Android6.0的Looper原始碼分析（1）
Android在Java標準執行緒模型的基礎上，提供了訊息驅動機制，用於多執行緒之間的通訊。而其具體實現就是Looper。
Android Looper的實現主要包括了3個概念：Message，MessageQueue，Handler，Looper。其中Message就是表示一個可執行的任務。訊息建立完畢通過訊息處理器Handler在任意執行緒中傳送新增至MessageQueue，最終在Looper執行緒逐個取出並呼叫handler.handleMessage()進行處理。
這裡可以嘗試分析Looper.java類的結構來推測Looper機制的實現原理。以下為Looper類的變數域：
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// static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>(); // 注意下面的static表示sMainLooper歸於Looper.Class private static Looper sMainLooper; //注意static資料，程序間並非共享 //Looper的每個執行緒例項都有一個MessageQueue final MessageQueue mQueue; final Thread mThread; |
第一個變數sThreadLocal為ThreadLocal<Looper>型別的變數，它主要由兩個方法，set()和get()；這裡通過泛型指定了需要執行緒賦值的變數型別為Looper。簡單理解sThreadLocal .set()就是將當前執行緒的Looper副本值設定為指定值。sThreadLocal.get()將得到Looper例項在當前執行緒下的副本。（ThreadLocal的實現還有待研究，初步猜測其內部存在雜湊Map，可以根據當前執行緒的執行緒號區分不同執行緒的變數）。通過ThreadLocal實現了執行緒級單例。
第二個變數為static的sMainLooper，存放的應該是主執行緒（即UI執行緒的Looper），型別設計為static，這樣通過Looper.getMainLooper()的方法在任何執行緒都能獲得該Looper，從而更新UI。
第三個引數為java層的Massage佇列，Handler.sendMessage()就是將Message新增到此佇列以供Looper.loop()。在接下來的分析將會發現，java層的MessageQueue的新建會導致Native層的NativeMessageQueue的建立，進而在導致Native層Looper的建立。
第四個引數，就是Looper所線上程的引用。
將一個執行緒改造成Looper執行緒很容易就可以實現，如下；
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class LooperThread extends Thread { public Handler mHandler; public void run() { Looper.prepare(); mHandler = new Handler() {//構造方法內部綁定了當前Looper執行緒 public void handleMessage(Message msg) { // 在這裡處理send進來的訊息 } }; Looper.loop(); } } |
首先分析Looper的準備工作prepare()。
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public static void prepare() { prepare(true); } private static void prepare(boolean quitAllowed) {//保證Looper的執行緒級單例 if (sThreadLocal.get() != null) { throw new RuntimeException("Only one Looper may be created per thread"); } sThreadLocal.set(new Looper(quitAllowed));//這裡建立了Looper的執行緒單例 } |
Looper執行緒單例的建立會導致MessageQueue的建立，MessageQueue內有一個Message型別的變數sMessages，因此可以想到MessageQueue在java層是通過連結串列實現的。以下為MessageQueue的建構函式：
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MessageQueue(boolean quitAllowed) { mQuitAllowed = quitAllowed; //通過JNI呼叫了Native層的相關函式，導致了NativeMessageQueue的建立 mPtr = nativeInit(); } |
可以看到MessageQueue在構造的時候通過JNI呼叫了Native層的C++函式，從而對Looper在Native層進行必要的初始化操作。同時java MessageQueue獲得了一個指向Native層的指標mPtr，從而可以通過mPtr方便的呼叫底層的相關方法。NativeInit對應android_os_MessageQueue.cpp中的以下函式。
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static jlong android_os_MessageQueue_nativeInit(JNIEnv* env, jclass clazz) { //在Native層又建立了NativeMessageQueue NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue(); if (!nativeMessageQueue) { jniThrowRuntimeException(env, "Unable to allocate native queue"); return 0; } nativeMessageQueue->incStrong(env); //這裡的返回給java層的mPtr，因此mPtr實際上是Java MessageQueue與 //nativeMessageQueue的橋樑，這裡比老版本實現更為簡潔 return reinterpret_cast<jlong>(nativeMessageQueue); } |
此時Java層和Native層的MessageQueue被mPtr連線起來了，NativeMessageQueue只是java層MessageQueue在Ntive層的體現，其本身並沒有實現Queue的資料結構，而是從其父類MessageQueue中繼承了mLooper變數。與java層類似，這個Looper也是執行緒級單例。以下為NativeMessageQueue的建構函式：
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NativeMessageQueue::NativeMessageQueue() : mPollEnv(NULL), mPollObj(NULL), mExceptionObj(NULL) { mLooper = Looper::getForThread(); if (mLooper == NULL) { mLooper = new Looper(false);//在Native層建立了Looper物件 Looper::setForThread(mLooper);//同樣是執行緒級單例 } } |
