之前從使用handler入手,順了下handler線程切換的流程。
位址:
http://blog.csdn.net/y1962475006/article/details/52243671
這篇需要詳細分析各部分的關系和工作。
一、handler、Looper、message、messagequeue的關系
先看一個UML圖:
可以看到它們之間都是依賴:
Handler -> Looper
Handler -> MessageQueue(其實這個依賴是取自Looper的)
Looper -> MessageQueue
Looper -> Thread
MessageQueue -> Message
Message -> Message
Message -> Handler
從下到上,從這個UML圖中可以看出:
- Message 是一個單連結清單結構;
- MessageQueue直接管理Message;
- Looper 持有着線程資訊;
- Looper 和 Handler 可以操作MessageQueue;
“MessageQueue”是由單連結清單實作的隊列。
從構造函數說起
1、 Looper
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
可以看到,在Looper構造的時候,執行個體了MessageQueue和Thread。同時,構造函數是私有的,是以不能由外部new 一個Looper。隻能通過prepare方法生成。
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));
}
ThreadLocal 是一個線程次元的資料結構,就是說,每個線程都有一份它存儲的變量的副本,互不幹擾。有興趣可以看一下它實作的源碼。
2、MessageQueue
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}
構造參數就一個quitAllowed,字面意思就是是否允許退出。這裡先不管他。重頭戲是
mPtr = nativeInit();
這是一個native方法。mPtr 是一個long型。那就看看這個native方法(frameworks/base/core/jni/android_os_MessageQueue.cpp):
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);
}
有趣的是,在nativeInit方法中,又new了一個NativeMessageQueue,也就是一個本地消息隊列,并且傳回了這個本地消息隊列的位址偏移量。于是,mPtr就被指派為這個偏移量。到此,Java層的MessageQueue就有了NativeMessage的對應關系,後續可以通過mPtr偏移量找到NativeMessageQueue。
再看看這個NativeMessageQueue的構造函數:
NativeMessageQueue::NativeMessageQueue() :
mPollEnv(NULL), mPollObj(NULL), mExceptionObj(NULL) {
mLooper = Looper::getForThread();
if (mLooper == NULL) {
mLooper = new Looper(false);
Looper::setForThread(mLooper);
}
}
啊哈?這裡又來了一個Looper?NO NO ,這并不是我們之前的java層的Looper,而是C++ native層的。而這個Looper,也是和線程綁定的。
到此,java層和native層各有一個MessageQueue 和Looper,而且都是和線程一對一的。隻不過java 層、native層兩者的關系相反:java層 Looper依賴MessageQueue ,native層MessageQueue依賴Looper。
發送過程
上一篇說到過,handler的發送,最後都走到了handler.enqueueMessage:
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
然後是queue.enqueueMessage(msg, uptimeMillis)。
來看:
boolean enqueueMessage(Message msg, long when) {
<!-- 如果target即messgae沒有綁定Handler,會直接抛異常退出;-->
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
<!-- 如果msg标記為正在使用,退出-->
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
synchronized (this) {
<!-- 如果線程正在退出,傳回false并回收msg-->
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;
}
<!-- 下面是正常流程-->
<!--标記msg為使用狀态-->
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
<!-- 如果隊列是空的,新來的msg作為單連結清單的頭-->
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 {
<!--如果隊列不為空,msg消息插入隊列,是按照時間順序插入的,也就是說隊列是按時間由小到大排序-->
// 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;
<!--找到第一個時間比入隊時間大的位置-->
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.
<!-- 如果有必要就喚醒隊列-->
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
插入的Message按時間排了序,也就是說,MessageQueue是由單連結清單實作的按時間排序的隊列。這點很重要。
“如果有必要就喚醒隊列”是根據needWake這個字段判斷的。隻有在隊列目前處在阻塞狀态并且設定了屏障(target==null)、msg是異步消息的時候,才會去喚醒。看下native的實作:
static void android_os_MessageQueue_nativeWake(JNIEnv* env, jclass clazz, jlong ptr) {
NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
nativeMessageQueue->wake();
}
呀呀,NativeMessageQueue又通過java 層傳過來的mptr生成的,并且發現除了nativeInit()外所有的本地方法都是要傳mPtr過去的,這也證明了,java 層通過mptr這個偏移量找C++層的對應隊列。瞧一瞧NativeMessageQueue的wake():
void NativeMessageQueue::wake() {
mLooper->wake();
}
調用了Looper 的wake。
回顧發送的過程,可以發現,enqueueMessage的過程,隻是在Java層把Message執行個體插入了單連結清單,native層做的事隻是底層隊列的喚醒與否。
讀取Message
上一篇說到過,讀取message是通過Looper.loop(),開啟無限循環讀取的,loop()方法裡,是通過queue.next()拿到message的。我們看這個方法:
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;
<!--開始進入循環-->
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
<!--設定休眠,0表示不休眠-->
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;
<!-- 如果消息隊列不為空,且設定了屏障,周遊找出下一個異步的消息-->
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) {
<!--如果消息隊列不空且,目前要處理的消息設定的處理時間比現在要大,就是還沒到要處理它,設定等待時間。-->
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 {
<!--如果可以處理這個消息了,取出這個msg,并标記為使用中-->
// 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 {
<!--消息隊列為空,進入閑等待-->
// No more messages.
