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Android7.0 MessageQueue Android7.0 MessageQueue详解

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Android中的消息处理机制大量依赖于Handler。每个Handler都有对应的Looper,用于不断地从对应的MessageQueue中取出消息处理。

一直以来,觉得MessageQueue应该是Java层的抽象,然而事实上MessageQueue的主要部分在Native层中。
自己对MessageQueue在Native层的工作不太熟悉,借此机会分析一下。

一、MessageQueue的创建

当需要使用Looper时,我们会调用Looper的prepare函数:

public static void prepare() {
 prepare(true);
}

private static void prepare(boolean quitAllowed) {
 if (sThreadLocal.get() != null) {
 throw new RuntimeException("Only one Looper may be created per thread");
 }
 //sThreadLocal为线程本地存储区;每个线程仅有一个Looper
 sThreadLocal.set(new Looper(quitAllowed));
}

private Looper(boolean quitAllowed) {
 //创建出MessageQueue
 mQueue = new MessageQueue(quitAllowed);
 mThread = Thread.currentThread();
}

1 NativeMessageQueue

我们看看MessageQueue的构造函数:

MessageQueue(boolean quitAllowed) {
 mQuitAllowed = quitAllowed;
 //mPtr的类型为long?
 mPtr = nativeInit();
}

MessageQueue的构造函数中就调用了native函数,我们看看android_os_MessageQueue.cpp中的实现:

static jlong android_os_MessageQueue_nativeInit(JNIEnv* env, jclass clazz) {
 //MessageQueue的Native层实体
 NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue();
 ............
 //这里应该类似与将指针转化成long类型,放在Java层保存;估计Java层使用时,会在native层将long变成指针,就可以操作队列了
 return reinterpret_cast<jlong>(nativeMessageQueue);
}

我们跟进NativeMessageQueue的构造函数:

NativeMessageQueue::NativeMessageQueue() :
 mPollEnv(NULL), mPollObj(NULL), mExceptionObj(NULL) {
 //创建一个Native层的Looper,也是线程唯一的
 mLooper = Looper::getForThread();
 if (mLooper == NULL) {
 mLooper = new Looper(false);
 Looper::setForThread(mLooper);
 }
}

从代码来看,Native层和Java层均有Looper对象,应该都是操作MessageQueue的。MessageQueue在Java层和Native层有各自的存储结构,分别存储Java层和Native层的消息。

2 Native层的looper

我们看看Native层looper的构造函数:

Looper::Looper(bool allowNonCallbacks) :
 mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false),
 mPolling(false), mEpollFd(-1), mEpollRebuildRequired(false),
 mNextRequestSeq(0), mResponseIndex(0), mNextMessageUptime(LLONG_MAX) {
 //此处创建了个fd
 mWakeEventFd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);
 .......
 rebuildEpollLocked();
}

在native层中,MessageQueue中的Looper初始化时,还调用了rebuildEpollLocked函数,我们跟进一下:

void Looper::rebuildEpollLocked() {
 // Close old epoll instance if we have one.
 if (mEpollFd >= 0) {
 close(mEpollFd);
 }

 // Allocate the new epoll instance and register the wake pipe.
 mEpollFd = epoll_create(EPOLL_SIZE_HINT);
 ............
 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;
 //在mEpollFd上监听mWakeEventFd上是否有数据到来
 int result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeEventFd, & eventItem);
 ...........
 for (size_t i = 0; i < mRequests.size(); i++) {
 const Request& request = mRequests.valueAt(i);
 struct epoll_event eventItem;
 request.initEventItem(&eventItem);
 //监听request对应fd上数据的到来
 int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, request.fd, & eventItem);
 ............
 }
}

从native层的looper来看,我们知道Native层依赖于epoll来驱动事件处理。此处我们先保留一下大致的映像,后文详细分析。

二、使用MessageQueue

1 写入消息
 Android中既可以在Java层向MessageQueue写入消息,也可以在Native层向MessageQueue写入消息。我们分别看一下对应的操作流程。

1.1 Java层写入消息
Java层向MessageQueue写入消息,依赖于enqueueMessage函数:

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) {
  .....
  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;
  //在头部插入数据,如果之前MessageQueue是阻塞的,那么现在需要唤醒
  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;
  }
  //不是第一个异步消息时,needWake置为false
  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;
}

上述代码比较简单,主要就是将新加入的Message按执行时间插入到原有的队列中,然后根据情况调用nativeAwake函数。

我们跟进一下nativeAwake:

void NativeMessageQueue::wake() {
 mLooper->wake();
}

void Looper::wake() {
 uint64_t inc = 1;
 //就是向mWakeEventFd写入数据
 ssize_t nWrite = TEMP_FAILURE_RETRY(write(mWakeEventFd, &inc, sizeof(uint64_t)));
 .............
}

