葛泽续
2012 年 2 月加入去哪儿网，多次参与 Android 客户端的重构，插件化开发，客户端安全攻防，现负责国际酒店抓取系统架构升级维护。
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：
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）。
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 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。
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。
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");}
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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(); }}
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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));}
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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(); } ......}
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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);}
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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); }}
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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();}
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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)); } }}
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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; }}
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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);}
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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; }}
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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); }}
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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;}
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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。
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