Android 消息机制 -- Handler 的 Native 实现 ⚓︎

基于 Android android-14.0.0_r28 的源代码，来分析 Handler 消息处理机制在 Native 层的实现逻辑

源码路径

/

framework/base/core/java/andorid/os/MessageQueue.java

framework/base/core/jni/android_os_MessageQueue.cpp
framework/base/core/java/andorid/os/Looper.java

system/core/libutils/Looper.cpp
system/core/include/utils/Looper.h
system/core/libutils/RefBase.cpp

framework/base/native/android/looper.cpp
framework/native/include/android/looper.h

概述 ⚓︎

在 Android 消息机制 -- Handler 的 Java 实现中分析了 Handler 消息处理机制在 Java 层的实现机制，其中 MessageQueue 类作为连接 Java 层和 Native 层的纽带涉及到多个 native 方法. 实际上在 Native 层还有一整套由 Java 层驱动的消息机制实现，用于处理 native 的消息。

背景知识: epoll 机制

结构体和类 ⚓︎

Message ⚓︎

system/core/libutils/include/utils/Looper.h
struct Message {
    Message() : what(0) { }
    Message(int w) : what(w) { }

    /* The message type. (interpretation is left up to the handler) */
    int what;
};

system/core/libutils/include/utils/Looper.h
struct MessageEnvelope {
    MessageEnvelope() : uptime(0) { }

    MessageEnvelope(nsecs_t u, sp<MessageHandler> h, const Message& m)
        : uptime(u), handler(std::move(h)), message(m) {}

    nsecs_t uptime;
    sp<MessageHandler> handler;
    Message message;
};

Request & Response ⚓︎

system/core/libutils/include/utils/Looper.h
struct Request {
    int fd;
    int ident;
    int events;
    sp<LooperCallback> callback;
    void* data;

    uint32_t getEpollEvents() const;
};

struct Response {
    SequenceNumber seq;
    int events;
    Request request;
};

MessageHandler ⚓︎

system/core/libutils/include/utils/Looper.h
class MessageHandler : public virtual RefBase {
protected:
    virtual ~MessageHandler();

public:
    /**
     * Handles a message.
     */
    virtual void handleMessage(const Message& message) = 0;
};

class WeakMessageHandler : public MessageHandler {
protected:
    virtual ~WeakMessageHandler();

public:
    WeakMessageHandler(const wp<MessageHandler>& handler);
    virtual void handleMessage(const Message& message);

private:
    wp<MessageHandler> mHandler;
};

LooperCallback ⚓︎

system/core/libutils/include/utils/Looper.h
class LooperCallback : public virtual RefBase {
protected:
    virtual ~LooperCallback();

public:
    /**
     * Handles a poll event for the given file descriptor.
     * It is given the file descriptor it is associated with,
     * a bitmask of the poll events that were triggered (typically EVENT_INPUT),
     * and the data pointer that was originally supplied.
     *
     * Implementations should return 1 to continue receiving callbacks, or 0
     * to have this file descriptor and callback unregistered from the looper.
     */
    virtual int handleEvent(int fd, int events, void* data) = 0;
};

class SimpleLooperCallback : public LooperCallback {
protected:
    virtual ~SimpleLooperCallback();

public:
    SimpleLooperCallback(Looper_callbackFunc callback);
    virtual int handleEvent(int fd, int events, void* data);

private:
    Looper_callbackFunc mCallback;
};

映射关系

MessageQueue ⚓︎

private native static long nativeInit();
private native static void nativeDestroy(long ptr);
@UnsupportedAppUsage
private native void nativePollOnce(long ptr, int timeoutMillis); /*non-static for callbacks*/
private native static void nativeWake(long ptr);
private native static boolean nativeIsPolling(long ptr);
private native static void nativeSetFileDescriptorEvents(long ptr, int fd, int events);

nativeInit ⚓︎

在 Java 层创建 MessageQueue 对象时调用,用于创建 Native 层的 MessageQueue 以及 Looper. 具体调用链如下:

frameworks/base/core/jni/android_os_MessageQueue.cpp
NativeMessageQueue::NativeMessageQueue() :
        mPollEnv(NULL), mPollObj(NULL), mExceptionObj(NULL) {
    // 这里的 Looper 是 Native 的实现, 和 Java 层的对象无关
    // setForThread getForThread 等同于 Java 层的 ThreadLocal相关操作
    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),
      mEpollRebuildRequired(false),
      mNextRequestSeq(WAKE_EVENT_FD_SEQ + 1),
      mResponseIndex(0),
      mNextMessageUptime(LLONG_MAX) {
    // Native 层的 Looper 利用 epoll 机制实现运转
    // mWakeEventFd 就是用来接受写入事件的进而实现唤醒的载体(非阻塞/不可继承)
    mWakeEventFd.reset(eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC));
    LOG_ALWAYS_FATAL_IF(mWakeEventFd.get() < 0, "Could not make wake event fd: %s", strerror(errno));

