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android源码分析-深入MessageQueue

LeexMuller / 1591人阅读

摘要:相当于层的初始化。注意,这里是层层自己的消息,与层的没关系。好吧,这个过程基本上分析完毕了,其实就是通过不断的处理消息,并且调用消息的回调。

承接上文在looper中会在一开始就创建一个MessageQueue,并且在loop中每次都会从其中取出一个message处理。那么我们就来看看这个MessageQueue:

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

nativeInit,无可避免的又要进入c层进行分析。对应的文件是/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(nativeMessageQueue);
}

这里创建了一个新的NativeMessageQueue并返回他的指针。这个类的定义也在此文件中,看看他的构造做了什么:

NativeMessageQueue::NativeMessageQueue() :
        mPollEnv(NULL), mPollObj(NULL), mExceptionObj(NULL) {
    mLooper = Looper::getForThread();
    if (mLooper == NULL) {
        mLooper = new Looper(false);
        Looper::setForThread(mLooper);
    }
}

新建了一个Looper对象,这个肯定不是java层的那个了,但是前后都有getForThread和setForThread。那么他们分别在干什么呢?我的理解是在做tls线程本地变量的处理,确保本线程只有一个looper。具体的内容在这里不再论述,后续有机会可以剖析下。

我们下面来看看这个Looper是什么吧,他的构造函数如下:

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();
}

除了状态的值得设置外,就是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));
        }
    }
}

我们看到了什么?epoll。这不是linux中的epoll吗?就是这个玩意,为了控制多个fd(文件描述符)的读写等事件而诞生的,一般多用于网络开发,类似win上的完成端口。然后新建了一个eventItem用于监听mWakeEventFd,就是将唤醒的eventfd放到epoll的监听队列中,用于唤醒机制。然后呢,进行了一个循环,取出所有的request,并且都放到了epoll监听,首次调用这个for循环不会被执行,因为mRequests的size是0。这些request都是什么呢?看定义:

    struct Request {
        int fd;
        int ident;
        int events;
        int seq;
        sp callback;
        void* data;

        void initEventItem(struct epoll_event* eventItem) const;
    };

那么他们对应的具体内容又是什么呢?先放一放,往下看。

回到java层的loop函数中,每次调用next方法获取message,那么看看这个MessageQueue的next方法:

    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;
        }
    }

首先看到获取了mPtr,这个ptr就是c层的nativeMessageQueue的地址。然后进入了一个死循环,率先走了一个nativePollOnce(ptr, nextPollTimeoutMillis);内部调用了android_os_MessageQueue_nativePollOnce:

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

这里实际上还原了地址为NativeMessageQueue对象,并调用了pollOnce方法:

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;
    }
}

保留了pollObj对象,并且调用了Looper的pollOnce。相当于c层Looper的初始化。那么来看看pollOnce:

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);
    }
}

一个死循环,里面先是一个while,优先处理应答response(一个request对应一个response),并返回。如果没有response需要处理的时候,走pollInner。这个pollInner是个关键,代码比较多,我们节选看:

    ......
    struct epoll_event eventItems[EPOLL_MAX_EVENTS];
    int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
    ......
    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);
            }
        }
    }
    ......

epoll_wait在mEpollFd上阻塞等待,直到有事件发生。如果等到了就执行下面的for循环,枚举每一个epoll_event,如果等待到的消息是唤醒消息(fd==mWakeEventFd),则执行awoken唤醒,否则判断epollEvents是否含有相关事件,如果有填写生成好的events,这个应该是转换一下事件为了上层使用。然后进行了pushResponse的动作,这里终于有个response生成的过程了,继续看下去:

void Looper::pushResponse(int events, const Request& request) {
    Response response;
    response.events = events;
    response.request = request;
    mResponses.push(response);
}

看到了吧,就是个填充response的过程,并将其push到mResponses中。再回到pollInner中往下看:

......
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 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;
        }
    }
    ......

一上来就是一个while循环,处理一下之前堆积的事件。注意,这里是c层(native层)自己的消息,与java层的没关系。这里有个时间的对比,如果每个messageEnvelope的uptime<=now,也即是小于等于当前时间,那么这个uptime是个什么呢?我的理解是一个唤醒时间,也就是message的执行时间,因为message是允许被后置一段时间执行的。如果需要被执行的时间比当前时间晚,就调用这个message的handler的handleMessage。看起来很合理,就是为了清除一下之前堆积还未执行的事件的handle的回调。
之后又是一个for循环:

    ......
    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;
        }
    }
    ......

这里就是处理response了,就是走一个response.request.callback->handleEvent。
我们现在继续找线索下,在Looper的构造中出现了mWakeEventFd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);,这个eventfd就是用来支持进程或者线程间通讯的通道,类似管道。

好吧,这个过程基本上分析完毕了,其实就是通过epoll不断的处理消息,并且调用消息的回调。但是其实整个过程还有很多不是很明确的地方,例如:1.这个epoll绑定的fd到底是个什么东西?是管道吗?网上的文章基本上都是说管道,这里我没有找到线索,不好确定。2.这个c层的looper中的sendmessage已经很明确是根据传递进来的参数来设定messageEnvelope的handler。但是调用他的是哪个东西呢?怎么和java层结合起来呢?有不少问题。

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