncBarrier，它会阻拦同步消息，而选择队列中第一个异步消息处理，如果没有则会阻塞。这表示什么呢？这是表示第一个Message是SyncBarrier的时候，会只处理异步消息。而我们前面介绍了InputEvent的时候，它就是异步消息，在有SyncBarrier的时候就会被优先处理。所以在调用了scheduleTraversal的时候，就会只处理触摸事件这些消息了，保证用户体验。保证了触摸事件及时处理，实际上这也能减少ANR。如果这个时候MessageQueue中有很多Message，也能够及时处理那些触摸事件的Message了。

总结

MessageQueue是Android消息消息机制的内部核心，理解好MessageQueue更能理解好Android应用层的消息逻辑。MessageQueue大部分情况下插入消息是插入队列首部的。另外MessageQueue的代码一直在不断地变化，对照不同版本的代码，真的能领略代码改变时的目的，从演变中学习。