底层原理说明：
例如有ThreadA和ThreadB等2个子线程，如果ThreadA作为接收端，ThreadB是发送端。
在Linux系统中，管道是一种非常重要的通信手段方式，它有2个端，一个是读的端，一个是写的端口，一个进程向管道的写端写入数据，另一个进程就可以在管道的读端进行读取数据。handler的底层就是使用了管道作为通信的方式，接收方就是管道的读端，发送方就对应管道的写端，管道作为中间通知的媒介。管道和Epoll结合充当等待和唤醒的工具。
1、接收端代码解读
接收端需要执行以下三步骤
Looper.prepare()--->定义mHandler--->Looper.loop()
1）Looper.prepare()
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));
}
sThreadLocal是专属线程的缓存区域，这里新建了一个Looper对象，并保存在当前线程的sThreadLocal中。
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
Looper的构造函数中，定义了MessageQueue对象
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}
nativeInit()方法是一个netive方法，在android_os_MessageQueue.cpp中实现的
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);
}
这里定义了一个NativeMessageQueue对象，这是MessageQueue在c++层对应的消息队列，计数+1，并将NativeMessageQueue对象的地址返回给MessageQueue，赋值给MessageQueue的成员变量mPtr。继续看NativeMessageQueue构造方法：
NativeMessageQueue::NativeMessageQueue() : mInCallback(false), mExceptionObj(NULL) {
//在内部创建了一个Looper对象
mLooper = Looper::getForThread();
if (mLooper == NULL) {
mLooper = new Looper(false);
Looper::setForThread(mLooper);
}
}
这里首先查询当前线程中是否存有mLooper 对象，这里和java层中的Looper保存在sThreadLocal有同样效果。在这里开始会创建一个Looper对象，并将结果保存给当前的线程中。c层中的Looper的构造器：
Looper::Looper(bool allowNonCallbacks) :
mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false),
mResponseIndex(0), mNextMessageUptime(LLONG_MAX) {
int wakeFds[2];
int result = pipe(wakeFds);
mWakeReadPipeFd = wakeFds[0];
mWakeWritePipeFd = wakeFds[1];
result = fcntl(mWakeReadPipeFd, F_SETFL, O_NONBLOCK);
result = fcntl(mWakeWritePipeFd, F_SETFL, O_NONBLOCK);
mIdling = false;
// Allocate the epoll instance and register the wake pipe.
//来创建一个epoll专用的文件描述符
mEpollFd = epoll_create(EPOLL_SIZE_HINT);
struct epoll_event eventItem;
memset(& eventItem, 0, sizeof(epoll_event)); // zero out unused members of data field union
//监控mWakeReadPipeFd文件描述符的EPOLLIN事件，即当管道中有内容可读时，就唤醒当前正在等待管道中的内容的线程
eventItem.events = EPOLLIN;
eventItem.data.fd = mWakeReadPipeFd;
//告诉epoll要监控相应的文件描述符的什么事件
result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeReadPipeFd, & eventItem);
}
这里就引出了最终的线程通信的boss（管道），首先新建一个无名管道，wakeFds表示读写2端，接着调用fcntl设置管道读写端都为非阻塞I/O操作，后面接着调用epoll定义事件等，epoll主要处理等待和唤醒工作。
小结：在Looper.prepare()中，定义了一个Looper对象，并将Looper对象保存在ThreadLocal中，接着定义了MessageQueue对象，MessageQueue又引入了c层的NativeMessageQueue对象，将地址保存在java层的MessageQueue中，同时也对应的在c层启用了一个Looper对象，创建了管道和Epoll作为通信的媒介。
2）定义Handler
public Handler(Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
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());
}
}
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
Looper.myLooper()方法就是从sThreadLocal中取出Looper.prepare()中定义的looper对象。
3）Looper.loop()的执行
public static void loop() {
final Looper me = myLooper();
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;
}
msg.target.dispatchMessage(msg);
// Make sure that during the course of dispatching the
// identity of the thread wasn't corrupted.
final long newIdent = Binder.clearCallingIdentity();
msg.recycleUnchecked();
}
}
进入for的无限循环中，从MessageQueue中取出Message对象，如果没有了消息就返回了，跳出循环结束了。取出Message对象就会调用 msg.target.dispatchMessage(msg);进行消息的分发。msg.target是一个Handler对象，由发送方决定的这个处理消息的工具。这里的重点方法是
queue.next();该方法如下：
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
//表示如果当前消息队列中没有消息，它要等待的时候
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
//看看当前消息队列中有没有消息
nativePollOnce(ptr, nextPollTimeoutMillis);
......
}
因为主线程和子线程共用一个Handler，所以也是共用的MessageQueue对象，MessageQueue中的mMessages也是共享的资源，mMessages表示消息队列中第一个消息。
进入了一个for的无限循环，首先执行了 nativePollOnce(ptr, nextPollTimeoutMillis);这个方法。
static void android_os_MessageQueue_nativePollOnce(JNIEnv* env, jclass clazz,
jlong ptr, jint timeoutMillis) {
NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
nativeMessageQueue->pollOnce(env, timeoutMillis);
}
void NativeMessageQueue::pollOnce(JNIEnv* env, int timeoutMillis) {
mInCallback = true;
mLooper->pollOnce(timeoutMillis);
mInCallback = false;
if (mExceptionObj) {
env->Throw(mExceptionObj);
env->DeleteLocalRef(mExceptionObj);
mExceptionObj = NULL;
}
}
inline int pollOnce(int timeoutMillis) {
return pollOnce(timeoutMillis, NULL, NULL, NULL);
}
上面代码都是在c层执行的，调用关系也比较简单，一路执行，最后调用了Looper.cpp中的pollOnce方法。
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);
}
}
这里是一个for的无限循环，第一次进来mResponses是空的，mResponses是一个数组，存储的是Response的值，在for循环中调用pollInner(timeoutMillis)；pollInner方法代码比较长
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.
