1 enqueueMessage
handler發送一條消息
mHandler.sendEmptyMessage(1);Handler.java
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);
}最後調用到Handler私有的函數enqueueMessage，把handler對象賦值給msg.target，調用queue.enqueueMessage
Handler.javaprivate boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}下面是核心代碼，首先是獲得同步鎖，
MessageQueue.java
boolean enqueueMessage(Message msg, long when) {
if (msg.isInUse()) {
throw new AndroidRuntimeException(msg + " This message is already in use.");
}
if (msg.target == null) {
throw new AndroidRuntimeException("Message must have a target.");
}
synchronized (this) {
if (mQuitting) {
RuntimeException e = new RuntimeException(
msg.target + " sending message to a Handler on a dead thread");
Log.w("MessageQueue", e.getMessage(), e);
return false;
}
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;
}首先是獲得自身的同步鎖synchronized (this)，接着這個msg跟MessageQueue實例的頭結點Message進行觸發時間先後的比較，
如果觸發時間比現有的頭結點Message前，則這個新的Message作爲整個MessageQueue的頭結點，如果阻塞着，則立即喚醒線程處理
如果觸發時間比頭結點晚，則按照觸發時間先後，在消息隊列中間插入這個結點
接着如果需要喚醒，則調用nativeWake函數
在android_os_MessageQueue.cpp裏定義了nativeWake函數
static void android_os_MessageQueue_nativeWake(JNIEnv* env, jobject obj, jint ptr) {
NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
return nativeMessageQueue->wake();
}實際調用到mLooper->wake();
android_os_MessageQueue.cpp
void NativeMessageQueue::wake() {
mLooper->wake();
}framework/base/libs/utils/Looper.cpp
void Looper::wake() {
#if DEBUG_POLL_AND_WAKE
LOGD("%p ~ wake", this);
#endif
#ifdef LOOPER_STATISTICS
// FIXME: Possible race with awoken() but this code is for testing only and is rarely enabled.
if (mPendingWakeCount++ == 0) {
mPendingWakeTime = systemTime(SYSTEM_TIME_MONOTONIC);
}
#endif
ssize_t nWrite;
do {
nWrite = write(mWakeWritePipeFd, "W", 1);
} while (nWrite == -1 && errno == EINTR);
if (nWrite != 1) {
if (errno != EAGAIN) {
LOGW("Could not write wake signal, errno=%d", errno);
}
}
}是不是很熟悉？基本就是上一講epoll原型的喚醒函數，向mWakeWritePipeFD寫入1字節，喚醒監聽block在mWakeReadPipeFD端口的epoll_wait
2 dequeueMessage
首先dequeueMessage只是我取的一個叫法，當java層的Looper進行loop的時候，就已經在不停地讀取MessageQueue裏的Message了
Looper.java
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.recycle();
}
}調用queue.next()讀取下一條消息（在loop調用的線程中），如果讀取到了就msg,target.dispatchMessage，
下面來看看queue.next()如何實現
MessageQueue.java
Message next() {
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
// We can assume mPtr != 0 because the loop is obviously still running.
// The looper will not call this method after the loop quits.
nativePollOnce(mPtr, 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 (false) Log.v("MessageQueue", "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("MessageQueue", "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;
}
}首先是個包內函數，所以在同一個包中(android.os)的Looper對象能調用到
nativePollOnce(mPtr, nextPollTimeoutMillis);函數待會展開，功能是調用上一講的epoll_wait，
nextPollTimeoutMillis超時時間爲下一條Message的觸發時間，如果沒有消息則會一直阻塞到超過超時時間
被喚醒後，我們暫時先忽略barrier類型的Message（這是android4.1後加入的一個特性Choreographer,http://blog.csdn.net/innost/article/details/8272867），
如果頭結點msg不爲null，就判斷現在到了這條msg觸發時間沒有，
如果沒到，則nextPollTimeoutMillis設置爲這個條消息需要執行的時間和現在的時間差，給for循環下一次調用nativePollOnce時使用
如果到了甚至超過了，則取出這條msg，退出for循環返回這條msg，給上面上的handler進行dispatch
那麼nativePollOnce具體是如何實現的呢？
android_os_MessageQueue.cpp
static void android_os_MessageQueue_nativePollOnce(JNIEnv* env, jobject obj,
jint ptr, jint timeoutMillis) {
NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
nativeMessageQueue->pollOnce(timeoutMillis);
}android_os_MessageQueue.cpp
void NativeMessageQueue::pollOnce(int timeoutMillis) {
mLooper->pollOnce(timeoutMillis);
}同樣，在framework/base/libs/utils/Looper.cpp中
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++);
if (! response.request.callback) {
#if DEBUG_POLL_AND_WAKE
LOGD("%p ~ pollOnce - returning signalled identifier %d: "
"fd=%d, events=0x%x, data=%p", this,
response.request.ident, response.request.fd,
response.events, response.request.data);
#endif
if (outFd != NULL) *outFd = response.request.fd;
if (outEvents != NULL) *outEvents = response.events;
if (outData != NULL) *outData = response.request.data;
return response.request.ident;
}
}
if (result != 0) {
#if DEBUG_POLL_AND_WAKE
LOGD("%p ~ pollOnce - returning result %d", this, result);
#endif
if (outFd != NULL) *outFd = 0;
if (outEvents != NULL) *outEvents = NULL;
if (outData != NULL) *outData = NULL;
return result;
}
result = pollInner(timeoutMillis);
}
}調用到pollInner
framework/base/libs/utils/Looper.cpp
nt Looper::pollInner(int timeoutMillis) {
#if DEBUG_POLL_AND_WAKE
LOGD("%p ~ pollOnce - waiting: timeoutMillis=%d", this, timeoutMillis);
#endif
int result = ALOOPER_POLL_WAKE;
mResponses.clear();
mResponseIndex = 0;
#ifdef LOOPER_STATISTICS
nsecs_t pollStartTime = systemTime(SYSTEM_TIME_MONOTONIC);
#endif
#ifdef LOOPER_USES_EPOLL
struct epoll_event eventItems[EPOLL_MAX_EVENTS];
int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
bool acquiredLock = false;
#else
// Wait for wakeAndLock() waiters to run then set mPolling to true.
