Android6.0的Looper源码分析（1）
1 Looper简介
Android在Java标准线程模型的基础上，提供了消息驱动机制，用于多线程之间的通信。而其具体实现就是Looper。
Android Looper的实现主要包括了3个概念：Message，MessageQueue，Handler，Looper。其中Message就是表示一个可执行的任务。消息创建完毕通过消息处理器Handler在任意线程中发送添加至MessageQueue，最终在Looper线程逐个取出并调用handler.handleMessage()进行处理。
2 Looper的初始化
这里可以尝试分析Looper.java类的结构来推测Looper机制的实现原理。以下为Looper类的变量域：
| //这里可以简单的将ThreadLocal类型的变量想象成一个Map,键值为线程号 static final ThreadLocal // 注意下面的static表示sMainLooper归于Looper.Class private static Looper sMainLooper; //注意static数据，进程间并非共享 //Looper的每个线程实例都有一个MessageQueue final MessageQueue mQueue; final Thread mThread; |
第一个变量sThreadLocal为ThreadLocal
第二个变量为static的sMainLooper，存放的应该是主线程（即UI线程的Looper），类型设计为static，这样通过Looper.getMainLooper()的方法在任何线程都能获得该Looper，从而更新UI。
第三个参数为java层的Massage队列，Handler.sendMessage()就是将Message添加到此队列以供Looper.loop()。在接下来的分析将会发现，java层的MessageQueue的新建会导致Native层的NativeMessageQueue的创建，进而在导致Native层Looper的创建。
第四个参数，就是Looper所在线程的引用。
将一个线程改造成Looper线程很容易就可以实现，如下；
| class LooperThread extends Thread { public Handler mHandler;
public void run() { Looper.prepare();
mHandler = new Handler() {//构造方法内部绑定了当前Looper线程 public void handleMessage(Message msg) { // 在这里处理send进来的消息 } };
Looper.loop(); } } |
首先分析Looper的准备工作prepare()。
| public static void prepare() { prepare(true); }
private static void prepare(boolean quitAllowed) {//保证Looper的线程级单例 if (sThreadLocal.get() != null) { throw new RuntimeException("Only one Looper may be created per thread"); } sThreadLocal.set(new Looper(quitAllowed));//这里创建了Looper的线程单例 } |
Looper线程单例的创建会导致MessageQueue的创建，MessageQueue内有一个Message类型的变量sMessages，因此可以想到MessageQueue在java层是通过链表实现的。以下为MessageQueue的构造函数：
| MessageQueue(boolean quitAllowed) { mQuitAllowed = quitAllowed; //通过JNI调用了Native层的相关函数，导致了NativeMessageQueue的创建 mPtr = nativeInit(); } |
可以看到MessageQueue在构造的时候通过JNI调用了Native层的C++函数，从而对Looper在Native层进行必要的初始化操作。同时java MessageQueue获得了一个指向Native层的指针mPtr，从而可以通过mPtr方便的调用底层的相关方法。NativeInit对应android_os_MessageQueue.cpp中的以下函数。
| static jlong android_os_MessageQueue_nativeInit(JNIEnv* env, jclass clazz) { //在Native层又创建了NativeMessageQueue NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue(); if (!nativeMessageQueue) { jniThrowRuntimeException(env, "Unable to allocate native queue"); return 0; }
nativeMessageQueue->incStrong(env); //这里的返回给java层的mPtr，因此mPtr实际上是Java MessageQueue与 //nativeMessageQueue的桥梁，这里比老版本实现更为简洁 return reinterpret_cast } |
|
此时Java层和Native层的MessageQueue被mPtr连接起来了，NativeMessageQueue只是java层MessageQueue在Ntive层的体现，其本身并没有实现Queue的数据结构，而是从其父类MessageQueue中继承了mLooper变量。与java层类似，这个Looper也是线程级单例。以下为NativeMessageQueue的构造函数：
| NativeMessageQueue::NativeMessageQueue() : mPollEnv(NULL), mPollObj(NULL), mExceptionObj(NULL) { mLooper = Looper::getForThread(); if (mLooper == NULL) { mLooper = new Looper(false);//在Native层创建了Looper对象 Looper::setForThread(mLooper);//同样是线程级单例 } } |
可以看到在Java层Looper的创建导致了MessageQueue的创建，而在Native层则刚好相反：NativeMessageQueue的创建导致了Looper的创建。而且Native层的Looper创建和Java层的也完全不一样。它利用了Linux的epoll机制监测了Input的fd和唤醒fd。从功能上来讲，这个唤醒fd才是真正处理java Message和Native Message的钥匙。（注意5.0以上版本Looper的定义在System/core下）。
| Looper::Looper(bool allowNonCallbacks) : mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false), mPolling(false), mEpollFd(-1), mEpollRebuildRequired(false), mNextRequestSeq(0), mResponseIndex(0), mNextMessageUptime(LLONG_MAX) { //这是linux后来才有的东西，负责线程通信,替换了老版本的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(); } |