    AutoMutex _l(mLock);
    rebuildEpollLocked();
}

void Looper::rebuildEpollLocked() {
    // 清理老的文件描述符
    // Close old epoll instance if we have one.
    if (mEpollFd >= 0) {
#if DEBUG_CALLBACKS
        ALOGD("%p ~ rebuildEpollLocked - rebuilding epoll set", this);
#endif
        mEpollFd.reset();
    }

    // 调用 epoll_create1 创建 mEpollFd 
    // Allocate the new epoll instance and register the WakeEventFd.
    mEpollFd.reset(epoll_create1(EPOLL_CLOEXEC));
    LOG_ALWAYS_FATAL_IF(mEpollFd < 0, "Could not create epoll instance: %s", strerror(errno));

    // 并且监听 mWakeEventFd 的可读(EPOLLIN)事件
    epoll_event wakeEvent = createEpollEvent(EPOLLIN, WAKE_EVENT_FD_SEQ);
    int result = epoll_ctl(mEpollFd.get(), EPOLL_CTL_ADD, mWakeEventFd.get(), &wakeEvent);
    LOG_ALWAYS_FATAL_IF(result != 0, "Could not add wake event fd to epoll instance: %s",
                        strerror(errno));

    // 监听所有的 mRequests 队列
    for (const auto& [seq, request] : mRequests) {
        epoll_event eventItem = createEpollEvent(request.getEpollEvents(), seq);

        int epollResult = epoll_ctl(mEpollFd.get(), 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));
        }
    }
}

由此 Native 层的 MessageQueue 和 Looper 对象就创建好了,并且完成了 epoll 的创建和监听

nativePollOnce ⚓︎

Native 层的 Looper 并不能独立运转,需要依赖 Java 层驱动. nativePollOnce 就是在 Java Looper.loop 过程中用来驱动 Native 层的 Looper 的桥梁. 具体调用链如下:

frameworks/base/core/jni/android_os_MessageQueue.cpp
void NativeMessageQueue::pollOnce(JNIEnv* env, jobject pollObj, int timeoutMillis) {
    // 直接调用 Looper->pollOnce 有一场就抛出
    mPollEnv = env;
    mPollObj = pollObj;
    mLooper->pollOnce(timeoutMillis);
    mPollObj = NULL;
    mPollEnv = NULL;

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

system/core/libutils/Looper.cpp
int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) {
    /**
        result 返回值
        1. POLL_WAKE(-1) 表示 epoll_wait() 是被 eventfd 唤醒的
        2. POLL_CALLBACK(-2) 表示消息或请求是通过回调处理的
        3. POLL_TIMEOUT(-3) 表示epoll_wait()超时
        4. POLL_ERROR(-4) 表示epoll_wait()出错
    */
    int result = 0;
    for (;;) {
        // 优先处理 mResponses 找到 request.ident >= 0 (非 callback 处理)的消息
        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 (outFd != nullptr) *outFd = fd;
                if (outEvents != nullptr) *outEvents = events;
                if (outData != nullptr) *outData = data;
                return ident;
            }
        }

        // 检查&处理返回值,这里的 result 一定是负数
        if (result != 0) {
            if (outFd != nullptr) *outFd = 0;
            if (outEvents != nullptr) *outEvents = 0;
            if (outData != nullptr) *outData = nullptr;
            return result;
        }

        // pollInner 返回 epoll 等待并处理事件的结果
        result = pollInner(timeoutMillis);
    }
}

int Looper::pollInner(int timeoutMillis) {
    // 根据当前时间和下一条消息的执行时间,调整超时时间
    // 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;
        }
    }

    // Poll.
    int result = POLL_WAKE;
    mResponses.clear();
    mResponseIndex = 0;

    // We are about to idle.
    mPolling = true;

    struct epoll_event eventItems[EPOLL_MAX_EVENTS];
    // epoll_wait 等待事件发生, 说 nativePollOnce 阻塞, 实际是在等事件发生
    int eventCount = epoll_wait(mEpollFd.get(), eventItems, EPOLL_MAX_EVENTS, timeoutMillis);

    // epoll_wait 一旦返回,就说明有事可干了 也就不是空闲状态
    // No longer idling.
    mPolling = false;

    // Acquire lock.
    mLock.lock();

    // 如果有必要, 重新创建 epoll
    // addFd 和 removeSequenceNumberLocked 会触发 Looper::scheduleEpollRebuildLocked() 修改 mEpollRebuildRequired
    // 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) {
        result = POLL_TIMEOUT;
        goto Done;
    }

