Android 线程间通讯机制

前言

之前研究Fragment遇到进程间通讯的一些东东，趁着最近有空，就在重新研究一下Android相关的代码。并且这些代码确实非常简单。之前研究过，但是遇到一些问题就没深究，这次我们就彻底搞懂他吧

正文

最简单的使用当然是在activity的oncreat中直接使用：

Handler handler = new Handler();
handler.post(new runable{
	Log.e("mainactivity","我是handler post的log");
})

但是我们的主线程初始化时候帮我们干了一大堆事情，研究不够典型，我们直接看新线程如何通讯，直接用HandlerThread来作为源码研究的调用开端

myHandlerThread = new HandlerThread( "handler-thread") ;
        //开启一个线程
myHandlerThread.start();
        //在这个线程中创建一个handler对象
handler = new Handler(myHandlerThread.getLooper())
handler.post(new runable{
	Log.e("mainactivity","我是handler post的log");
})

相信你第一眼目光就集中在run()函数中了，内容如下

@Override
    public void run() {
        mTid = Process.myTid();
        Looper.prepare();
        synchronized (this) {
            mLooper = Looper.myLooper();
            notifyAll();
        }
        Process.setThreadPriority(mPriority);
        onLooperPrepared();
        Looper.loop();
        mTid = -1;
    }

这个才是真的进程间通讯的核心,这里我们就一点一点阅读：

    /** Initialize the current thread as a looper.
      * This gives you a chance to create handlers that then reference
      * this looper, before actually starting the loop. Be sure to call
      * {@link #loop()} after calling this method, and end it by calling
      * {@link #quit()}.
      */
    public static void prepare() {
        prepare(true);
    }
    
    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));
    }

注释解释的很明白。就是初始化一个Looper。其实可能有疑惑的地方是sThreadLocal.set()这是维持在当前线程的一个变量，可以在任何线程中，调用myLooper。得到唯一的一个实例。暂时对我们问题空间没有影响。我们只要关心new Looper(quitAllowed)即可

    private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }

有些出乎意料，太简单了吧！可是短小的都是精华，脚下就是万丈深渊

    MessageQueue(boolean quitAllowed) {
        mQuitAllowed = quitAllowed;
        mPtr = nativeInit();
    }
        private native static long nativeInit();

emmm忽然进入native层了，有些让人望而却步。不过我们看过以前大佬的书和博客，还是很容易定为到源码的位置aosp\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<jlong>(nativeMessageQueue);
}

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

貌似和java层差不多，都是初始化了一个Looper。所有的等待和定时其实都是通过c层的Looper来实现的。我们这里就一一分析

Looper::Looper(bool allowNonCallbacks)
    {
    //初始化一个文件，作为中断的一端
    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);
    //关键部分
    rebuildEpollLocked();
}

这里所以的东西都是通过epoll来实现的休眠定时唤醒等操作。epoll大概原理是类似一个回调，但是属于系统层的。epoll可以监听一个文件是否变化，变化时候，可以回调函数，若文件不变，则线程会一直休眠。我们来看下相关代码

void Looper::rebuildEpollLocked() {

    // Allocate the new epoll instance and register the wake pipe.
    mEpollFd.reset(epoll_create1(EPOLL_CLOEXEC));

    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.get();
    int result = epoll_ctl(mEpollFd.get(), EPOLL_CTL_ADD, mWakeEventFd.get(), &eventItem);

//这个是为了系统层的一些东西准备的，这都没被调用所以可以注释掉
/**
    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.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));
        }
    }
**/
}

通过epoll_create1初始化。epoll_ctl注册回调监听.所有的事情都干完了。
下面进入真的loop函数了

/**
     * Run the message queue in this thread. Be sure to call
     * {@link #quit()} to end the loop.
     */
    public static void loop() {
        final Looper me = myLooper();
       
        final MessageQueue queue = me.mQueue;

        for (;;) {
            Message msg = queue.next(); // might block
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }

            try {
                msg.target.dispatchMessage(msg);
            } finally {
            }
            msg.recycleUnchecked();
        }
    }