mIdling = true;
//定义一个事件数组
struct epoll_event eventItems[EPOLL_MAX_EVENTS];
//监控的文件描述符是否有IO事件发生
int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
// No longer idling.
mIdling = false;
// Acquire lock.
mLock.lock();
// Check for poll error.
if (eventCount < 0) { //出现错误
if (errno == EINTR) {
goto Done;
}
ALOGW("Poll failed with an unexpected error, errno=%d", errno);
result = POLL_ERROR;
goto Done;
}
// Check for poll timeout.
if (eventCount == 0) { //表示超时了
result = POLL_TIMEOUT;
goto Done;
}
// Handle all events.
for (int i = 0; i < eventCount; i++) { //有事件需要处理
int fd = eventItems[i].data.fd;
uint32_t epollEvents = eventItems[i].events;
if (fd == mWakeReadPipeFd) {
if (epollEvents & EPOLLIN) {
//在mWakeReadPipeFd文件描述符上发生了EPOLLIN就说明应用程序中的消息队列里面有新的消息需要处理了
awoken();
} else {
ALOGW("Ignoring unexpected epoll events 0x%x on wake read pipe.", 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();
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;
int callbackResult = response.request.callback->handleEvent(fd, events, data);
if (callbackResult == 0) {
removeFd(fd);
}
// 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;
}
首先重新定义了消息处理的时间，根据时间定义了一个事件存储器，调用epoll_wait方法取出事件数量，epoll_wait函数中最后一个timeout参数表示阻塞的时间，当为-1时用久等待，0就立刻继续执行，大于0就等待过了这个时间就继续执行，这个方法是阻塞的，当管道中没有操作事件的时候，线程就会在这里进行等待，直到有事件来了才开始处理
如果出现错误或者超时了就跳过事件处理，否则进行事件的处理
如果对应是管道的读端就调用awoken();
void Looper::awoken() {
char buffer[16];
ssize_t nRead;
do {
nRead = read(mWakeReadPipeFd, buffer, sizeof(buffer));
} while ((nRead == -1 && errno == EINTR) || nRead == sizeof(buffer));
}
管道有事件操作，唤醒Epoll中等待的线程，继续执行，这时候就回到java层的MessageQueue.next()中继续执行
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;
}
}
从消息队列中取出消息，如果没有消息，nextPollTimeoutMillis为-1，再次调用epoll_wait时将会进入等待状态，有消息时需要判断第一个消息的执行时间还没有到，那将继续循环在epoll_wait中进行等待，只有当这个消息的时间到了才返回，否则就一次一次的循环，没有消息的情况就会让线程进入用久等待状态。取出消息以后就会立即返回，这样取消息的总流程就已经结束了。
IdleHandler是线程空闲的时候执行的，在next方法中当取不到消息的时候会处理IdleHandler等，执行完就会删除，只执行一次。
取出消息以后就比较简单了，回到Looper.loop()中，
try {
msg.target.dispatchMessage(msg);
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
直接调用msg.target处理消息，在Handler发送消息的时候已经有了定义
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
于是由Handler处理消息，对消息做最后的分发处理，处理消息也有优先级
/**
* 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);
}
}
如果消息自己设置了回调，则优先有Message自己处理回调的消息，如果Handler设置了回调，则再处理Handler的回调，如果都没有设置回调，则由handleMessage()处理。
至此，消息的读取这一部分分析结束了。
2、发送端代码解读 Handler.sendEmptyMessage()--->Handler.sendMessageAtTime()--->Handler.enqueueMessage()--->MessageQueue.enqueueMessage()
上面是一些简单的调用过程，就不具体分析了
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;
}
Message本身设计是一种单向链表的结构，含有重要成员属性next。设置msg执行的时间when属性，而且Message本身排序按照when的时间先后顺序进行排序，when时间越大，将执行的时间越长。
上面代码中首先判断mMessages是否为null，
1）当为null时表示当前的MessageQueue中没有Message，加入的Message为第一个message，设置next为null,并将传入消息保存在mMessages属性中，这时的needWake由mBlocked决定，mBlocked是因为线程取消息的时候可能设置为true。
2）当MessageQueue中有Message的时候，需要将当前的Message插入到MessageQueue中。
插入Message到MessageQueue中以后就根据needWake决定是否需要唤醒nativeWake(mPtr);
static void android_os_MessageQueue_nativeWake(JNIEnv* env, jclass clazz, jlong ptr) {
NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
return nativeMessageQueue->wake();
}
void NativeMessageQueue::wake() {
mLooper->wake();
}
void Looper::wake() {
ssize_t nWrite;
do {
nWrite = write(mWakeWritePipeFd, "W", 1);
} while (nWrite == -1 && errno == EINTR);
}
这里write命令是向管道的写端口写入一个w字符，当对管道进行写入操作的时候，读取管道的线程处于epoll_wait()等待期的时候就会得到唤醒,继续执行后续的代码，获取消息。这样发送消息端就已经分析完毕了。
总结：
至此，大部分流程已经走通，留待同仁参考，具体的细节请参考源码，这里做简单的总结。
1）Looper是属于哪一个线程，handler发送的消息就属于哪一个线程进行处理。
2）两个线程之间传递数据是因为在同一个进程中，两个线程进行资源的共享，这里的资源共享就是消息队列和里面的消息
3）两个线程之间等待和唤醒是使用了无名管道，并且使用了Epoll机制进行线程的阻塞和唤醒