可以看到在Java層Looper的建立導致了MessageQueue的建立，而在Native層則剛好相反：NativeMessageQueue的建立導致了Looper的建立。而且Native層的Looper建立和Java層的也完全不一樣。它利用了Linux的epoll機制監測了Input的fd和喚醒fd。從功能上來講，這個喚醒fd才是真正處理java Message和Native Message的鑰匙。（注意5.0以上版本Looper的定義在System/core下）。
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Looper::Looper(bool allowNonCallbacks) : mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false), mPolling(false), mEpollFd(-1), mEpollRebuildRequired(false), mNextRequestSeq(0), mResponseIndex(0), mNextMessageUptime(LLONG_MAX) { //這是linux後來才有的東西，負責執行緒通訊,替換了老版本的pipe mWakeEventFd = eventfd(0, EFD_NONBLOCK); LOG_ALWAYS_FATAL_IF(mWakeEventFd < 0, "Could not make wake event fd. errno=%d", errno); AutoMutex _l(mLock); rebuildEpollLocked(); } |
進入rebuildEpollLocked
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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. //採用linux的Epoll,與Select功能其實有點類似 mEpollFd = epoll_create(EPOLL_SIZE_HINT); LOG_ALWAYS_FATAL_IF(mEpollFd < 0, "Could not create epoll instance. errno=%d", errno); struct epoll_event eventItem; memset(& eventItem, 0, sizeof(epoll_event)); // 清空 eventItem.events = EPOLLIN;//關注EPOLLIN事件，也就是可讀 eventItem.data.fd = mWakeEventFd;//設定Fd // 將mWakeEventFd的event新增到監聽佇列,這裡其實只是為epoll_ctl放置一個喚醒機制 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. errno=%d", errno); //這裡主要新增的是Input事件如鍵盤，感測器輸入，這裡基本上由系統負責，很少主動去新增 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, errno=%d", request.fd, errno); } } } |
這裡一定要明白的是，新增的這些fd除了mWakeEventFd負責解除阻塞讓程式繼續執行，從而處理Native Message和Java Message外，其他fd與Message的處理其實，毫無關係（知道這點非常重要）。此時Java層與Native層的聯絡如下圖所示：
建立訊息和傳送訊息一般是在Looper執行緒之外的另一個執行緒通過Handler傳送。以下是Handler的滿參構造方法。
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public Handler(Callback callback, boolean async) { if (FIND_POTENTIAL_LEAKS) {//除錯介面，預設為false 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()); } } //Handler綁定當前執行緒的Looper例項 mLooper = Looper.myLooper(); if (mLooper == null) { throw new RuntimeException( "Can't create handler inside thread that has not called Looper.prepare()"); } mQueue = mLooper.mQueue;//sendMessage的目標佇列就是Looper的MessageQueue mCallback = callback;//Handler指定callback mAsynchronous = async;//是否非同步 } |
在每一個Handler的構造過程中，Handler通過“mLooper =Looper.myLooper();”悄悄的持有了當前所在的looper執行緒的一個引用。我們已經知道每個Looper都會有一個MessageQueue，這樣Handler，Looper，MessageQueue就被關聯起來了。
利用Handler傳送訊息之前需要新建一個Message。獲取Message一般可以採用Message類的static方法obtain()。此方法有很多過載方法，零參實現如下（多參過載只是對零參時未賦值的變數進行了賦值）
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public static Message obtain() { synchronized (sPoolSync) { if (sPool != null) { Message m = sPool; sPool = m.next; m.next = null; m.flags = 0; // clear in-use flag sPoolSize--; return m; } } return new Message(); } |
接著就可以呼叫Handler（非Looper執行緒持有Handler引用）的sendMessage（msg）方法。前面已經提到，Handler內部持有一個Looper的引用，Looper內部有一個MessageQueue。這樣就實現了執行緒間的訊息傳遞。當然除了sendMessage（msg）之外還有其他類似的傳送訊息的函式。其本質就是往MessageQueue裡面新增Message。這裡就不詳述了。
特別要指出的是Looper.loop（）在訊息佇列為空的情況下並不是阻塞在這個MessageQueue上，而是阻塞在Native層的epoll_wait上面。這樣會存在很多問題，一個最為重要的問題就是如果在阻塞的時候，突然接收到java Message，程式怎麼立馬去處理這個Message？前面提到epoll監聽了Input的fd和mWakeEventFd。答案就在mWakeEventFd。
先來看每個sendMessage()或其他Send方法都會最終呼叫以下的這個方法。
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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; } |
可以看到以上函式才是真正新增Message的實幹函式。在每次新增完畢之後都在需needWake的時候去呼叫NativeWake(mPtr)。我們已經知道mPtr指向了Native層的NativeMessageQueue。NativeWake(mPtr)最終呼叫了該類的wake()方法。此方法向mWakeEventFd寫入了一個位元組的內容。到底是什麼內容並不重要，重要的是fd存在內容了，換句話說就是mWakeEventFd可讀了！因此epoll_wait返回。首先遍歷Native訊息佇列（此時基本上為空遍歷），接著遍歷活動fd，這裡只有一個活動fd就是mWakeEventFd，讀掉這一個位元組的資料解除掉mWakeEventFd的可讀狀態。此時mWakeEventFd功成身退。程式已經從阻塞狀態解除了出來。程式返回到java層的MessageQueue.next()函式中，next函式返回即從MessageQueue中返回此msg，以做後續的處理。