mLock.lock();
while (mWaiters != 0) {
mResume.wait(mLock);
}
mPolling = true;
mLock.unlock();
size_t requestedCount = mRequestedFds.size();
int eventCount = poll(mRequestedFds.editArray(), requestedCount, timeoutMillis);
#endif
if (eventCount < 0) {
if (errno == EINTR) {
goto Done;
}
LOGW("Poll failed with an unexpected error, errno=%d", errno);
result = ALOOPER_POLL_ERROR;
goto Done;
}
if (eventCount == 0) {
#if DEBUG_POLL_AND_WAKE
LOGD("%p ~ pollOnce - timeout", this);
#endif
result = ALOOPER_POLL_TIMEOUT;
goto Done;
}
#if DEBUG_POLL_AND_WAKE
LOGD("%p ~ pollOnce - handling events from %d fds", this, eventCount);
#endif
#ifdef LOOPER_USES_EPOLL
for (int i = 0; i < eventCount; i++) {
int fd = eventItems[i].data.fd;
uint32_t epollEvents = eventItems[i].events;
if (fd == mWakeReadPipeFd) {
if (epollEvents & EPOLLIN) {
awoken();
} else {
LOGW("Ignoring unexpected epoll events 0x%x on wake read pipe.", epollEvents);
}
} else {
if (! acquiredLock) {
mLock.lock();
acquiredLock = true;
}
ssize_t requestIndex = mRequests.indexOfKey(fd);
if (requestIndex >= 0) {
int events = 0;
if (epollEvents & EPOLLIN) events |= ALOOPER_EVENT_INPUT;
if (epollEvents & EPOLLOUT) events |= ALOOPER_EVENT_OUTPUT;
if (epollEvents & EPOLLERR) events |= ALOOPER_EVENT_ERROR;
if (epollEvents & EPOLLHUP) events |= ALOOPER_EVENT_HANGUP;
pushResponse(events, mRequests.valueAt(requestIndex));
} else {
LOGW("Ignoring unexpected epoll events 0x%x on fd %d that is "
"no longer registered.", epollEvents, fd);
}
}
}
if (acquiredLock) {
mLock.unlock();
}
Done: ;
#else
for (size_t i = 0; i < requestedCount; i++) {
const struct pollfd& requestedFd = mRequestedFds.itemAt(i);
short pollEvents = requestedFd.revents;
if (pollEvents) {
if (requestedFd.fd == mWakeReadPipeFd) {
if (pollEvents & POLLIN) {
awoken();
} else {
LOGW("Ignoring unexpected poll events 0x%x on wake read pipe.", pollEvents);
}
} else {
int events = 0;
if (pollEvents & POLLIN) events |= ALOOPER_EVENT_INPUT;
if (pollEvents & POLLOUT) events |= ALOOPER_EVENT_OUTPUT;
if (pollEvents & POLLERR) events |= ALOOPER_EVENT_ERROR;
if (pollEvents & POLLHUP) events |= ALOOPER_EVENT_HANGUP;
if (pollEvents & POLLNVAL) events |= ALOOPER_EVENT_INVALID;
pushResponse(events, mRequests.itemAt(i));
}
if (--eventCount == 0) {
break;
}
}
}
Done:
// Set mPolling to false and wake up the wakeAndLock() waiters.