    // 虽说不提倡 goto,该用还是用
    // 前面几个 goto Done 会忽略当前发生的事件
    // 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++) {
        const SequenceNumber seq = eventItems[i].data.u64;
        uint32_t epollEvents = eventItems[i].events;
        // WAKE_EVENT_FD_SEQ 是唤醒 mWakeEventFd 携带的数据,表示这次是被唤醒的
        // awoken() 会清理唤醒时候写入的数据
        if (seq == WAKE_EVENT_FD_SEQ) {
            if (epollEvents & EPOLLIN) {
                awoken();
            } else {
                ALOGW("Ignoring unexpected epoll events 0x%x on wake event fd.", epollEvents);
            }
        } else {
            // 不是 mWakeEventFd 就看看是哪个 Request 有事情,构造出 Response
            const auto& request_it = mRequests.find(seq);
            if (request_it != mRequests.end()) {
                const auto& request = request_it->second;
                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;
                mResponses.push({.seq = seq, .events = events, .request = request});
            } else {
                ALOGW("Ignoring unexpected epoll events 0x%x for sequence number %" PRIu64
                      " that is no longer registered.",
                      epollEvents, seq);
            }
        }
    }
Done: ;

    // 这里才是真正在处理 MessageEnvelope
    // 类似 Java 层的逻辑,从前往后遍历,找到可处理的消息来处理
    // 队列中剩下的都是不可处理的消息,第一个消息决定下次唤醒的时间
    // 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();

    // 处理掉需要通过 callback 执行的 request 产生的 response
    // 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) {
                AutoMutex _l(mLock);
                removeSequenceNumberLocked(response.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;
}

nativeWake ⚓︎

Java 层的 MessageQueue 在 1. 退出; 2. 删除了队列头部的 Barrier; 3. 有新消息来,且满足一定条件; 会通过 nativeWake 唤醒 Native 层 Looper. 具体调用链如下:

system/core/libutils/Looper.cpp
void Looper::wake() {
    uint64_t inc = 1;
    // 通过不断向 mWakeEventFd 写入 1, 触发 epoll_wait 返回
    ssize_t nWrite = TEMP_FAILURE_RETRY(write(mWakeEventFd.get(), &inc, sizeof(uint64_t)));
    if (nWrite != sizeof(uint64_t)) {
        if (errno != EAGAIN) {
            LOG_ALWAYS_FATAL("Could not write wake signal to fd %d (returned %zd): %s",
                             mWakeEventFd.get(), nWrite, strerror(errno));
        }
    }
}

nativeDestroy ⚓︎

Java 层队列退出或者销毁的时候通过 dispose() 调用:

system/core/libutils/RefBase.cpp
void RefBase::decStrong(const void* id) const
{
    weakref_impl* const refs = mRefs;
    refs->removeStrongRef(id); // 移除强引用
    const int32_t c = refs->mStrong.fetch_sub(1, std::memory_order_release);
    LOG_ALWAYS_FATAL_IF(BAD_STRONG(c), "decStrong() called on %p too many times",
            refs);
    if (c == 1) {
        std::atomic_thread_fence(std::memory_order_acquire);
        refs->mBase->onLastStrongRef(id);
        int32_t flags = refs->mFlags.load(std::memory_order_relaxed);
        if ((flags&OBJECT_LIFETIME_MASK) == OBJECT_LIFETIME_STRONG) {
            delete this;
            // The destructor does not delete refs in this case.
        }
    }
    refs->decWeak(id); // 移除弱引用
}

Looper ⚓︎

addFd ⚓︎

Native 层的 RenderThread surfaceflinger的vsync 等通过 addFd 方式设置 Request,在由 Looper 机制驱动回调处理

system/core/libutils/Looper.cpp
int Looper::addFd(int fd, int ident, int events, Looper_callbackFunc callback, void* data) {
    sp<SimpleLooperCallback> looperCallback;
    if (callback) {
        looperCallback = sp<SimpleLooperCallback>::make(callback);
    }
    return addFd(fd, ident, events, looperCallback, data);
}

int Looper::addFd(int fd, int ident, int events, const sp<LooperCallback>& callback, void* data) {
#if DEBUG_CALLBACKS
    ALOGD("%p ~ addFd - fd=%d, ident=%d, events=0x%x, callback=%p, data=%p", this, fd, ident,
            events, callback.get(), data);
#endif

    if (!callback.get()) {
        if (! mAllowNonCallbacks) {
            ALOGE("Invalid attempt to set NULL callback but not allowed for this looper.");
            return -1;
        }

        if (ident < 0) {
            ALOGE("Invalid attempt to set NULL callback with ident < 0.");
            return -1;
        }
    } else {
        ident = POLL_CALLBACK;
    }

