Android 应用程序是通过消息来驱动的,系统为每一个应用程序维护一个消息队例,应用程序的主线程不断地从这个消息队例中获取消息(Looper),然后对这些消息进行处理(Handler),这样就实现了通过消息来驱动应用程序的执行。先了解一下涉及到的几个概念:
Message
消息(Message)代表一个行为(what)或者一串动作(Runnable),每一个消息在加入消息队列时,都有明确的目标(Handler)。
MessageQueue
以队列的形式存放消息对象,其内部结构是以链表的形式存储消息。对外提供插入和删除操作。
Looper
Looper 是循环的意思,它负责从 MessageQueue 中循环的取出 Message 然后交给目标(Handler)处理。
Handler
消息的真正处理者,具备获取消息、发送消息、处理消息、移除消息等功能。
ThreadLocal
作用是为了线程隔离,内部实现相当于Map以当前线程为key,存入的值作为 value。
Looper 不断从 MessageQueue 中取出一个 Message,然后交给其对应的 Handler 处理。
我们平时接触到的 Looper、Message、Handler 都是用 JAVA 实现的,Android 是一个基于 Linux 的系统,底层用C、C++实现的,而且还有 NDK 的存在,Android 消息驱动的模型为了消息的及时性、高效性,在 Native 层也设计了 Java 层对应的类如 Looper、MessageQueue 等。
在 ActivityThread 的 main 函数里面调用主线程的 loop 方法开启消息循环监听,这个 loop 方法会一直运行,伴随应用的整个生命周期。
以下是 ActitivyThread 的 main 的实现:
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
// CloseGuard defaults to true and can be quite spammy. We
// disable it here, but selectively enable it later (via
// StrictMode) on debug builds, but using DropBox, not logs.
CloseGuard.setEnabled(false);
Environment.initForCurrentUser();
// Set the reporter for event logging in libcore
EventLogger.setReporter(new EventLoggingReporter());
// Make sure TrustedCertificateStore looks in the right place for CA certificates
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
prepareMainLooper 做的事情其实就是在线程中创建一个 Looper 对象:
/** * 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(); }}
先调用 prepare 来进行主要成员变量的初始化,传传入参数 false 最终会传到 MessageQueue 的构造函数中。初始化完成后,接着调用 myLooper 方法将返回值赋给成员变量 sMainLooper,它也是一个 Looper 类型的成员变量,接着再来看一下 prepare 方法的实现,源码如下:
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));}
这个 Looper 对象是存放在 sThreadLocal 成员变量里面的。线程创建 Looper 对象的工作是由 prepare 函数来完成的,而在创建 Looper 对象的时候,会同时创建一个消息队列 MessageQueue,保存在 Looper 的成员变量 mQueue 中,后续消息就是存放在这个队列中去。消息队列在 Android 应用程序消息处理机制中最重要的组件,以下是它的创建过程:
public class MessageQueue { ...... // True if the message queue can be quit. private final boolean mQuitAllowed; private int mPtr; // used by native code private native void nativeInit(); MessageQueue(boolean quitAllowed) { mQuitAllowed = quitAllowed; mPtr = nativeInit(); } ......}
它的初始化工作都交给 JNI 方法 nativeInit 实现:
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);
}
在 JNI 中,也相应地创建了一个消息队列 NativeMessageQueue,接着把 C++ 里面的这个指针转成 jlong 类型返回给 java 层,赋值给前面我们在 Java 层创建的 MessageQueue 对象的 mPtr 成员变量。继续看 NativeMessageQueue 的创建过程:
NativeMessageQueue::NativeMessageQueue() : mPollEnv(NULL), mPollObj(NULL), mExceptionObj(NULL) { mLooper = Looper::getForThread(); if (mLooper == NULL) { mLooper = new Looper(false); Looper::setForThread(mLooper); }}
它主要就是在内部创建了一个 Looper 对象,这里的 Looper 跟 java 层的是对应的。继续看 Looper 对象的创建过程:
Looper::Looper(bool allowNonCallbacks) :
mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false),
mPolling(false), mEpollFd(-1), mEpollRebuildRequired(false),
mNextRequestSeq(0), mResponseIndex(0), mNextMessageUptime(LLONG_MAX) {
mWakeEventFd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);
LOG_ALWAYS_FATAL_IF(mWakeEventFd < 0, "Could not make wake event fd: %s",
strerror(errno));
AutoMutex _l(mLock);
rebuildEpollLocked();
}
该方法中首先调用 eventfd 系统函数,该函数返回一个文件描述符,与打开的其他文件一样,可以进行读写操作。然后调用 rebuildEpollLocked 函数继续进行后续的初始化,继续看 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.
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;
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));
}
}
}
为我们的主线程创建 Epoll 循环的结构体。该方法执行完,我们的 epoll 节点添加进去之后,那么初始化的工作就结束了。framework 中为我们创建好的 java 层的 Looper、MessageQueue 和 native 层的 Looper、NativeMessageQueue 都已经准备好了,epoll 机制相应的节点也注册好了。
下面我们接着来分析 ActivityThread 类的 main 方法中的 Looper.loop()的实现。先调用 myLooper 方法来判断前面的准备工作是否完成,如果准备工作都出错,那就直接抛出运行时异常。接着一个 for (;;) 无限循环取消息。queue.next()取下一个消息,该方法可能会阻塞,如果取到的 msg 为空,则说明消息循环要退出了,则直接 return。取到下一个消息 msg 之后,就调用 msg.target.dispatchMessage(msg) 将它分发给目标进行处理,msg 的成员变量 target 的类型为 Handler,它是在我们往当前的 MessageQueue 消息队列上发送消息时指定的,分发完成后调用 recycleUnchecked() 来将当前的 msg 回收掉。Message 对象的构建也是使用了一个缓存池,因为消息循环是非常频繁的,所以使用缓存池可以有效的减少无用内存的分配,非常必要。接下来重点看一下 queue.next() 是如何取到下一条消息的,该方法的实现在 MessageQueue 类中,方法的源码如下:
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
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;
}
}
主要是一个 for (;;) 无限循环,所有发送过来的消息最终都会存储在成员变量 mMessages 上,它的类型为 Message,Message 类又有一个类型为 Message 的成员变量 next,相当于 Message 类就是单向链表,所以我们发送过来的消息会不断的往上挂,从 mMessages 上取下一个消息 msg,如果当前消息时间未到,那么就需要休眠,休眠的时间长短取决于 nextPollTimeoutMillis;否则处理该消息,则将该消息返回给 Looper 类的 loop 方法中进行处理。下面看一下 nativePollOnce 的实现:
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);}
取到在 native 层创建的 NativeMessageQueue,然后调用它的 pollOnce 继续处理,pollOnce 方法的源码如下:
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;
}
}
调用 native 层的 Looper 类的 pollOnce 继续处理,源码如下:
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);
}
}
调用 pollInner 进一步处理,pollInner 方法的源码如下:
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;
// 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.
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
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;
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;
}
该方法的参数 timeoutMillis 就是下一个消息的等待时间,在调用 epollwait 系统函数时,就会将当前的线程休眠。休眠时间到之后,epollwait 就会返回,再次检查消息队列时,就会有符合要求的消息了。