在native层的looper初始化时,我们提到过native层的looper将利用epoll来驱动事件,其中构造出的epoll句柄就监听了mWakeEventFd。
实际上从MessageQueue中取出数据时,若没有数据到来,就会利用epoll进行等待;因此当Java层写入消息时,将会将唤醒处于等待状态的MessageQueue。
在后文介绍从MessageQueue中提取消息时,将再次分析这个问题。

1.2 Native层写入消息
Native层写入消息,依赖于Native层looper的sendMessage函数:

void Looper::sendMessage(const sp<MessageHandler>& handler, const Message& message) {
 nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);
 sendMessageAtTime(now, handler, message);
}

void Looper::sendMessageAtTime(nsecs_t uptime, const sp<MessageHandler>& handler,
 const Message& message) {
 size_t i = 0;
 {
 AutoMutex _l(mLock);

 //同样需要按时间插入
 size_t messageCount = mMessageEnvelopes.size();
 while (i < messageCount && uptime >= mMessageEnvelopes.itemAt(i).uptime) {
  i += 1;
 }

 //将message包装成一个MessageEnvelope对象
 MessageEnvelope messageEnvelope(uptime, handler, message);
 mMessageEnvelopes.insertAt(messageEnvelope, i, 1);

 // Optimization: If the Looper is currently sending a message, then we can skip
 // the call to wake() because the next thing the Looper will do after processing
 // messages is to decide when the next wakeup time should be. In fact, it does
 // not even matter whether this code is running on the Looper thread.
 if (mSendingMessage) {
  return;
 }
 }
 // Wake the poll loop only when we enqueue a new message at the head.
 if (i == 0) {
 //若插入在队列头部,同样利用wake函数触发epoll唤醒
 wake();
 }
}

以上就是向MessageQueue中加入消息的主要流程,接下来我们看看从MessageQueue中取出消息的流程。

2、提取消息
当Java层的Looper对象调用loop函数时,就开始使用MessageQueue提取消息了:

public static void loop() {
 final Looper me = myLooper();
 .......
 for (;;) {
 Message msg = queue.next(); // might block
 .......
 try {
  //调用Message的处理函数进行处理
  msg.target.dispatchMessage(msg);
 }........
 }
}

此处我们看看MessageQueue的next函数:

Message next() {
 //mPtr保存了NativeMessageQueue的指针
 final long ptr = mPtr;
 .......
 int pendingIdleHandlerCount = -1; // -1 only during first iteration
 int nextPollTimeoutMillis = 0;

 for (;;) {
 if (nextPollTimeoutMillis != 0) {
  //会调用Native函数,最终调用IPCThread的talkWithDriver,将数据写入Binder驱动或者读取一次数据
  //不知道在此处进行这个操作的理由?
  Binder.flushPendingCommands();
 }

 //处理native层的数据,此处会利用epoll进行blocked
 nativePollOnce(ptr, nextPollTimeoutMillis);

 synchronized (this) {
  final long now = SystemClock.uptimeMillis();
  Message prevMsg = null;
  Message msg = mMessages;
  //下面其实就是找出下一个异步处理类型的消息;异步处理类型的消息,才含有对应的执行函数
  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;
   //完成next记录的存储
   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;
  }

  //MessageQueue中引入了IdleHandler接口,即当MessageQueue没有数据处理时,调用IdleHandler进行一些工作

  //pendingIdleHandlerCount表示待处理的IdleHandler,初始为-1
  if (pendingIdleHandlerCount < 0
   && (mMessages == null || now < mMessages.when)) {
  //mIdleHandlers的size默认为0,调用接口addIdleHandler才能增加
  pendingIdleHandlerCount = mIdleHandlers.size();
  }

  if (pendingIdleHandlerCount <= 0) {
  // No idle handlers to run. Loop and wait some more.
  mBlocked = true;
  continue;
  }

  //将待处理的IdleHandler加入到PendingIdleHandlers中
  if (mPendingIdleHandlers == null) {
  mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
  }
  //调用ArrayList.toArray(T[])节省每次分配的开销;毕竟对于Message.Next这样调用频率较高的函数,能省一点就是一点
  mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
 }

 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 {
  //执行实现类的queueIdle函数,返回值决定是否继续保留
  keep = idler.queueIdle();
  } catch (Throwable t) {
  Log.wtf(TAG, "IdleHandler threw exception", t);
  }

  if (!keep) {
  synchronized (this) {
   mIdleHandlers.remove(idler);
  }
  }
 }
 pendingIdleHandlerCount = 0;
 nextPollTimeoutMillis = 0;
 }
}

整个提取消息的过程,大致上如上图所示。
可以看到在Java层,Looper除了要取出MessageQueue的消息外,还会在队列空闲期执行IdleHandler定义的函数。