mLock.lock();
mPolling = false;
if (mWaiters != 0) {
mAwake.broadcast();
}
mLock.unlock();
#endif
#ifdef LOOPER_STATISTICS
nsecs_t pollEndTime = systemTime(SYSTEM_TIME_MONOTONIC);
mSampledPolls += 1;
if (timeoutMillis == 0) {
mSampledZeroPollCount += 1;
mSampledZeroPollLatencySum += pollEndTime - pollStartTime;
} else if (timeoutMillis > 0 && result == ALOOPER_POLL_TIMEOUT) {
mSampledTimeoutPollCount += 1;
mSampledTimeoutPollLatencySum += pollEndTime - pollStartTime
- milliseconds_to_nanoseconds(timeoutMillis);
}
if (mSampledPolls == SAMPLED_POLLS_TO_AGGREGATE) {
LOGD("%p ~ poll latency statistics: %0.3fms zero timeout, %0.3fms non-zero timeout", this,
0.000001f * float(mSampledZeroPollLatencySum) / mSampledZeroPollCount,
0.000001f * float(mSampledTimeoutPollLatencySum) / mSampledTimeoutPollCount);
mSampledPolls = 0;
mSampledZeroPollCount = 0;
mSampledZeroPollLatencySum = 0;
mSampledTimeoutPollCount = 0;
mSampledTimeoutPollLatencySum = 0;
}
#endif
for (size_t i = 0; i < mResponses.size(); i++) {
const Response& response = mResponses.itemAt(i);
if (response.request.callback) {
#if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS
LOGD("%p ~ pollOnce - invoking callback: fd=%d, events=0x%x, data=%p", this,
response.request.fd, response.events, response.request.data);
#endif
int callbackResult = response.request.callback(
response.request.fd, response.events, response.request.data);
if (callbackResult == 0) {
removeFd(response.request.fd);
}
result = ALOOPER_POLL_CALLBACK;
}
}
return result;
}主要看#ifdef LOOPER_USES_EPOLL部分
int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
等待所有attach到mEpollFd上的事件，如果收到喚醒信號繼續執行，否則阻塞等待
之後的#ifdef LOOPER_USES_EPOLL部分
#ifdef LOOPER_USES_EPOLL
for (int i = 0; i < eventCount; i++) {
int fd = eventItems[i].data.fd;
uint32_t epollEvents = eventItems[i].events;
if (fd == mWakeReadPipeFd) {
if (epollEvents & EPOLLIN) {
awoken();
} else {
LOGW("Ignoring unexpected epoll events 0x%x on wake read pipe.", epollEvents);
}
} else {
if (! acquiredLock) {
mLock.lock();
acquiredLock = true;
}
ssize_t requestIndex = mRequests.indexOfKey(fd);
if (requestIndex >= 0) {
int events = 0;
if (epollEvents & EPOLLIN) events |= ALOOPER_EVENT_INPUT;
if (epollEvents & EPOLLOUT) events |= ALOOPER_EVENT_OUTPUT;
if (epollEvents & EPOLLERR) events |= ALOOPER_EVENT_ERROR;
if (epollEvents & EPOLLHUP) events |= ALOOPER_EVENT_HANGUP;
pushResponse(events, mRequests.valueAt(requestIndex));
} else {
LOGW("Ignoring unexpected epoll events 0x%x on fd %d that is "
"no longer registered.", epollEvents, fd);
}
}
}
if (acquiredLock) {
mLock.unlock();
}
Done: ;對所有attach在mEpollFd上的事件進行遍歷，如果對象文件描述符有mWakeReadPipeFd，則awoken()
framework/base/libs/utils/Looper.cpp
void Looper::awoken() {
#if DEBUG_POLL_AND_WAKE
LOGD("%p ~ awoken", this);
#endif
#ifdef LOOPER_STATISTICS
if (mPendingWakeCount == 0) {
LOGD("%p ~ awoken: spurious!", this);
} else {
mSampledWakeCycles += 1;
mSampledWakeCountSum += mPendingWakeCount;
mSampledWakeLatencySum += systemTime(SYSTEM_TIME_MONOTONIC) - mPendingWakeTime;
mPendingWakeCount = 0;
mPendingWakeTime = -1;
if (mSampledWakeCycles == SAMPLED_WAKE_CYCLES_TO_AGGREGATE) {
LOGD("%p ~ wake statistics: %0.3fms wake latency, %0.3f wakes per cycle", this,
0.000001f * float(mSampledWakeLatencySum) / mSampledWakeCycles,
float(mSampledWakeCountSum) / mSampledWakeCycles);
mSampledWakeCycles = 0;
mSampledWakeCountSum = 0;
mSampledWakeLatencySum = 0;
}
}
#endif
char buffer[16];
ssize_t nRead;
do {
nRead = read(mWakeReadPipeFd, buffer, sizeof(buffer));
} while ((nRead == -1 && errno == EINTR) || nRead == sizeof(buffer));
}awoken()即上一講中得awoken()函數，用於把mWakeReadPipeFd上的數據讀取乾淨，因爲mWakeWriteReadPipeFd可能寫入多次
讀取乾淨後下一次epoll_wait時就會等待mWakeWriteReadPipeFd寫入，如果沒有讀取乾淨，即通知epoll內核和mWakeReadPipeFd這個事件相關的處理完畢了，
否則epoll_wait就一直會觸發對應的事件了（不等待新的寫入，一直不阻塞）
3 總結
那麼至此，enqueueMessage和定義dequeueMessage都解釋清楚，感覺豁然開朗了有木有！！！！
下一講講nativeapp的線程消息循環處理過程（主要解讀android_native_app_glue.c）
歡迎各位指正！！
4 reference
android sdk sourcecode
android framework sourcecode