关于Android应用程序消息循环Looper和Handler简析

android version:7.1.1_r6

Android应用程序的消息处理机制也是由消息循环、消息发送和消息处理这三个部分组成

1. 消息循环

frameworks/base/core/java/android/app/ActivityThread.java

public static void main(String[] args) {
......

Looper.prepareMainLooper();

ActivityThread thread = new ActivityThread();
thread.attach(false);

if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}

......

Looper.loop();

......
}

frameworks/base/core/java/android/os/Looper.java

public final class Looper {
/*
* API Implementation Note:
*
* This class contains the code required to set up and manage an event loop
* based on MessageQueue.  APIs that affect the state of the queue should be
* defined on MessageQueue or Handler rather than on Looper itself.  For example,
* idle handlers and sync barriers are defined on the queue whereas preparing the
* thread, looping, and quitting are defined on the looper.
*/

private static final String TAG = "Looper";

// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static Looper sMainLooper;  // guarded by Looper.class

final MessageQueue mQueue;
final Thread mThread;

private Printer mLogging;
private long mTraceTag;

/** 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));
}

/**
* Initialize the current thread as a looper, marking it as an
* application's main looper. The main looper for your application
* is created by the Android environment, so you should never need
* to call this function yourself.  See also: {@link #prepare()}
*/
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}

/**
* Returns the application's main looper, which lives in the main thread of the application.
*/
public static Looper getMainLooper() {
synchronized (Looper.class) {
return sMainLooper;
}
}

/**
* 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 traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
try {
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}

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();
}
}

/**
* Return the Looper object associated with the current thread.  Returns
* null if the calling thread is not associated with a Looper.
*/
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}

/**
* Return the {@link MessageQueue} object associated with the current
* thread.  This must be called from a thread running a Looper, or a
* NullPointerException will be thrown.
*/
public static @NonNull MessageQueue myQueue() {
return myLooper().mQueue;
}

private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}

/**
* Returns true if the current thread is this looper's thread.
*/
public boolean isCurrentThread() {
return Thread.currentThread() == mThread;
}

......

/**
* Quits the looper.
* <p>
* Causes the {@link #loop} method to terminate without processing any
* more messages in the message queue.
* </p><p>
* Any attempt to post messages to the queue after the looper is asked to quit will fail.
* For example, the {@link Handler#sendMessage(Message)} method will return false.
* </p><p class="note">
* Using this method may be unsafe because some messages may not be delivered
* before the looper terminates.  Consider using {@link #quitSafely} instead to ensure
* that all pending work is completed in an orderly manner.
* </p>
*
* @see #quitSafely
*/
public void quit() {
mQueue.quit(false);
}

/**
* Quits the looper safely.
* <p>
* Causes the {@link #loop} method to terminate as soon as all remaining messages
* in the message queue that are already due to be delivered have been handled.
* However pending delayed messages with due times in the future will not be
* delivered before the loop terminates.
* </p><p>
* Any attempt to post messages to the queue after the looper is asked to quit will fail.
* For example, the {@link Handler#sendMessage(Message)} method will return false.
* </p>
*/
public void quitSafely() {
mQueue.quit(true);
}

/**
* Gets the Thread associated with this Looper.
*
* @return The looper's thread.
*/
public @NonNull Thread getThread() {
return mThread;
}

/**
* Gets this looper's message queue.
*
* @return The looper's message queue.
*/
public @NonNull MessageQueue getQueue() {
return mQueue;
}

......
}

frameworks/base/core/java/android/os/MessageQueue.java

private long mPtr; // used by native code
......

private native static long nativeInit();
......

MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}

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);
}
}

system/core/libutils/Looper.cpp

Looper::Looper(bool allowNonCallbacks) :
mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false),
mPolling(false), mEpollFd(-1), mEpollRebuildRequired(false),
mNextRequestSeq(0), mResponseIndex(0), mNextMessageUptime(LLONG_MAX) {
mWakeEventFd = ｑ(0, EFD_NONBLOCK | EFD_CLOEXEC);
LOG_ALWAYS_FATAL_IF(mWakeEventFd < 0, "Could not make wake event fd: %s",
strerror(errno));

AutoMutex _l(mLock);
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);
}
/**
使用Linux系统的epoll机制，首先要通过epoll_create来创建一个epoll专用的文件描述符
*/
// 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: %s", strerror(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;
/**
这里就是告诉mEpollFd，它要监控mWakeEventFd文件描述符的EPOLLIN事件，即当mWakeEventFd中有内容可读时，就唤醒当前正在等待mWakeEventFd中的内容的线程
*/
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: %s",
strerror(errno));

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: %s",
request.fd, strerror(errno));
}
}
}

总结

C++层的这个Looper对象创建完成后，就返回到JNI层的NativeMessageQueue的构造函数，最后就返回到Java层的消息队列MessageQueue的创建过程，这样，Java层的Looper对象就准备好了

在Java层，创建了一个Looper对象，这个Looper对象是用来进入消息循环的，它的内部有一个消息队列MessageQueue对象mQueue；

在JNI层，创建了一个NativeMessageQueue对象，这个NativeMessageQueue对象保存在Java层的消息队列对象mQueue的成员变量mPtr中；

在C++层，创建了一个Looper对象，保存在JNI层的NativeMessageQueue对象的成员变量mLooper中，这个对象的作用是，当Java层的消息队列中没有消息时，就使Android应用程序主线程进入等待状态，而当Java层的消息队列中来了新的消息后，就唤醒Android应用程序的主线程来处理这个消息。

frameworks/base/core/java/android/os/Looper.java

public final class Looper {
......

/**
* 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;

......

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 {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}

......

msg.recycleUnchecked();
}
}

......
}

frameworks/base/core/java/android/os/MessageQueue.java

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();
}
/**
查看当前消息队列中有没有消息，这里传入的参数mPtr指向前面在JNI层创建的NativeMessageQueue对象,
而参数nextPollTimeoutMillis则表示如果当前消息队列中没有消息，它要等待的时间，for循环开始时，
传入的值为0，表示不等待。
*/
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;
}
}

frameworks/base/core/jni/android_os_MessageQueue.cpp

static void android_os_MessageQueue_nativePollOnce(JNIEnv* env, jobject obj,
jlong ptr, jint timeoutMillis) {
NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
nativeMessageQueue->pollOnce(env, obj, timeoutMillis);
}

void NativeMessageQueue::pollOnce(JNIEnv* env, jobject pollObj, int timeoutMillis) {
mPollEnv = env;
mPollObj = pollObj;
mLooper->pollOnce(timeoutMillis);
mPollObj = NULL;
mPollEnv = NULL;

if (mExceptionObj) {
env->Throw(mExceptionObj);
env->DeleteLocalRef(mExceptionObj);
mExceptionObj = NULL;
}
}

/system/core/include/utils/Looper.h

inline int pollOnce(int timeoutMillis) {
return pollOnce(timeoutMillis, NULL, NULL, NULL);
}

system/core/libutils/Looper.cpp

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 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) {
#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);
}
}

system/core/libutils/Looper.cpp

[Looper.cpp–>Looper::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];
/**
当mEpollFd所监控的文件描述符发生了要监控的IO事件后或者监控时间超时后，线程就从epoll_wait返回了，否则线程就会在epoll_wait函数中进入睡眠状态
*/
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.返回后如果eventCount等于0，就说明是超时
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
//如果eventCount不等于0，就说明发生要监控的事件
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) {
/**
如果在mWakeEventFd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，接下来它就会先调用awoken函数清空
管道中的内容，以便下次再调用pollInner函数时，知道自从上次处理完消
息队列中的消息后，有没有新的消息加进来
*/
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;
}

void Looper::awoken() {
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ awoken", this);
#endif

uint64_t counter;
//把管道中的内容都读取出来
TEMP_FAILURE_RETRY(read(mWakeEventFd, &counter, sizeof(uint64_t)));
}

