Android 8.0系统源码分析--Message发送、处理过程源码分析
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上节我们分析了应用进程中的Looper和MessageQueue创建过程,接下来我们来看看Message是如何发送到当前的MessageQueue上并且它是如何得到处理的。
一、Message的发送过程
发送一个Message对于应用来说,非常简单,就是调用handler.sendMessage方法,就可以将一个封装好的Message发送出去了,或者调用handler.post(Runnable r)也可以,两种调用往下的实现是完全一样的。我们就以sendMessage为入口来看一下Message发送的过程。该方法的实现在Handler.java类中,目录路径为frameworks\base\core\java\android\os\Handler.java,sendMessage方法的源码如下:
public final boolean sendMessage(Message msg) { return sendMessageDelayed(msg, 0); } public final boolean sendMessageDelayed(Message msg, long delayMillis) { if (delayMillis < 0) { delayMillis = 0; } return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis); } 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); }
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }
该方法的第一句就是给当前的message的target赋值,表示该message最终由谁处理,一般我们都会构造自己的handler,所以最终该消息得到分发的时候,目标就是我们重写的handler了,接着调用queue局部变量的enqueueMessage方法来将该message入队,queue局部变量的类型为MessageQueue,它指向的就是执行当前逻辑的Looper对象的mQueue成员变量,也就是上一节我们分析Looper创建过程中创建好的。MessageQueue类的目录路径为frameworks\base\core\java\android\os\MessageQueue.java,enqueueMessage方法的源码如下:
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; }
该方法中前面的逻辑的目的就是寻找到一个合适的位置,把当前的message对象挂载进去,上一节我们已经说了MessageQueue类的mMessages成员变量,它的类型为Message,而Message又是一个单向链表,可以不断的往它的成员变量next上指定下一个message,所以只要发送message,就会不断的往上面一个合适的节点挂载。挂载完成后根据局部变量needWake的值来判断是否要唤醒当前的Looper循环,如果我们发送的message需要延迟,而且时间没到,那么就不需要,MessageQueue类的next方法中就会去修改局部变量nextPollTimeoutMillis的值,让Looper循环继续休眠,否则说明消息的处理时间到了,那么就接着调用nativeWake函数来唤醒Looper循环。nativeWake方法的实现在android_os_MessageQueue.cpp文件中,目录路径为frameworks\base\core\jni\android_os_MessageQueue.cpp,nativeWake方法的源码如下:
static void android_os_MessageQueue_nativeWake(JNIEnv* env, jclass clazz, jlong ptr) { NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr); nativeMessageQueue->wake();}
这里就是直接调用NativeMessageQueue类的wake方法继续处理,wake方法的源码如下:
void NativeMessageQueue::wake() { mLooper->wake();}
这里也是直接调用Looper类的wake方法继续处理,Looper类的目录路径为system\core\libutils\Looper.cpp,wake方法的源码如下:
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)); } }}
这里的逻辑比较简单,就是调用write系统函数往native层的Looper对象初始化时创建的Event文件描述符mWakeEventFd上写入一个整数1,老罗的博客上也说了,这里写入什么内容其实无关紧要,因为该方法的目的是唤醒该线程的Looper循环,而要处理的Message已经保存的MessageQueue对象的成员变量mMessages链表中了。Linux的event机制在发现mWakeEventFd文件描述符上有事件发生时,那么就会从Looper类的pollInner方法中的epoll_wait唤醒,继续处理消息。我们再来看一下Looper类的pollInner方法,源码如下:
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: %s", strerror(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;}
当epoll_wait逻辑返回时,就会得到当前的消息数量eventCount,当前场景下,该值大于0,那么就会在for (int i = 0; i < eventCount; i++) 循环中取消息进行处理,当前事件的fd、epollEvents分别为mWakeEventFd、EPOLLIN,它们都是在Looper对象初始化时,在rebuildEpollLocked方法中构造struct epoll_event eventItem结构体时赋值的,所以就执行awoken函数,该函数的源码如下:
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)));}
