版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/sinat_22657459/article/details/78880793
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。