2. 消息的发送

frameworks/base/core/java/android/app/ActivityThread.java

// we use token to identify this activity without having to send the
// activity itself back to the activity manager. (matters more with ipc)
@Override
public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,
ActivityInfo info, Configuration curConfig, Configuration overrideConfig,
CompatibilityInfo compatInfo, String referrer, IVoiceInteractor voiceInteractor,
int procState, Bundle state, PersistableBundle persistentState,
List<ResultInfo> pendingResults, List<ReferrerIntent> pendingNewIntents,
boolean notResumed, boolean isForward, ProfilerInfo profilerInfo) {

updateProcessState(procState, false);
/**
把相关的参数都封装成一个ActivityClientRecord对象r，然后调用sendMessage方法来往应用程序的消息队列中加入一个新的消息（H.LAUNCH_ACTIVITY）
*/
ActivityClientRecord r = new ActivityClientRecord();

r.token = token;
r.ident = ident;
r.intent = intent;
r.referrer = referrer;
r.voiceInteractor = voiceInteractor;
r.activityInfo = info;
r.compatInfo = compatInfo;
r.state = state;
r.persistentState = persistentState;

r.pendingResults = pendingResults;
r.pendingIntents = pendingNewIntents;

r.startsNotResumed = notResumed;
r.isForward = isForward;

r.profilerInfo = profilerInfo;

r.overrideConfig = overrideConfig;
updatePendingConfiguration(curConfig);

sendMessage(H.LAUNCH_ACTIVITY, r);
}

private void sendMessage(int what, Object obj) {
sendMessage(what, obj, 0, 0, false);
}

private void sendMessage(int what, Object obj, int arg1) {
sendMessage(what, obj, arg1, 0, false);
}

private void sendMessage(int what, Object obj, int arg1, int arg2) {
sendMessage(what, obj, arg1, arg2, false);
}

private void sendMessage(int what, Object obj, int arg1, int arg2, boolean async) {
if (DEBUG_MESSAGES) Slog.v(
TAG, "SCHEDULE " + what + " " + mH.codeToString(what)
+ ": " + arg1 + " / " + obj);
Message msg = Message.obtain();
msg.what = what;
msg.obj = obj;
msg.arg1 = arg1;
msg.arg2 = arg2;
if (async) {
msg.setAsynchronous(true);
}
mH.sendMessage(msg);
}

private void sendMessage(int what, Object obj, int arg1, int arg2, int seq) {
if (DEBUG_MESSAGES) Slog.v(
TAG, "SCHEDULE " + mH.codeToString(what) + " arg1=" + arg1 + " arg2=" + arg2 +
"seq= " + seq);
Message msg = Message.obtain();
msg.what = what;
SomeArgs args = SomeArgs.obtain();
args.arg1 = obj;
args.argi1 = arg1;
args.argi2 = arg2;
args.argi3 = seq;
msg.obj = args;
/**
mH是ActivityThread类的成员变量，它的类型为H，继承于Handler
在ActivityThread的main()方法中，创建的H对象,因为H继承Handler,
因此会调用Handler的构造方法
*/
mH.sendMessage(msg);
}

frameworks/base/core/java/android/os/Handler.java

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());
}
}
/**
myLooper是Looper类的静态成员函数，通过它来获得一个Looper对象，这个Looper对象就是前面我们在分
析消息循环时，在ActivityThread类的main函数中通过Looper.prepareMainLooper函数创建的
*/
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
/**
通过Looper.mQueue来访问应用程序的消息队列
*/
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}

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) {
/**
设置这个消息的目标对象target，即这个消息最终是由谁来处理的
这里将它赋值为this，即表示这个消息最终由这个Handler对象
来处理，即由ActivityThread对象的mH成员变量来处理。
*/
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}

frameworks/base/core/java/android/os/MessageQueue.java

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) {
/**
当前消息队列为空时，这时候应用程序的主线程一般就是处于空闲等待状态了，这时候就要唤醒它
只要把消息放在消息队列头就可以了
*/
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
/**
应用程序的消息队列不为空，这时候就不需要唤醒应用程序的主线程了，因为这时候它一定是在忙着
处于消息队列中的消息，因此不会处于空闲等待的状态.
当往消息队列中发送消息时，是可以指定消息的处理时间的，而消息队列中的消息，就是按照这个时间从
小到大来排序的，因此，当把新的消息加入到消息队列时，就要根据它的处理时间来找到合适的位置，然
后再放进消息队列中去
*/
// 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;
}