首先來看Looper.loop()。
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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 (;;) {//無限迴圈直到quit() Message msg = queue.next();//獲取下一個java Message 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);//java層的Message處理在這裡 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(); } } |
這裡直接進入MessageQueue.next()
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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; if (ptr == 0) { return null; } int pendingIdleHandlerCount = -1; // -1 only during first iteration int nextPollTimeoutMillis = 0;//這個引數向Native層epoll_wait指定時超時時間 for (;;) { if (nextPollTimeoutMillis != 0) {//此處作用有待研究 Binder.flushPendingCommands(); } nativePollOnce(ptr, nextPollTimeoutMillis);//一般都是阻塞在這個函式 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 (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; } } |
上面函式中最為重要的變數為nextPollTimeoutMillis。這個引數為Native層的epoll_wait指定了超時時間。為什麼會存在這個epoll_wait超時時間呢？不是已經有一個mWakeEventFd已經可以喚醒epoll_wait了麼？回答這個問題需要對Message加以分析，存在多種Message，其中一種Message為需要立即執行的訊息。這樣的訊息通過mWakeEventFd喚醒就可以了。另一種訊息是延時訊息，或者是在指定時間執行的訊息。這樣的訊息新增到MessageQueue後一般不需要立即執行，而是等一段時間才會去執行，通過一些必要的計算給epoll_wait()指定超時時間可以使得在需要執行這些定時任務的時候epoll_wait()返回。此函式就是實現了這樣的邏輯。
接著上面的之前的分析，Looper.loop()呼叫MessageQueue.next()。next()呼叫NativePollOnce從而進入Native層處理input和Native Message。NativePollOnce經過幾次轉調最終會落在mLooper.PollOnce()，如下：
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int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) { int result = 0; for (;;) {//首先對fd對應的的responses進行處理，後面會發現responses裡都是活動fd while (mResponseIndex < mResponses.size()) { const Response& response = mResponses.itemAt(mResponseIndex++); int ident = response.request.ident; if (ident >= 0) {//這裡大於0標示沒有指定callback直接返回即可，有為-2 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) {//注意這裡處於迴圈內部，改變result的值是在後面的pollInner #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);//內部epoll_wait } } |
接著進入pollInner
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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; // 獲得鎖，在Native Message的處理和新增邏輯上需要同步 mLock.lock(); //如果需要，重建epoll 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; } // epoll超時 if (eventCount == 0) { #if DEBUG_POLL_AND_WAKE ALOGD("%p ~ pollOnce - timeout", this); #endif result = POLL_TIMEOUT;//此值返回PollOnce，從而導致java定時Message執行 goto Done; } // Handle all events. #if DEBUG_POLL_AND_WAKE ALOGD("%p ~ pollOnce - handling events from %d fds", this, eventCount); #endif //首先處理活動的input裝置和mWakeEventFd for (int i = 0; i < eventCount; i++) { int fd = eventItems[i].data.fd; uint32_t epollEvents = eventItems[i].events; if (fd == mWakeEventFd) {//若果是喚醒fd有反應 if (epollEvents & EPOLLIN) { awoken();//內部就是read,從而使fd可讀狀態被清除 } else { ALOGW("Ignoring unexpected epoll events 0x%x on wake event fd.", epollEvents); } } else {//其他input fd處理，其實就是講活動fd放入到responses佇列中，等待處理 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: ; // 這裡應該是處理Native層的Message 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);//處理Native 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(); // 處理之前新增進responses的活動Input裝置 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. //這裡處理了有callback的fd,沒有fd的處理可以推後到下次迴圈的pollOnce 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; } |