    { // acquire lock
        AutoMutex _l(mLock);
        // There is a sequence number reserved for the WakeEventFd.
        if (mNextRequestSeq == WAKE_EVENT_FD_SEQ) mNextRequestSeq++;
        const SequenceNumber seq = mNextRequestSeq++;

        Request request;
        request.fd = fd;
        request.ident = ident;
        request.events = events;
        request.callback = callback;
        request.data = data;

        epoll_event eventItem = createEpollEvent(request.getEpollEvents(), seq);
        auto seq_it = mSequenceNumberByFd.find(fd);
        if (seq_it == mSequenceNumberByFd.end()) {
            int epollResult = epoll_ctl(mEpollFd.get(), EPOLL_CTL_ADD, fd, &eventItem);
            if (epollResult < 0) {
                ALOGE("Error adding epoll events for fd %d: %s", fd, strerror(errno));
                return -1;
            }
            mRequests.emplace(seq, request);
            mSequenceNumberByFd.emplace(fd, seq);
        } else {
            int epollResult = epoll_ctl(mEpollFd.get(), EPOLL_CTL_MOD, fd, &eventItem);
            if (epollResult < 0) {
                if (errno == ENOENT) {
                    epollResult = epoll_ctl(mEpollFd.get(), EPOLL_CTL_ADD, fd, &eventItem);
                    if (epollResult < 0) {
                        ALOGE("Error modifying or adding epoll events for fd %d: %s",
                                fd, strerror(errno));
                        return -1;
                    }
                    scheduleEpollRebuildLocked();
                } else {
                    ALOGE("Error modifying epoll events for fd %d: %s", fd, strerror(errno));
                    return -1;
                }
            }
            const SequenceNumber oldSeq = seq_it->second;
            mRequests.erase(oldSeq);
            mRequests.emplace(seq, request);
            seq_it->second = seq;
        }
    } // release lock
    return 1;
}

sendMessage ⚓︎

MessageQueue::postMessage 以及 Looper::sendMessage 和 Looper::sendMessageDelayed 最终都是调用 Looper::sendMessageAtTime 来完成消息插入.

system/core/libutils/Looper.cpp
void Looper::sendMessageAtTime(nsecs_t uptime, const sp<MessageHandler>& handler,
        const Message& message) {
#if DEBUG_CALLBACKS
    ALOGD("%p ~ sendMessageAtTime - uptime=%" PRId64 ", handler=%p, what=%d",
            this, uptime, handler.get(), message.what);
#endif

    size_t i = 0;
    { // acquire lock
        AutoMutex _l(mLock);

        // 与 Java 层逻辑类似, 将 message 插入基于 uptime 排序的队列
        size_t messageCount = mMessageEnvelopes.size();
        while (i < messageCount && uptime >= mMessageEnvelopes.itemAt(i).uptime) {
            i += 1;
        }

        MessageEnvelope messageEnvelope(uptime, handler, message);
        mMessageEnvelopes.insertAt(messageEnvelope, i, 1);

        // Optimization: If the Looper is currently sending a message, then we can skip
        // the call to wake() because the next thing the Looper will do after processing
        // messages is to decide when the next wakeup time should be.  In fact, it does
        // not even matter whether this code is running on the Looper thread.
        //如果当前正在发送消息，那么不再需要调用 wake()，直接返回。
        if (mSendingMessage) {
            return;
        }
    } // release lock

    // message 插入队头, 主动唤醒一次
    // Wake the poll loop only when we enqueue a new message at the head.
    if (i == 0) {
        wake();
    }
}

removeMessages ⚓︎

同样与 Java 层逻辑类似, 将满足条件的 message 从队列中移除

system/core/libutils/Looper.cpp
void Looper::removeMessages(const sp<MessageHandler>& handler) {
#if DEBUG_CALLBACKS
    ALOGD("%p ~ removeMessages - handler=%p", this, handler.get());
#endif

    { // acquire lock
        AutoMutex _l(mLock);

        for (size_t i = mMessageEnvelopes.size(); i != 0; ) {
            const MessageEnvelope& messageEnvelope = mMessageEnvelopes.itemAt(--i);
            if (messageEnvelope.handler == handler) {
                mMessageEnvelopes.removeAt(i);
            }
        }
    } // release lock
}

void Looper::removeMessages(const sp<MessageHandler>& handler, int what) {
#if DEBUG_CALLBACKS
    ALOGD("%p ~ removeMessages - handler=%p, what=%d", this, handler.get(), what);
#endif

    { // acquire lock
        AutoMutex _l(mLock);

        for (size_t i = mMessageEnvelopes.size(); i != 0; ) {
            const MessageEnvelope& messageEnvelope = mMessageEnvelopes.itemAt(--i);
            if (messageEnvelope.handler == handler
                    && messageEnvelope.message.what == what) {
                mMessageEnvelopes.removeAt(i);
            }
        }
    } // release lock
}