Dubbo存在内存泄漏

书唐瑞

发布于 2022-06-02 14:58:07

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发布于 2022-06-02 14:58:07

文章被收录于专栏：Netty历险记Netty历险记

[前提]

本文阐述的内容基于Dubbo 2.7.3版本

[正文]

如上图, 在上一篇 com.alibaba.fastjson存在内存泄漏文章中, 我们解释了线程的threadLocals中存在内存泄漏的情况, 仔细观察上图, 还有一个地方, 在threadLocalMap属性的内部也存在662.19KB的内存, 这个地方也不正常.

查看threadLocalMap的内部属性

在org.apache.dubbo.rpc.FutureContext内部的result属性`持有`651.93KB内存, 这个result的内容实际是Dubbo接口的返回值. 而这个FutureContext对象也是在调用外部Dubbo接口的时候创建的.

我们简单分析下一个业务线程调用Dubbo接口的过程.

当业务线程需要调用外部Dubbo接口的时候, 会创建一个DefaultFuture, 每个DefaultFuture对象都会有唯一的一个Id与之对应, 并把这个关系放到Map中

private DefaultFuture(Channel channel, Request request, int timeout) {
    this.channel = channel;
    this.request = request;
    this.id = request.getId();
    this.timeout = timeout > 0 ? timeout : channel.getUrl().getPositiveParameter(TIMEOUT_KEY, DEFAULT_TIMEOUT);
    // 存储 id <-> DefaultFuture关系
    FUTURES.put(id, this);
    CHANNELS.put(id, channel);
}

由于接口调用都会有超时, 那么如何实现这个超时机制呢?

将一个超时任务放入到时间轮上.

// org.apache.dubbo.remoting.exchange.support.DefaultFuture#newFuture
public static DefaultFuture newFuture(Channel channel, Request request, int timeout) {
    final DefaultFuture future = new DefaultFuture(channel, request, timeout);
    // 超时检查
    timeoutCheck(future);
    return future;
}

// org.apache.dubbo.remoting.exchange.support.DefaultFuture#timeoutCheck
private static void timeoutCheck(DefaultFuture future) {
    TimeoutCheckTask task = new TimeoutCheckTask(future.getId());
    future.timeoutCheckTask = TIME_OUT_TIMER.newTimeout(task, future.getTimeout(), TimeUnit.MILLISECONDS);
}

在我之前的 Netty中的时间轮(v3.10.7) 文章中介绍了时间轮, 通过时间轮的方式, 检测任务是否超时到期了.

接下来就是将DefaultFuture等信息组装成一个FutureContext放入到线程的ThreadLocalMap中.

// org.apache.dubbo.rpc.protocol.dubbo.DubboInvoker#doInvoke
protected Result doInvoke(final Invocation invocation) throws Throwable {
    try {
        boolean isOneway = RpcUtils.isOneway(getUrl(), invocation);
        int timeout = getUrl().getMethodPositiveParameter(methodName, TIMEOUT_KEY, DEFAULT_TIMEOUT);
        if (isOneway) {
            boolean isSent = getUrl().getMethodParameter(methodName, Constants.SENT_KEY, false);
            currentClient.send(inv, isSent);
            return AsyncRpcResult.newDefaultAsyncResult(invocation);
        } else {
            AsyncRpcResult asyncRpcResult = new AsyncRpcResult(inv);
            CompletableFuture<Object> responseFuture = currentClient.request(inv, timeout);
            asyncRpcResult.subscribeTo(responseFuture);
            // 
            FutureContext.getContext().setCompatibleFuture(responseFuture);
            return asyncRpcResult;
        }
    }
}

// org.apache.dubbo.rpc.FutureContext
public class FutureContext {
    private static InternalThreadLocal<FutureContext> futureTL = new InternalThreadLocal<FutureContext>() {
        @Override
        protected FutureContext initialValue() {
            return new FutureContext();
        }
    };

    public static FutureContext getContext() {
        return futureTL.get();
    }
}

综上, id和DefaultFuture存到Map中, 设置好定时任务, DefaultFuture放到线程ThreadLocalMap中之后, 线程就可以被阻塞了.

线程调用get方法一直被阻塞.

当Dubbo的提供方返回数据之后, Dubbo调用方的线程就可以处理响应了.

如上图, Dubbo调用方的Dubbo线程开始处理响应.

// org.apache.dubbo.remoting.exchange.support.DefaultFuture#received(org.apache.dubbo.remoting.Channel, org.apache.dubbo.remoting.exchange.Response, boolean)
public static void received(Channel channel, Response response, boolean timeout) {
    try {
      // 从Map中移除id <-> DefaultFuture的关系
        DefaultFuture future = FUTURES.remove(response.getId());
        if (future != null) {
            Timeout t = future.timeoutCheckTask;
            if (!timeout) {
                // 将时间轮上的超时任务取消掉
                t.cancel();
            }
            // 
            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());
    }
}

首先从Map中移除id <-> DefaultFuture的关系, 将时间轮上的超时任务取消掉.

接下来就是把响应数据设置到DefaultFuture上, 并唤醒之前阻塞的线程.

// org.apache.dubbo.remoting.exchange.support.DefaultFuture#doReceived
private void doReceived(Response res) {
    if (res.getStatus() == Response.OK) {
        this.complete(res.getResult());
    }
}

// java.util.concurrent.CompletableFuture#complete
public boolean complete(T value) {
    // 将结果设置到DefaultFuture
    boolean triggered = completeValue(value);
    // 唤醒阻塞线程
    postComplete();
    return triggered;
}

使用到了异步编程

被唤醒的阻塞线程就可以从DefaultFuture中拿到已设置好的数据,继续后续的业务处理.