进入rebuildEpollLocked
| 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. //采用linux的Epoll,与Select功能其实有点类似 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;//关注EPOLLIN事件，也就是可读 eventItem.data.fd = mWakeEventFd;//设置Fd // 将mWakeEventFd的event添加到监听队列,这里其实只是为epoll_ctl放置一个唤醒机制 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); //这里主要添加的是Input事件如键盘，传感器输入，这里基本上由系统负责，很少主动去添加 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); } } } |
这里一定要明白的是，添加的这些fd除了mWakeEventFd负责解除阻塞让程序继续运行，从而处理Native Message和Java Message外，其他fd与Message的处理其实，毫无关系（知道这点非常重要）。此时Java层与Native层的联系如下图所示：
3 创建消息并发送消息
创建消息和发送消息一般是在Looper线程之外的另一个线程通过Handler发送。以下是Handler的满参构造方法。
| public Handler(Callback callback, boolean async) { if (FIND_POTENTIAL_LEAKS) {//调试接口，默认为false final Class 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绑定当前线程的Looper实例 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的目标队列就是Looper的MessageQueue mCallback = callback;//Handler指定callback mAsynchronous = async;//是否异步 } |
在每一个Handler的构造过程中，Handler通过“mLooper =Looper.myLooper();”悄悄的持有了当前所在的looper线程的一个引用。我们已经知道每个Looper都会有一个MessageQueue，这样Handler，Looper，MessageQueue就被关联起来了。
利用Handler发送消息之前需要新建一个Message。获取Message一般可以采用Message类的static方法obtain()。此方法有很多重载方法，零参实现如下（多参重载只是对零参时未赋值的变量进行了赋值）
| 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(); } |
接着就可以调用Handler（非Looper线程持有Handler引用）的sendMessage（msg）方法。前面已经提到，Handler内部持有一个Looper的引用，Looper内部有一个MessageQueue。这样就实现了线程间的消息传递。当然除了sendMessage（msg）之外还有其他类似的发送消息的函数。其本质就是往MessageQueue里面添加Message。这里就不详述了。
特别要指出的是Looper.loop（）在消息队列为空的情况下并不是阻塞在这个MessageQueue上，而是阻塞在Native层的epoll_wait上面。这样会存在很多问题，一个最为重要的问题就是如果在阻塞的时候，突然接收到java Message，程序怎么立马去处理这个Message？前面提到epoll监听了Input的fd和mWakeEventFd。答案就在mWakeEventFd。
先来看每个sendMessage()或其他Send方法都会最终调用以下的这个方法。
| 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; } |
可以看到以上函数才是真正添加Message的实干函数。在每次添加完毕之后都在需needWake的时候去调用NativeWake(mPtr)。我们已经知道mPtr指向了Native层的NativeMessageQueue。NativeWake(mPtr)最终调用了该类的wake()方法。此方法向mWakeEventFd写入了一个字节的内容。到底是什么内容并不重要，重要的是fd存在内容了，换句话说就是mWakeEventFd可读了！因此epoll_wait返回。首先遍历Native消息队列（此时基本上为空遍历），接着遍历活动fd，这里只有一个活动fd就是mWakeEventFd，读掉这一个字节的数据解除掉mWakeEventFd的可读状态。此时mWakeEventFd功成身退。程序已经从阻塞状态解除了出来。程序返回到java层的MessageQueue.next()函数中，next函数返回即从MessageQueue中返回此msg，以做后续的处理。
首先来看Looper.loop()。
| 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 (;;) {//无限循环直到quit() Message msg = queue.next();//获取下一个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层的Message处理在这里
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(); } } |
这里直接进入MessageQueue.next()
| 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;//这个参数向Native层epoll_wait指定时超时时间 for (;;) { if (nextPollTimeoutMillis != 0) {//此处作用有待研究 Binder.flushPendingCommands(); }
nativePollOnce(ptr, nextPollTimeoutMillis);//一般都是阻塞在这个函数
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) { // 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; } } |
上面函数中最为重要的变量为nextPollTimeoutMillis。这个参数为Native层的epoll_wait指定了超时时间。为什么会存在这个epoll_wait超时时间呢？不是已经有一个mWakeEventFd已经可以唤醒epoll_wait了么？回答这个问题需要对Message加以分析，存在多种Message，其中一种Message为需要立即执行的消息。这样的消息通过mWakeEventFd唤醒就可以了。另一种消息是延时消息，或者是在指定时间执行的消息。这样的消息添加到MessageQueue后一般不需要立即执行，而是等一段时间才会去执行，通过一些必要的计算给epoll_wait()指定超时时间可以使得在需要执行这些定时任务的时候epoll_wait()返回。此函数就是实现了这样的逻辑。
接着上面的之前的分析，Looper.loop()调用MessageQueue.next()。next()调用NativePollOnce从而进入Native层处理input和Native Message。NativePollOnce经过几次转调最终会落在mLooper.PollOnce()，如下：
