Asop 之 消息处理机制

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 epoll wait 系统函数时，就会将当前的线程休眠。休眠时间到之后，epoll wait 就会返回，再次检查消息队列时，就会有符合要求的消息了。

END