Looper Handler
Looper
概述
Looper结合内部的MessageQueue会维护一个线程内单例的消息队列，它借助了epoll系统调用来等待兴趣fd列表的事件。这份兴趣Fd列表不仅包含用于其他线程唤醒自己的fd，还可以被添加新的关注fd或者移除。这对于关注vsync, inputEvent的fd都提供了方式。此外，它的消息队列在native侧和java侧是独立的，进而可以互不干扰地对两侧的消息进行分发。对于java侧的消息还有一则限制，即必须有handler作为message的target（仅有下文提到的唯一特殊情况例外）。
实现细节
Looper构造，仅可通过Looper.prepare()构造，构造方法中会创建新的MessageQueue。即MessageQueue和Looper一对一。
Looper.prepare，用于为线程设定Looper单例。Looper是线程内单例。cpp的代码主要是pthread_setspecific(gTLSKey, looper.get());，java侧则是sThreadLocal.set(new Looper(quitAllowed));
Looper.loop，一个死循环，直到有退出消息。从MessageQueue#next获取信息，阻塞当前线程。
// Looper.java
loop(){....
for (;;) {
if (!loopOnce(me, ident, thresholdOverride)) {
return;
}
}
}
// 精简过的LoopOnce
private static boolean loopOnce(final Looper me,
final long ident, final int thresholdOverride) {
Message msg = me.mQueue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return false;
}
// 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);
}
// Make sure the observer won't change while processing a transaction.
final Observer observer = sObserver;
try {
msg.target.dispatchMessage(msg);
if (observer != null) {
observer.messageDispatched(token, msg);
}
} catch (Exception exception) {
if (observer != null) {
observer.dispatchingThrewException(token, msg, exception);
}
throw exception;
}
if (logging != null) {
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
}
msg.recycleUnchecked();
return true;
}
MessageQueue.next实现如下：
// MessageQueue.java 已精简
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;
}
for (;;) {
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();
}
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
}
}
nativePollOnce本质是调用了Looper#pollOnce->pollInner。
// Looper.cpp 已精简
int Looper::pollInner(int timeoutMillis) {
mResponses.clear();
mResponseIndex = 0;
struct epoll_event eventItems[EPOLL_MAX_EVENTS];
int eventCount = epoll_wait(mEpollFd.get(), eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
mPolling = false;
for (int i = 0; i < eventCount; i++) {
const SequenceNumber seq = eventItems[i].data.u64;
uint32_t epollEvents = eventItems[i].events;
if (seq == WAKE_EVENT_FD_SEQ) {
if (epollEvents & EPOLLIN) {
awoken();
} else {
ALOGW("Ignoring unexpected epoll events 0x%x on wake event fd.", epollEvents);
}
} else {
const auto& request_it = mRequests.find(seq);
if (request_it != mRequests.end()) {
const auto& request = request_it->second;
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;
mResponses.push({.seq = seq, .events = events, .request = request});
} else {
ALOGW("Ignoring unexpected epoll events 0x%x for sequence number %" PRIu64
" that is no longer registered.",
epollEvents, seq);
}
}
}
// 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();
handler->handleMessage(message);
} // release handler
mSendingMessage = false;
result = POLL_CALLBACK;
}
}
// 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);
// 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;
}
即本质是 int eventCount = epoll_wait(mEpollFd.get(), eventItems, EPOLL_MAX_EVENTS, timeoutMillis);，即epoll系统调用，可参阅“Linux”section。eventItems局部变量会接收到事件列表。而收到事件之后，这里的处理主要分三种情况：
- 如果是
wake事件，确认无误后，单纯唤醒。 - 如果有
mMessageEnvelopes处理，则处理message，特别注意，这里的message是通过MessageQueue.cpp的postMessage或者调用了Looper.cpp的sendMessage系列方法入队的信息。对于java侧的信息，并不会在这个vector之中（这一点让我当时困惑半天）。相当于native侧的handler.postMessage()。 - 如果有
mResponses处理，则处理这些回调。相当于native侧的postRunnable。
而对于java侧的MessageQueue.enqueueMessage方法，它的实现如下：
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
synchronized (this) {
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
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;
}
nativeWake。而nativeWake最终会调用到Looper.cpp的wake方法：
void Looper::wake() {
uint64_t inc = 1;
ssize_t nWrite = TEMP_FAILURE_RETRY(write(mWakeEventFd.get(), &inc, sizeof(uint64_t)));
...
}
mWakeEventFd毫无疑问是epoll_wait感兴趣的fd之一，由此结合前面MessageQueue.java的next方法，可以看到，mMessages链表表头是直接在java侧完成的多线程修改，仅仅只是借助native侧在wake的fd上写入下wake事件而已。
Handler
概述
这里主要概述下Handler和Looper的关系。一个Looper可以对应多个Handler。Handler构造时需要传入Looper，以明确Handler的工作MessageQueue。后续使用Handler时，即可完成从Handler.send/postMessage的诸多签名方法调用发起线程，发送消息到此Handler关联的Looper所在的线程。此消息的target依然是自身handler实例，即此handler实例从线程A调用了send/post等方法后在其对应Looper线程中得到回调执行。
Handler可以在任意线程发起信息、Callback到其定义时所声明的所处线程处理消息、执行回调。
Async 与 Sync区别
Handler有async和sync两种方式，在创建时指定。这个属性最终会反映在此handler所发送的Message实例的flag上用于标识Message的async和sync区分。
async 和 sync的message具体作用依赖于MesssageQueue#PostSyncBarrier。这个方法实际上是插入一个message，target为空（message.target为空也就这一种情况）。在MessageQueue#next方法里，如果发现了target=null的message，则会认为是遇到了“同步栅栏”即syncBarrier，会遍历链表找到async为true的message来处理。而不再处理同步消息。直到MessageQueue#removeSyncBarrier发生。
Message
概述
这里主要想提一下Message.obtain，Message类维护了静态的链表作为复用池，池子尺寸上限50个。
回答曾经的一个疑问
一开始我以为Handler/Looper出现仅是为了解决线程之间通信。但考虑到有类似BlockingQueue这种jdk的线程集合，为何不能用此来通信呢？
我认为包含如下几点原因：
- Looper的机制涵盖了java和native两侧的线程间通信。对于不包含jvm的进程下，依然可以使用此机制（当然这个理由还不主要）。
- fd的订阅，可以方便用于方便捕获更多IO事件。比如Vsync, input。
- 两者异步的原理也并不相同(epoll vs 线程状态链表+park/unpark)，这里的优劣我暂时无法得出评判。
BlockingQueue的介绍请参阅“Java”section。
拓展阅读
- Looper MessageQueue的源码们。
- async sync的message差异