consumer的DubboResponseTimeoutScanTimer线程

考虑这样一种情况，由于网络延时，consumer先抛出超时异常，一段时间后又收到了已经超时的响应，dubbo是怎么处理的？

拆分为3步看：

1. consumer的DubboResponseTimeoutScanTimer进行扫描

DubboResponseTimeoutScanTimer负责扫描响应，如果发现超时，自行构造一个超时响应，并处理。

Future，Request，Response共用同一个id

//DefaultFuture内部类
private static class RemotingInvocationTimeoutScan implements Runnable {

    public void run() {
        while (true) {
            try {
                for (DefaultFuture future : FUTURES.values()) {
                    if (future == null || future.isDone()) {
                        continue;
                    }
                    if (System.currentTimeMillis() - future.getStartTimestamp() > future.getTimeout()) {
                        // consumer创建一个超时响应
                        // create exception response.
                        Response timeoutResponse = new Response(future.getId());
                        // set timeout status.
                        timeoutResponse.setStatus(future.isSent() ? Response.SERVER_TIMEOUT : Response.CLIENT_TIMEOUT);
                        timeoutResponse.setErrorMessage(future.getTimeoutMessage(true));
                        // handle response.
                        DefaultFuture.received(future.getChannel(), timeoutResponse);
                    }
                }
                Thread.sleep(30);
            } catch (Throwable e) {
                logger.error("Exception when scan the timeout invocation of remoting.", e);
            }
        }
    }
}

//DefaultFuture类
public static void received(Channel channel, Response response) {
    try {
        //首先删除future对象
        DefaultFuture future = FUTURES.remove(response.getId());
        if (future != null) {
            future.doReceived(response);
        } else {
            logger.warn("The timeout response finally returned at " 
                        + (new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS").format(new Date())) 
                        + ", response " + response 
                        + (channel == null ? "" : ", channel: " + channel.getLocalAddress() 
                            + " -> " + channel.getRemoteAddress()));
        }
    } finally {
        CHANNELS.remove(response.getId());
    }
}

private void doReceived(Response res) {
    lock.lock();
    try {
        response = res;
        if (done != null) {
            //唤醒等待的线程（也许有，也许没有）
            done.signal();
        }
    } finally {
        lock.unlock();
    }
    if (callback != null) {
        invokeCallback(callback);
    }
}

2. consumer因为超时抛异常

//DefaultFuture
public Object get(int timeout) throws RemotingException {
    if (timeout <= 0) {
        timeout = Constants.DEFAULT_TIMEOUT;
    }
    if (! isDone()) {
        long start = System.currentTimeMillis();
        lock.lock();
        try {
            while (! isDone()) {
　　             // 被DubboResponseTimeoutScanTimer线程唤醒，但是有个超时的响应，所以isDone返回true　　　　　　
                done.await(timeout, TimeUnit.MILLISECONDS);
                if (isDone() || System.currentTimeMillis() - start > timeout) {
                    break;
                }
            }
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        } finally {
            lock.unlock();
        }
        if (! isDone()) {
            throw new TimeoutException(sent > 0, channel, getTimeoutMessage(false));
        }
    }
    // isDone返回true，进入returnFromResponse
    return returnFromResponse();
}

private Object returnFromResponse() throws RemotingException {
    Response res = response;
    if (res == null) {
        throw new IllegalStateException("response cannot be null");
    }
    if (res.getStatus() == Response.OK) {
        return res.getResult();
    }
    if (res.getStatus() == Response.CLIENT_TIMEOUT || res.getStatus() == Response.SERVER_TIMEOUT) {
        //此处抛异常
        throw new TimeoutException(res.getStatus() == Response.SERVER_TIMEOUT, channel, res.getErrorMessage());
    }
    throw new RemotingException(channel, res.getErrorMessage());
}

3. 迟到的请求到来时

//DefaultFuture
public static void received(Channel channel, Response response) {
    try {
        //DubboResponseTimeoutScanTimer已经删除了迟到的请求
        //所以走else分支
        DefaultFuture future = FUTURES.remove(response.getId());
        if (future != null) {
            future.doReceived(response);
        } else {
            logger.warn("The timeout response finally returned at " 
                        + (new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS").format(new Date())) 
                        + ", response " + response 
                        + (channel == null ? "" : ", channel: " + channel.getLocalAddress() 
                            + " -> " + channel.getRemoteAddress()));
        }
    } finally {
        CHANNELS.remove(response.getId());
    }
}

静态分析代码时，以为先是步骤2，然后是步骤1，但实际调试时，结果是步骤1，步骤2.