很长的代码，但是貌似其他的内容都是线程安全和性能分析的trace的。所以我们就分析一下这些，其实看名字就容易理解。looper是一个终极循环，loop函数是终极循环的循环体。messagequeue是一个消息队列如果看注释。在queue的next方法中，他注释是可能阻塞，也就是没消息时候，就阻塞一下。整个逻辑就更清晰了。
我们还是看我们的queue的next方法，这里阻塞是如何实现的：

    Message next() {
   
        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;
                }

             }
             
            // 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;
        }
    }

主要函数还是这个nativePollOnce(ptr, ia)。ptr是cpp底层的messagequeue保存一个looper。其实最终就是调用了lopper的pollonce。我们直接看最终代码

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 (outFd != NULL) *outFd = fd;
                if (outEvents != NULL) *outEvents = events;
                if (outData != NULL) *outData = data;
                return ident;
            }
        }

        if (result != 0) {
            if (outFd != NULL) *outFd = 0;
            if (outEvents != NULL) *outEvents = 0;
            if (outData != NULL) *outData = NULL;
            return result;
        }

        result = pollInner(timeoutMillis);
    }
}

两个循环让人摸不着头脑，但是我们很容易确定mResponses里面没内容。所以就是等待pollInner函数返回部为0的值。继续追pollInner:

int Looper::pollInner(int timeoutMillis) {

    // 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];
    int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);

    // No longer idling.
    mPolling = false;

    // Acquire lock.
    mLock.lock();

    // Handle all events.

    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 {
        /**这是为了适应屏幕点击事件等其他的事情，才设置的，我们不关注即可
            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();

                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;
            // 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;
}

真个流程比较复杂，但是真的有用的地方只有那个int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);他是阻塞线程的核心，后面的就是处理一些屏幕点击事件唤醒线程的东东，完全可以无视，所以就返回了int result = POLL_WAKE
wait才是真正的阻塞，初始化的时候，timeoutMillis都为0，所以是不会阻塞的，直接返回到Looper中的loop函数中。
loop函数中。再次进入cpp层，timeoutMillis变成了-1.然后就可以睡眠了，等待监听的文件有写入操作，继续进入loop函数的无限循环。
下面我们分析handlerThread的post发送消息。
这里很容易进入核心的函数调用到了messageQueue中的enqueueMessage

    boolean enqueueMessage(Message msg, long when) {
        
        synchronized (this) {
        
            msg.markInUse();
            msg.when = when;
            Message p = mMessages;
            boolean needWake;
            if (p == null || when == 0 || when < p.when) {
                // New head, wake up the event queue if blocked.
                msg.next = p;
                mMessages = msg;
                needWake = mBlocked;
            } else {
                // Inserted within the middle of the queue.  Usually we don't have to wake
                // up the event queue unless there is a barrier at the head of the queue
                // and the message is the earliest asynchronous message in the queue.
                needWake = mBlocked && p.target == null && msg.isAsynchronous();
                Message prev;
                for (;;) {
                    prev = p;
                    p = p.next;
                    if (p == null || when < p.when) {
                        break;
                    }
                    if (needWake && p.isAsynchronous()) {
                        needWake = false;
                    }
                }
                msg.next = p; // invariant: p == prev.next
                prev.next = msg;
            }

            // We can assume mPtr != 0 because mQuitting is false.
            if (needWake) {
                nativeWake(mPtr);
            }
        }
        return true;
    }

其实这个代码会比较简单，就是判断消息队列mMessages是否为空，为空就吧自己当成消息队列的开头，然后通过一个for循环，把当前的消息插入合适的位置，ps只有当当前消息的when小于消息队列中的最后一个，或者消息队列后面已经没内容的时候，就把消息插入到当前的位置，然后调用唤醒线程的操作nativeWake(mPtr);
具体的代码如下：

void Looper::wake() {

    uint64_t inc = 1;
    ssize_t nWrite = TEMP_FAILURE_RETRY(write(mWakeEventFd, &inc, sizeof(uint64_t)));
}

忽然觉得代码太简单了吧，可是道理就这样，你只要对那个mWakeEventFd写入一个内容，刚才pollInner中就会的wait就会被唤醒，然后就开始执行刚才我们插入的一个内容。
关于退出就不在详细分析，觉得计较简单。

后记

这个东东早就了解代码，但是总是在cpp层的epoll就完全搞不懂了，这次了解整个原理，内心还是很开心的。以后有空自己写一个简化版的handler机制。敬请期待。