前言
之前研究Fragment遇到进程间通讯的一些东东，趁着最近有空，就在重新研究一下Android相关的代码。并且这些代码确实非常简单。之前研究过，但是遇到一些问题就没深究，这次我们就彻底搞懂他吧
正文
最简单的使用当然是在activity的oncreat中直接使用：
Handler handler = new Handler();
handler.post(new runable{
Log.e("mainactivity","我是handler post的log");
})
但是我们的主线程初始化时候帮我们干了一大堆事情，研究不够典型，我们直接看新线程如何通讯，直接用HandlerThread来作为源码研究的调用开端
myHandlerThread = new HandlerThread( "handler-thread") ;
//开启一个线程
myHandlerThread.start();
//在这个线程中创建一个handler对象
handler = new Handler(myHandlerThread.getLooper())
handler.post(new runable{
Log.e("mainactivity","我是handler post的log");
})
相信你第一眼目光就集中在run()函数中了，内容如下
@Override
public void run() {
mTid = Process.myTid();
Looper.prepare();
synchronized (this) {
mLooper = Looper.myLooper();
notifyAll();
}
Process.setThreadPriority(mPriority);
onLooperPrepared();
Looper.loop();
mTid = -1;
}
这个才是真的进程间通讯的核心,这里我们就一点一点阅读：
/** Initialize the current thread as a looper.
* This gives you a chance to create handlers that then reference
* this looper, before actually starting the loop. Be sure to call
* {@link #loop()} after calling this method, and end it by calling
* {@link #quit()}.
*/
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.set(new Looper(quitAllowed));
}
注释解释的很明白。就是初始化一个Looper。其实可能有疑惑的地方是sThreadLocal.set()这是维持在当前线程的一个变量，可以在任何线程中，调用myLooper。得到唯一的一个实例。暂时对我们问题空间没有影响。我们只要关心new Looper(quitAllowed)即可
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
有些出乎意料，太简单了吧！可是短小的都是精华，脚下就是万丈深渊
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}
private native static long nativeInit();
emmm忽然进入native层了，有些让人望而却步。不过我们看过以前大佬的书和博客，还是很容易定为到源码的位置aosp\frameworks\base\core\jni\android_os_MessageQueue.cpp
static jlong android_os_MessageQueue_nativeInit(JNIEnv* env, jclass clazz) {
NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue();
if (!nativeMessageQueue) {
jniThrowRuntimeException(env, "Unable to allocate native queue");
return 0;
}
nativeMessageQueue->incStrong(env);
return reinterpret_cast<jlong>(nativeMessageQueue);
}
NativeMessageQueue::NativeMessageQueue() :
mPollEnv(NULL), mPollObj(NULL), mExceptionObj(NULL) {
mLooper = Looper::getForThread();
if (mLooper == NULL) {
mLooper = new Looper(false);
Looper::setForThread(mLooper);
}
}
貌似和java层差不多，都是初始化了一个Looper。所有的等待和定时其实都是通过c层的Looper来实现的。我们这里就一一分析
Looper::Looper(bool allowNonCallbacks)
{
//初始化一个文件，作为中断的一端
mWakeEventFd.reset(eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC));
LOG_ALWAYS_FATAL_IF(mWakeEventFd.get() < 0, "Could not make wake event fd: %s", strerror(errno));
AutoMutex _l(mLock);
//关键部分
rebuildEpollLocked();
}
这里所以的东西都是通过epoll来实现的休眠定时唤醒等操作。epoll大概原理是类似一个回调，但是属于系统层的。epoll可以监听一个文件是否变化，变化时候，可以回调函数，若文件不变，则线程会一直休眠。我们来看下相关代码
void Looper::rebuildEpollLocked() {
// Allocate the new epoll instance and register the wake pipe.