下面是Looper的處理結構圖。關鍵在於epoll。
這裡很明顯涉及到3類訊息的處理：
1，Java層的Message
2，Native層的Message
3，活動fd指向的Input裝置
下面將對著三類訊息一一進行分析。
4 Java層 Message的處理
首先需要明確的是Java層Message的執行時機。在上一節的分析中已經分析過了，它是在Native層Message和fd之後。Looper.loop()阻塞的位置在MassageQueue.next()->pollOnce()->pollInner()->epoll_wait()。
1， 如果三類訊息都為空，此時Java層send進來一個msg。sendMessage()將呼叫NativeWake喚醒epoll_wait()。從而回到Java層處理該msg。
2， 如果只有Java層有msg，且為定時任務，sendMessage時喚醒epoll_wait()。在下一次迴圈中為epoll_wait設定超時時間。（實際上邏輯更為複雜）。
3， 在迴圈時新增Java Message。epoll_wait立即返回。Msg在下一次迴圈被處理。
Java層Message的傳送和處理流程大致如下圖所示：
5 Native層 Message的處理
Native層Message的傳送和處理流程大致如下圖所示：
從圖中可以發現，Native訊息的傳送過程和處理與java層Message的處理比較類似。都是在任意執行緒中新建一個Message，然後sendMessage()，所不同的是Native層的Looper沒有Handler，因此sendMessage只能通過Looper.sendMessage()。並且需要在SendMessage（）時為該Message指定處理該Message的MessageHandler。而且Native層MessageQueue的實現mMessageEnvelopes本質上是Vector，這一點和Java層MessageQueue是不同的。同樣需要在sendMessage()的時候wake()。邏輯和Java層類似就不贅述了。
6 活動fd對應的Input裝置的處理
這類訊息由epoll直接監聽fd，當input裝置有活動時，epoll_wait()檢測到對應的fd可讀（或可寫）。從而對fd做處理。這類訊息的處理比較分散，首先來看pollInner()。
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int Looper::pollInner(int timeoutMillis) { …… int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis); …… 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; //將活動的fd對應mRequests包裝成responses佇列 pushResponse(events, mRequests.valueAt(requestIndex)); } else { ALOGW("Ignoring unexpected epoll events 0x%x on fd %d that is " "no longer registered.", epollEvents, fd); } } } …… } // 帶callback的responses處理 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; } |
可以看到，對於活躍fd已經包含了callback的response，直接呼叫了此callback的HandlerEvent()函式。那對於沒有指定Callback的活動responses在那處理呢？在下一次訓話中的PollOnce（）。也就是下一次epoll_wait()之前。
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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) {//這裡大於0標示沒有指定callback直接返回即可，有為-2 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;//對沒有callback的response直接返回ident(“沒有callback”) } } 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); } } |
注意pollOnce傳入此函式的後三個引數為指標，因此也可以被認為是“返回值”，上層由此獲得了一個活動fd的副本，以做後續處理。而此活動fd被responses.clear()掉。
接著還是來繼續分析自帶callback的request。這裡面臨兩個問題：1，誰添加了這些request？2，這些request的callback->handleEvent（）到底指向了那個函式？
對於第一個為題，可從後往前分析。epoll使用的是fd。這些fd在NativeInit中具體一點就是在Native Looper的構建中被新增進epoll監聽佇列中，如下
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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. mEpollFd = epoll_create(EPOLL_SIZE_HINT); LOG_ALWAYS_FATAL_IF(mEpollFd < 0, "Could not create epoll instance. errno=%d", errno); struct epoll_event eventItem; memset(& eventItem, 0, sizeof(epoll_event)); // zero out unused members of data field union eventItem.events = EPOLLIN; eventItem.data.fd = mWakeEventFd; 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. errno=%d", errno); //就是這裡 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, errno=%d", request.fd, errno); } } } |
從以上程式可以發現這些fd都是mRequests中取出來的。而mRequests由Looper.addFd()新增。檢視此函式的呼叫者發現，很多地方都有呼叫此函式。因此推測在Native層可以直接使用此函式，向epoll新增監聽fd。那java層能向epoll新增fd麼？發現NativeInit在Native層對應的函式android_os_MessageQueue_nativeInit有一個鄰居如下。
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static void android_os_MessageQueue_nativeSetFileDescriptorEvents(JNIEnv* env, jclass clazz, jlong ptr, jint fd, jint events) { NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr); nativeMessageQueue->setFileDescriptorEvents(fd, events); } |
進入setFileDescriptorEvents()