| int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) { int result = 0; for (;;) {//首先对fd对应的的responses进行处理，后面会发现responses里都是活动fd while (mResponseIndex < mResponses.size()) { const Response& response = mResponses.itemAt(mResponseIndex++); int ident = response.request.ident; if (ident >= 0) {//这里大于0标示没有指定callback直接返回即可，有为-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) {//注意这里处于循环内部，改变result的值是在后面的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);//内部epoll_wait } } |
接着进入pollInner
| 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;
// 获得锁，在Native Message的处理和添加逻辑上需要同步 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超时 if (eventCount == 0) { #if DEBUG_POLL_AND_WAKE ALOGD("%p ~ pollOnce - timeout", this); #endif result = POLL_TIMEOUT;//此值返回PollOnce，从而导致java定时Message执行 goto Done; }
// Handle all events. #if DEBUG_POLL_AND_WAKE ALOGD("%p ~ pollOnce - handling events from %d fds", this, eventCount); #endif //首先处理活动的input设备和mWakeEventFd for (int i = 0; i < eventCount; i++) { int fd = eventItems[i].data.fd; uint32_t epollEvents = eventItems[i].events; if (fd == mWakeEventFd) {//若果是唤醒fd有反应 if (epollEvents & EPOLLIN) { awoken();//内部就是read,从而使fd可读状态被清除 } else { ALOGW("Ignoring unexpected epoll events 0x%x on wake event fd.", epollEvents); } } else {//其他input fd处理，其实就是讲活动fd放入到responses队列中，等待处理 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: ;
// 这里应该是处理Native层的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 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);//处理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();
// 处理之前添加进responses的活动Input设备 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. //这里处理了有callback的fd,没有fd的处理可以推后到下次循环的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的处理结构图。关键在于epoll。
这里很明显涉及到3类消息的处理：
1，Java层的Message
2，Native层的Message
3，活动fd指向的Input设备
下面将对着三类消息一一进行分析。
4 Java层 Message的处理
首先需要明确的是Java层Message的执行时机。在上一节的分析中已经分析过了，它是在Native层Message和fd之后。Looper.loop()阻塞的位置在MassageQueue.next()->pollOnce()->pollInner()->epoll_wait()。
1， 如果三类消息都为空，此时Java层send进来一个msg。sendMessage()将调用NativeWake唤醒epoll_wait()。从而回到Java层处理该msg。
2， 如果只有Java层有msg，且为定时任务，sendMessage时唤醒epoll_wait()。在下一次循环中为epoll_wait设置超时时间。（实际上逻辑更为复杂）。
3， 在循环时添加Java Message。epoll_wait立即返回。Msg在下一次循环被处理。
Java层Message的发送和处理流程大致如下图所示：
5 Native层 Message的处理
Native层Message的发送和处理流程大致如下图所示：
从图中可以发现，Native消息的发送过程和处理与java层Message的处理比较类似。都是在任意线程中新建一个Message，然后sendMessage()，所不同的是Native层的Looper没有Handler，因此sendMessage只能通过Looper.sendMessage()。并且需要在SendMessage（）时为该Message指定处理该Message的MessageHandler。而且Native层MessageQueue的实现mMessageEnvelopes本质上是Vector，这一点和Java层MessageQueue是不同的。同样需要在sendMessage()的时候wake()。逻辑和Java层类似就不赘述了。
6 活动fd对应的Input设备的处理
这类消息由epoll直接监听fd，当input设备有活动时，epoll_wait()检测到对应的fd可读（或可写）。从而对fd做处理。这类消息的处理比较分散，首先来看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; //将活动的fd对应mRequests包装成responses队列 pushResponse(events, mRequests.valueAt(requestIndex)); } else { ALOGW("Ignoring unexpected epoll events 0x%x on fd %d that is " "no longer registered.", epollEvents, fd); } } } …… }
// 带callback的responses处理 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，对于活跃fd已经包含了callback的response，直接调用了此callback的HandlerEvent()函数。那对于没有指定Callback的活动responses在那处理呢？在下一次训话中的PollOnce（）。也就是下一次epoll_wait()之前。
| 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) {//这里大于0标示没有指定callback直接返回即可，有为-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;//对没有callback的response直接返回ident(“没有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传入此函数的后三个参数为指针，因此也可以被认为是“返回值”，上层由此获得了一个活动fd的副本，以做后续处理。而此活动fd被responses.clear()掉。