nextPollTimeoutMillis = -1;
}
<!--後面的省略。。。-->
}
注釋中解釋了大部分和消息擷取相關的代碼。其中nativePollOnce 是一個native方法,表示等待或者說休眠多久。nextPollTimeoutMillis 表示下次循環的時候,需要等待的時間,0表示不等待,-1表示無限等待。但是無限等待不就是死循環嗎?那不就導緻ANR了?這裡應該了解忙等待和閑等待。舉個例子,現在有的手扶電梯為了節能,在沒有人的時候會是停着的,這就是閑等待,如果有人站上去,就會喚醒運作;如果電梯上沒有位置了,那麼後來的人就要等待,這就是忙等待。
看nativePollOnce的代碼:
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);
}
對應的:
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;
}
}
發現最後調用了Looper的pollOnce方法。
native 層Looper
到這裡估計就有人問了。為什麼MessageQueuen的native 方法分析了,不管發送和讀取到Looper這就停了?
其實是故意的哈哈。上面分析到,發送消息,在java層,就是msg丢到單連結清單裡,然後native層Looper喚醒線程;讀取消息,在java層,MessageQueue取出需要處理的消息,然後native層Looper設定等待。最後都落在了Looper,由Looper處理線程的喚醒和等待。
那麼就看一下native層Looper(5.0以上在system/core/libutils/Looper.cpp)的構造方法:
Looper::Looper(bool allowNonCallbacks) :
mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false),
mPolling(false), mEpollFd(-1), mEpollRebuildRequired(false),
mNextRequestSeq(0), mResponseIndex(0), mNextMessageUptime(LLONG_MAX) {
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();
}
可以看到,這裡用的是linux系統中的epoll機制。epoll是一個Linux下的IO多路複用的機制,可以監聽fd以及fd上的事件例如讀寫打開關閉,當有事件時傳回,沒事件時阻塞等待喚醒或者逾時。更詳細的這裡不作詳解。但是要留意mWakeEventFd,這個就是和喚醒有關的fd。
直接看Looper->pollOnce() 和 Looper->wake()。
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);
}
}
前面那些可以先不管,直接到最後一句result = pollInner(timeoutMillis); 因為隻有這一句涉及到了時間:
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];
<!--阻塞到這裡,如果有事件就緒或者逾時,會從阻塞狀态推出,eventcount>0,eventItems包含了就緒的事件fd-->
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;
}
<!--小于0認為出錯-->
// Check for poll error.
if (eventCount < 0) {
if (errno == EINTR) {
goto Done;
}
ALOGW("Poll failed with an unexpected error: %s", strerror(errno));
result = POLL_ERROR;
goto Done;
}
<!--等于零認為逾時-->
// 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
<!--大于0的時候,周遊就緒的fd,如果有mWakeEventFd,且事件是EPOLLIN(可讀取或者socket關閉),去讀它。-->
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;
}
awoken方法:
void Looper::awoken() {
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ awoken", this);
#endif
uint64_t counter;
TEMP_FAILURE_RETRY(read(mWakeEventFd, &counter, sizeof(uint64_t)));
}
最後對mWakeEventFd做了讀取。
是以到這裡,阻塞從jave的Looper.loop()->MessageQueue.next()->nativePollOnce()->native Looper.pollOnce()->Looper.pollInner()->epoll_wait();最終停在了epoll這裡。
再看Looper.wake():
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) {
LOG_ALWAYS_FATAL("Could not write wake signal to fd %d: %s",
mWakeEventFd, strerror(errno));
}
}
}
這個代碼很簡單,隻要往mWakeEventFd寫入。就緒時,核心會通知給epoll_wait,程序再次調用時,epoll_wait會傳回這個fd。再回顧一下取消息的過程(僞代碼):
Looper.loop(){
while(true){
Message msg = Message.Next(){
while(true){
設定逾時阻塞;
取出單連結清單頭消息;
if(消息是就緒的){
立即傳回該消息;
}else{
更新下次阻塞時間;
continue;
}
}
}
分發msg
}
}
發現花括号裡兩個無限循環,由于message單連結清單是按時間由小到大排序的,就緒的消息一出隊列就立馬傳回分發了,剩下未就緒的通過epoll_wait 等待設定時間逾時(傳回0)或者有新的可就緒消息入隊列(mWakeEventFd進行寫操作),這個過程CPU去做其他事,是個閑等待,這裡說的阻塞是指阻塞了操作MessageQueue的線程。當時間逾時或者有新的消息入隊列的時候,epoll_wait 會退出阻塞,線程變為活躍态,循環繼續。
這個過程中你也許看到了native層的Message會Handler。沒錯,native層也是有的,而且作用和java層的一樣,隻不過是用來處理其他硬體裝置的fd的,我們不用考慮。
總結
到此為止,分析了java 層和native層的消息發送和讀取的源碼過程:
java 層負責對Message這個單連結清單進行插入和删除操作,native負責線程的喚醒和休眠。
最後上一個圖:
綠色是發送流程,黃色是讀取流程。