mEpollFd.reset(epoll_create1(EPOLL_CLOEXEC));
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.get();
int result = epoll_ctl(mEpollFd.get(), EPOLL_CTL_ADD, mWakeEventFd.get(), &eventItem);
//这个是为了系统层的一些东西准备的，这都没被调用所以可以注释掉
/**
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.get(), EPOLL_CTL_ADD, request.fd, &eventItem);
if (epollResult < 0) {
ALOGE("Error adding epoll events for fd %d while rebuilding epoll set: %s",
request.fd, strerror(errno));
}
}
**/
}
通过epoll_create1初始化。epoll_ctl注册回调监听.所有的事情都干完了。
下面进入真的loop函数了
/**
* 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();
final MessageQueue queue = me.mQueue;
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
try {
msg.target.dispatchMessage(msg);
} finally {
}
msg.recycleUnchecked();
}
}
很长的代码，但是貌似其他的内容都是线程安全和性能分析的trace的。所以我们就分析一下这些，其实看名字就容易理解。looper是一个终极循环，loop函数是终极循环的循环体。messagequeue是一个消息队列如果看注释。在queue的next方法中，他注释是可能阻塞，也就是没消息时候，就阻塞一下。整个逻辑就更清晰了。
我们还是看我们的queue的next方法，这里阻塞是如何实现的：
Message next() {
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;
}
}
// 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;
}
}
主要函数还是这个nativePollOnce(ptr, ia)。ptr是cpp底层的messagequeue保存一个looper。其实最终就是调用了lopper的pollonce。我们直接看最终代码
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) {
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;
}
}
if (result != 0) {
if (outFd != NULL) *outFd = 0;
if (outEvents != NULL) *outEvents = 0;
if (outData != NULL) *outData = NULL;
return result;
}
result = pollInner(timeoutMillis);
}
}
两个循环让人摸不着头脑，但是我们很容易确定mResponses里面没内容。所以就是等待pollInner函数返回部为0的值。继续追pollInner:
int Looper::pollInner(int timeoutMillis) {
// 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;
}
}
// 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();
// Handle all events.
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 {
/**这是为了适应屏幕点击事件等其他的事情，才设置的，我们不关注即可
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();
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;
// 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;
}
真个流程比较复杂，但是真的有用的地方只有那个int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);他是阻塞线程的核心，后面的就是处理一些屏幕点击事件唤醒线程的东东，完全可以无视，所以就返回了int result = POLL_WAKE
wait才是真正的阻塞，初始化的时候，timeoutMillis都为0，所以是不会阻塞的，直接返回到Looper中的loop函数中。
loop函数中。再次进入cpp层，timeoutMillis变成了-1.然后就可以睡眠了，等待监听的文件有写入操作，继续进入loop函数的无限循环。
下面我们分析handlerThread的post发送消息。
这里很容易进入核心的函数调用到了messageQueue中的enqueueMessage
boolean enqueueMessage(Message msg, long when) {
synchronized (this) {
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;
}
其实这个代码会比较简单，就是判断消息队列mMessages是否为空，为空就吧自己当成消息队列的开头，然后通过一个for循环，把当前的消息插入合适的位置，ps只有当当前消息的when小于消息队列中的最后一个，或者消息队列后面已经没内容的时候，就把消息插入到当前的位置，然后调用唤醒线程的操作nativeWake(mPtr);
具体的代码如下：
void Looper::wake() {
uint64_t inc = 1;
ssize_t nWrite = TEMP_FAILURE_RETRY(write(mWakeEventFd, &inc, sizeof(uint64_t)));
}
忽然觉得代码太简单了吧，可是道理就这样，你只要对那个mWakeEventFd写入一个内容，刚才pollInner中就会的wait就会被唤醒，然后就开始执行刚才我们插入的一个内容。
关于退出就不在详细分析，觉得计较简单。
后记
这个东东早就了解代码，但是总是在cpp层的epoll就完全搞不懂了，这次了解整个原理，内心还是很开心的。以后有空自己写一个简化版的handler机制。敬请期待。