接着还是来继续分析自带callback的request。这里面临两个问题：1，谁添加了这些request？2，这些request的callback->handleEvent（）到底指向了那个函数？
对于第一个为题，可从后往前分析。epoll使用的是fd。这些fd在NativeInit中具体一点就是在Native Looper的构建中被添加进epoll监听队列中，如下
| 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); //就是这里 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); } } } |
从以上程序可以发现这些fd都是mRequests中取出来的。而mRequests由Looper.addFd()添加。查看此函数的调用者发现，很多地方都有调用此函数。因此推测在Native层可以直接使用此函数，向epoll添加监听fd。那java层能向epoll添加fd么？发现NativeInit在Native层对应的函数android_os_MessageQueue_nativeInit有一个邻居如下。
| static void android_os_MessageQueue_nativeSetFileDescriptorEvents(JNIEnv* env, jclass clazz, jlong ptr, jint fd, jint events) { NativeMessageQueue* nativeMessageQueue = reinterpret_cast nativeMessageQueue->setFileDescriptorEvents(fd, events); } |
进入setFileDescriptorEvents()
| void NativeMessageQueue::setFileDescriptorEvents(int fd, int events) { if (events) {//从这里判断是添加还是删除 int looperEvents = 0; if (events & CALLBACK_EVENT_INPUT) { looperEvents |= Looper::EVENT_INPUT; } if (events & CALLBACK_EVENT_OUTPUT) { looperEvents |= Looper::EVENT_OUTPUT; } mLooper->addFd(fd, Looper::POLL_CALLBACK, looperEvents, this, reinterpret_cast } else { mLooper->removeFd(fd);//这里删除fd } } |
因此在Java层也是可以向epoll添加fd的
| private void updateOnFileDescriptorEventListenerLocked(FileDescriptor fd, int events, OnFileDescriptorEventListener listener) { final int fdNum = fd.getInt$();
int index = -1; FileDescriptorRecord record = null; if (mFileDescriptorRecords != null) { index = mFileDescriptorRecords.indexOfKey(fdNum); if (index >= 0) { record = mFileDescriptorRecords.valueAt(index); if (record != null && record.mEvents == events) { return; } } }
if (events != 0) { events |= OnFileDescriptorEventListener.EVENT_ERROR; if (record == null) { if (mFileDescriptorRecords == null) { mFileDescriptorRecords = new SparseArray } record = new FileDescriptorRecord(fd, events, listener); mFileDescriptorRecords.put(fdNum, record); } else { record.mListener = listener; record.mEvents = events; record.mSeq += 1; } nativeSetFileDescriptorEvents(mPtr, fdNum, events);//添加或删除fd } else if (record != null) { record.mEvents = 0; mFileDescriptorRecords.removeAt(index);//猜测是java层的fd记录 } } |
由于在addFd时候指定自己也就是this是callback，因此到最后该fd处理的时候会进入NativeMessageQueue的handlerEvent()方法。
| int NativeMessageQueue::handleEvent(int fd, int looperEvents, void* data) { int events = 0; if (looperEvents & Looper::EVENT_INPUT) { events |= CALLBACK_EVENT_INPUT; } if (looperEvents & Looper::EVENT_OUTPUT) { events |= CALLBACK_EVENT_OUTPUT; } if (looperEvents & (Looper::EVENT_ERROR | Looper::EVENT_HANGUP | Looper::EVENT_INVALID)) { events |= CALLBACK_EVENT_ERROR; } int oldWatchedEvents = reinterpret_cast int newWatchedEvents = mPollEnv->CallIntMethod(mPollObj,//调用java层代码 gMessageQueueClassInfo.dispatchEvents, fd, events); if (!newWatchedEvents) { return 0; // unregister the fd } if (newWatchedEvents != oldWatchedEvents) { setFileDescriptorEvents(fd, newWatchedEvents); } return 1; } |
需要注意的是gMessageQueueClassInfo指向了java层的MessageQueue，因此MessageQueue的dispatchEvents()方法被调用。Message在java层被处理。
当然在Native层就可以实现fd的添加和处理。貌似这也是主要的途径。Android6.0有好些个专门的类处理Input设备。如android_view_InputQueue、android_view_InputEventSender、android_view_InputEventReceiver等等。这里就不详述了。留待以后研究。