2.1 nativePollOnce
现在唯一的疑点是nativePollOnce是如何处理Native层数据的,我们看看对应的native函数:

static void android_os_MessageQueue_nativePollOnce(JNIEnv* env, jobject obj,
 jlong ptr, jint timeoutMillis) {
 //果然Java层调用native层MessageQueue时,将long类型的ptr变为指针
 NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
 nativeMessageQueue->pollOnce(env, obj, timeoutMillis);
}

void NativeMessageQueue::pollOnce(JNIEnv* env, jobject pollObj, int timeoutMillis) {
 mPollEnv = env;
 mPollObj = pollObj;
 //最后还是进入到Native层looper的pollOnce函数
 mLooper->pollOnce(timeoutMillis);
 mPollObj = NULL;
 mPollEnv = NULL;

 if (mExceptionObj) {
 .........
 }
}

看看native层looper的pollOnce函数:

//timeoutMillis为超时等待时间。值为-1时,表示无限等待直到有事件到来;值为0时,表示无需等待
//outFd此时为null,含义是:存储产生事件的文件句柄
//outEvents此时为null,含义是:存储outFd上发生了哪些事件,包括可读、可写、错误和中断
//outData此时为null,含义是:存储上下文数据,其实调用时传入的参数
int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) {
 int result = 0;
 for (;;) {
 //处理response,目前我们先不关注response的内含
 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 (outFd != NULL) *outFd = fd;
  if (outEvents != NULL) *outEvents = events;
  if (outData != NULL) *outData = data;
  return ident;
  }
 }

 //根据pollInner的结果,进行操作
 if (result != 0) {
  if (outFd != NULL) *outFd = 0;
  if (outEvents != NULL) *outEvents = 0;
  if (outData != NULL) *outData = NULL;
  return result;
 }

 //主力还是靠pollInner
 result = pollInner(timeoutMillis);
 }
}

跟进一下pollInner函数:

int Looper::pollInner(int timeoutMillis) {
 // Adjust the timeout based on when the next message is due.
 //timeoutMillis是Java层事件等待事件
 //native层维持了native message的等待时间
 //此处其实就是选择最小的等待时间
 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;
 }
 }

 int result = POLL_WAKE;
 //pollInner初始就清空response
 mResponses.clear();
 mResponseIndex = 0;

 // We are about to idle.
 mPolling = true;

 //利用epoll等待mEpollFd监控的句柄上事件到达
 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();

 //重新调用rebuildEpollLocked时,将使得epoll句柄能够监听新加入request对应的fd
 if (mEpollRebuildRequired) {
 mEpollRebuildRequired = false;
 rebuildEpollLocked();
 goto Done;
 }

 // Check for poll error.
 if (eventCount < 0) {
 if (errno == EINTR) {
  goto Done;
 }
 ......
 result = POLL_ERROR;
 goto Done;
 }

 // Check for poll timeout.
 if (eventCount == 0) {
 result = POLL_TIMEOUT;
 goto Done;
 }

 for (int i = 0; i < eventCount; i++) {
 if (fd == mWakeEventFd) {
  if (epollEvents & EPOLLIN) {
  //前面已经分析过,当java层或native层有数据写入队列时,将写mWakeEventFd,以触发epoll唤醒
  //awoken将读取并清空mWakeEventFd上的数据
  awoken();
  } else {
  .........
  }
 } else {
  //epoll同样监听的request对应的fd
  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;
  //存储这个fd对应的response
  pushResponse(events, mRequests.valueAt(requestIndex));
  } else {
  ..........
  }
 }
 }

Done:

 // Invoke pending message callbacks.
 mNextMessageUptime = LLONG_MAX;
 //处理Native层的Message
 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.
  {
  sp<MessageHandler> handler = messageEnvelope.handler;
  Message message = messageEnvelope.message;
  mMessageEnvelopes.removeAt(0);
  mSendingMessage = true;
  mLock.unlock();

  //处理Native Message
  handler->handleMessage(message);
  }
  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();

 //处理带回调函数的response
 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;

  //调用response的callback
  int callbackResult = response.request.callback->handleEvent(fd, events, data);
  if (callbackResult == 0) {
  removeFd(fd, response.request.seq);
  }

  response.request.callback.clear();
  result = POLL_CALLBACK;
 }
 }
 return result;
}

说实话native层的代码写的很乱,该函数的功能比较多。
如上图所示,在nativePollOnce中利用epoll监听是否有数据到来,然后处理native message、native response。

最后,我们看看如何在native层中加入request。

3 添加监控请求
native层增加request依赖于looper的接口addFd:

//fd表示需要监听的句柄
//ident的含义还没有搞明白
//events表示需要监听的事件,例如EVENT_INPUT、EVENT_OUTPUT、EVENT_ERROR和EVENT_HANGUP中的一个或多个
//callback为事件发生后的回调函数
//data为回调函数对应的参数
int Looper::addFd(int fd, int ident, int events, Looper_callbackFunc callback, void* data) {
 return addFd(fd, ident, events, callback ? new SimpleLooperCallback(callback) : NULL, data);
}

结合上文native层轮询队列的操作,我们大致可以知道:addFd的目的,就是让native层的looper监控新加入的fd上是否有指定事件发生。
如果发生了指定的事件,就利用回调函数及参数构造对应的response。
native层的looper处理response时,就可以执行对应的回调函数了。

看看实际的代码:

int Looper::addFd(int fd, int ident, int events, const sp<LooperCallback>& callback, void* data) {
 ........
 {
 AutoMutex _l(mLock);

 //利用参数构造一个request
 Request request;
 request.fd = fd;
 request.ident = ident;
 request.events = events;
 request.seq = mNextRequestSeq++;
 request.callback = callback;
 request.data = data;
 if (mNextRequestSeq == -1) mNextRequestSeq = 0; // reserve sequence number -1

 struct epoll_event eventItem;
 request.initEventItem(&eventItem);

 //判断之前是否已经利用该fd构造过Request
 ssize_t requestIndex = mRequests.indexOfKey(fd);
 if (requestIndex < 0) {
  //mEpollFd新增一个需监听fd
  int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, fd, & eventItem);
  .......
  mRequests.add(fd, request);
 } else {
  //mEpollFd修改旧的fd对应的监听事件
  int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_MOD, fd, & eventItem);
  if (epollResult < 0) {
  if (errno == ENOENT) {
   // Tolerate ENOENT because it means that an older file descriptor was
   // closed before its callback was unregistered and meanwhile a new
   // file descriptor with the same number has been created and is now
   // being registered for the first time. 
   epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, fd, & eventItem);
   .......
  }
  //发生错误重新加入时,安排EpollRebuildLocked,将让epollFd重新添加一次待监听的fd
  scheduleEpollRebuildLocked();
  }
  mRequests.replaceValueAt(requestIndex, request);
 }
 }
}

对加入监控请求的处理,在上文介绍pollInner函数时已做分析,此处不再赘述。

三、总结

1、流程总结


MessageQueue的整个流程包括了Java部分和Native部分,从图中可以看出Native层的比重还是很大的。我们结合上图回忆一下整个MessageQueue对应的处理流程:
1、Java层创建Looper对象时,将会创建Java层的MessageQueue;Java层的MessageQueue初始化时,将利用Native函数创建出Native层的MessageQueue。

2、Native层的MessageQueue初始化后,将创建对应的Native Looper对象。Native对象初始化时,将创建对应epollFd和WakeEventFd。其中,epollFd将作为epoll的监听句柄,初始时epollFd仅监听WakeEventFd。

3、图中红色线条为Looper从MessageQueue中取消息时,处理逻辑的流向。
3.1、当Java层的Looper开始循环时,首先需要通过JNI函数调用Native Looper进行pollOnce的操作。

3.2、Native Looper开始运行后,需要等待epollFd被唤醒。当epollFd等待超时或监听的句柄有事件到来,Native Looper就可以开始处理事件了。

3.3、在Native层,Native Looper将先处理Native MessageQueue中的消息,再调用Response对应的回调函数。

3.4、本次循环中,Native层事件处理完毕后,才开始处理Java层中MessageQueue的消息。若MessageQueue中没有消息需要处理,并且MessageQueue中存在IdleHandler时,将调用IdleHandler定义的处理函数。

图中蓝色部分为对应的函数调用:
在Java层:
利用MessageQueue的addIdleHandler,可以为MessageQueue增加IdleHandler;
利用MessageQueue的enqueueMessage,可以向MessageQueue增加消息;必要时将利用Native函数向Native层的WakeEventFd写入消息,以唤醒epollFd。

在Native层:
利用looper:sendMessage,可以为Native MessageQueue增加消息;同样,要时将向Native层的WakeEventFd写入消息,以唤醒epollFd;
利用looper:addFd,可以向Native Looper注册监听请求,监听请求包含需监听的fd、监听的事件及对应的回调函数等,监听请求对应的fd将被成为epollFd监听的对象。当被监听的fd发生对应的事件后,将会唤醒epollFd,此时将生成对应response加入的response List中,等待处理。一旦response被处理,就会调用对应的回调函数。

2、注意事项
MessageQueue在Java层和Native层有各自的存储结构,可以分别增加消息。从处理逻辑来看,会优先处理native层的Message,然后处理Native层生成的response,最后才是处理Java层的Message。

以上就是本文的全部内容,希望对大家的学习有所帮助,也希望大家多多支持。

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