frameworks/base/core/jni/android_os_MessageQueue.cpp

static void android_os_MessageQueue_nativeWake(JNIEnv* env, jclass clazz, jlong ptr) {
NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
nativeMessageQueue->wake();
}

void NativeMessageQueue::wake() {
mLooper->wake();
}

system/core/libutils/Looper.cpp

/**
wake函数很简单，只是通过打开文件描述符mWakeEventFd往写入一个1。写入内容的目的是为了唤醒应用程序的主线程。
前面我们在分析应用程序的消息循环时说到，当应用程序的消息队列中没有消息处理时，应用程序的主线程就会进入空闲等待状
态，而这个空闲等待状态就是通过调用这个Looper类的pollInner函数来进入的，具体就是在pollInner函数中调用epoll_wait
函数来等待mWakeEventFd中有内容可读的。这时候既然mWakeEventFd有内容可读了，应用程序的主线程就会从这里的Looper类
的pollInner函数返回到JNI层的nativePollOnce函数，最后返回到Java层中的MessageQueue.next函数中去，这里它就会发
现消息队列中有新的消息需要处理了，于就会处理这个消息。
*/
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) {
ALOGW("Could not write wake signal: %s", strerror(errno));
}
}
}

3. 消息的处理

frameworks/base/core/java/android/os/Looper.java

public final class Looper {
......

/**
* 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;

......

for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
......

try {
/**
根据上文分析target指的是ActivityThread的内部类H,由于H没有实现dispatchMessage，因此会调用
父类的dispatchMessage
*/
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}

......

msg.recycleUnchecked();
}
}

......
}

frameworks/base/core/java/android/os/ Handler.java

/**
这里的消息对象msg的callback成员变量和Handler类的mCallBack成员变量一般都为null，于是，就会调用Handler类的
handleMessage函数来处理这个消息，由于H类在继承Handler类时，重写了handleMessage函数，因此，这里调用的实际上
是H类的handleMessage函数
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}

frameworks/base/core/java/android/app/ActivityThread.java

public void handleMessage(Message msg) {
if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
switch (msg.what) {
case LAUNCH_ACTIVITY: {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
final ActivityClientRecord r = (ActivityClientRecord) msg.obj;

r.packageInfo = getPackageInfoNoCheck(
r.activityInfo.applicationInfo, r.compatInfo);
handleLaunchActivity(r, null, "LAUNCH_ACTIVITY");
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
} break;
case RELAUNCH_ACTIVITY: {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityRestart");
ActivityClientRecord r = (ActivityClientRecord)msg.obj;
handleRelaunchActivity(r);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
} break;
......

因为前面在分析消息的发送时所举的例子中，发送的消息的类型为H.LAUNCH_ACTIVITY，因此，这里就会调用ActivityThread类的handleLaunchActivity函数来真正地处理这个消息了

总结：

Android应用程序的消息处理机制由消息循环、消息发送和消息处理三个部分组成的。

Android应用程序的主线程在进入消息循环过程前，会在内部创建一个Linux eventfd，这个eventfd的作用是使得Android应用程序主线程在消息队列为空时可以进入空闲等待状态，并

且使得当应用程序的消息队列有消息需要处理时唤醒应用程序的主线程。

Android应用程序的主线程进入空闲等待状态的方式实际上就是等待eventfd内容可读，具体来说就是是通过Linux系统的Epoll机制中的epoll_wait函数进行的。

当往Android应用程序的消息队列中加入新的消息时，会同时往eventfd写入内容，通过这种方式就可以唤醒正在等待消息到来的应用程序主线程。

当应用程序主线程在进入空闲等待前，会认为当前线程处理空闲状态，于是就会调用那些已经注册了的IdleHandler接口，使得应用程序有机会在空闲的时候处理一些事情。