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:

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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:

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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:

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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:

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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:

  1. 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。
  2. 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。
  3. 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:

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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:

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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:

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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:

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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:

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

而NativeMessageQueue的构造函数是:

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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:

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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:

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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:

  1. 调用epoll_wait方法等待所监听的fd的写入,其方法原型如下:
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    int epoll_wait(int epfd, struct epoll_event * events, intmaxevents, int timeout)

调用的方法参数为:

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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

  1. 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.
  2. 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。
  3. 处理NativeMessageQueue的消息,这些消息是native层的消息
  4. 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:

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//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

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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:

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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:

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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:

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//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:

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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。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。