该函数就是调用系统函数read将产生在mWakeEventFd文件描述符上的事件读取出来,清空管道,以免事件重复。pollInner方法中下面的逻辑就是判断mResponses中是否有回调需要处理,处理完成后就会一层层返回到Java层的MessageQueue类的next方法中了,那么下次再从mMessages链表中取消息时,就会有消息了。
二、Message的处理过程
经过上面的分析,我们知道当Looper循环中有消息需要处理时,那么MessageQueue类的next方法就会去找当前需要处理的Message消息,MessageQueue类的next方法的源码如下:
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; for (;;) { if (nextPollTimeoutMillis != 0) { Binder.flushPendingCommands(); } 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; 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; } }
它就是在当前的链表中查找是否有合适的Message,找到的话,就返回当前的Message对象,该方法返回后就会到Looper类的loop方法的无限循环中,Looper类的loop方法的源码如下:
/** * Run the message queue in this thread. Be sure to call * {@link #quit()} to end the 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 (; ; ) { 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 final Printer logging = me.mLogging; if (logging != null) { logging.println(">>>>> Dispatching to " + msg.target + " " + msg.callback + ": " + msg.what); } final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs; final long traceTag = me.mTraceTag; if (traceTag != 0 && Trace.isTagEnabled(traceTag)) { Trace.traceBegin(traceTag, msg.target.getTraceName(msg)); } final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis(); final long end; try { msg.target.dispatchMessage(msg); end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis(); } finally { if (traceTag != 0) { Trace.traceEnd(traceTag); } } if (slowDispatchThresholdMs > 0) { final long time = end - start; if (time > slowDispatchThresholdMs) { Slog.w(TAG, "Dispatch took " + time + "ms on " + Thread.currentThread().getName() + ", h=" + msg.target + " cb=" + msg.callback + " msg=" + msg.what); } } 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(); } }
Message msg = queue.next()逻辑返回后,我们就拿到的当前需要处理的msg,如果msg为空,那么loop方法也就结束了,说明没有该Looper循环已经完成使命了。所以大家可以回头看一下MessageQueue类的next方法,只有在mQuitting为true时才会返回null,其他场景下要么找到合适的msg进行处理,要么就是执行nativePollOnce进入休眠,拿到了目标msg,接着就调用sg.target.dispatchMessage(msg)对它进行处理,当前msg的成员变量target就是前面它的发送过程中赋值的,target的类型为Handler,我们接着来看一下Handler类的dispatchMessage方法,Handler类的的目录路径为:frameworks\base\core\java\android\os\Handler.java,dispatchMessage方法的源码如下:
/** * Handle system messages here. */ public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } }
首先判断当前msg的回调接口callback是否为空,如果不为空,则调用handleCallback处理,Message类的成员变量callback的类型为Runnable,也就是调用它的run方法去执行我们自己的逻辑,这里也就是发送message时,我们调用handler.post(Runnable run)对应上了;如果当前Message的callback为空,那么继续判断当前Handler的成员变量mCallback是否为空,该成员变量是在Handler对象的构造方法中传入的,相当于系统给我们多预留了一个出口,我们可以把当前Handler上所有的消息取出来放在自己定义的Callback中去处理,如果mCallback也为空,那么就调用handleMessage去处理了,我们一般也就是要重写该方法来处理我们自己的逻辑。
这里还需要说明一点,之前有些项目中,我有看到有些同事直接重写Handler类的dispatchMessage方法,这样其实很不妥,从上面的处理过程可以看到,系统给我们预留了足够多的地方去处理message,而dispatchMessage方法是从Looper类回调过来的入口,假如我们按照一般的逻辑去判断msg.what进行消息处理,那么就会导致那些post的消息无法分发了,所以还是老老实实重写handleMessage方法就可以了。
好,Looper、MessageQueue的消息循环我们就分析到这里了。
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