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。
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<Looper> sThreadLocal = new ThreadLocal<Looper>();
// 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<Looper>类型的变量，它主要由两个方法，set()和get()；这里通过泛型指定了需要线程赋值的变量类型为Looper。简单理解sThreadLocal .set()就是将当前线程的Looper副本值设定为指定值。sThreadLocal.get()将得到Looper实例在当前线程下的副本。（ThreadLocal的实现还有待研究，初步猜测其内部存在哈希Map，可以根据当前线程的线程号区分不同线程的变量）。通过ThreadLocal实现了线程级单例。
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<jlong>(nativeMessageQueue);
}
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：
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<? 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<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);//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。
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：
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。
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()。
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*>(ptr);
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<void*>(events));//添加fd，this指明callback为类自己
} 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<FileDescriptorRecord>();
}
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<intptr_t>(data);
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。
原文地址： http://blog.csdn.net/a34140974/article/details/50638089
