Dubbo源码分析：Dubbo协议客户端单一长连接RPC并发调用的结果获取

浅浅的花香味﹌ 2022-04-01 08:31 332阅读 0赞

### RPC并发调用的结果获取原理 ###

*  Dubbo协议在客户端针对所有的Service类，默认是使用单一Netty长连接来处理对这些Service类的方法的RPC调用请求的，即所有Service共享这个单一netty长连接。而在客户端，如在web环境中，任何一个时刻，可能存在多个线程并发对该Service进行并发调用，这些请求都是通过该单一Channel发送和获取结果的，而Netty所有请求都是异步，故dubbo如何保证这些并发线程能正确获取到自己的请求结果，而不会造成数据混乱呢？核心实现为：

1.  客户端Request通过AtomicLong生成的当前进程全局唯一id，服务端响应回传该id；
2.  客户端通过FUTURES静态ConcurrentHashMap保存调用id和异步结果DefaultFuture之间的关系，服务端响应时，查询根据Response的回传请求id，获取该response对应的DefaultFuture，通过await和signal机制实现请求发起线程和结果获取线程之间的通信，最终请求发起线程得到最终的结果。

### 源码实现 ###

*  当客户端发起对服务端的RPC调用时，使用的是DubboInvoker的doInvoker方法：

protected Result doInvoke(final Invocation invocation) throws Throwable { 
        RpcInvocation inv = (RpcInvocation) invocation;
        final String methodName = RpcUtils.getMethodName(invocation);
        inv.setAttachment(Constants.PATH_KEY, getUrl().getPath());
        inv.setAttachment(Constants.VERSION_KEY, version);
        ExchangeClient currentClient;
        if (clients.length == 1) { 
            currentClient = clients[0];
        } else { 
            currentClient = clients[index.getAndIncrement() % clients.length];
        }
        try { 
            boolean isAsync = RpcUtils.isAsync(getUrl(), invocation);
            boolean isAsyncFuture = RpcUtils.isGeneratedFuture(inv) || RpcUtils.isFutureReturnType(inv);
            boolean isOneway = RpcUtils.isOneway(getUrl(), invocation);
            int timeout = getUrl().getMethodParameter(methodName, Constants.TIMEOUT_KEY, Constants.DEFAULT_TIMEOUT);
            if (isOneway) { 
                boolean isSent = getUrl().getMethodParameter(methodName, Constants.SENT_KEY, false);
                currentClient.send(inv, isSent);
                RpcContext.getContext().setFuture(null);
                return new RpcResult();
            } else if (isAsync) { 
                ResponseFuture future = currentClient.request(inv, timeout);
                // For compatibility
                FutureAdapter<Object> futureAdapter = new FutureAdapter<>(future);
                RpcContext.getContext().setFuture(futureAdapter);
                Result result;
                if (isAsyncFuture) { 
                    // register resultCallback, sometimes we need the asyn result being processed by the filter chain.
                    result = new AsyncRpcResult(futureAdapter, futureAdapter.getResultFuture(), false);
                } else { 
                    result = new SimpleAsyncRpcResult(futureAdapter, futureAdapter.getResultFuture(), false);
                }
                return result;
            } else { 
                RpcContext.getContext().setFuture(null);
                return (Result) currentClient.request(inv, timeout).get();
            }
        } catch (TimeoutException e) { 
            throw new RpcException(RpcException.TIMEOUT_EXCEPTION, "Invoke remote method timeout. method: " + invocation.getMethodName() + ", provider: " + getUrl() + ", cause: " + e.getMessage(), e);
        } catch (RemotingException e) { 
            throw new RpcException(RpcException.NETWORK_EXCEPTION, "Failed to invoke remote method: " + invocation.getMethodName() + ", provider: " + getUrl() + ", cause: " + e.getMessage(), e);
        }
    }

核心关注：调用HeaderExchangeClient发送请求，获取future，这个future是DefaultFuture类，然后封装成FutureAdapter，构造AsyncRpcResult的result：

// 代码1
    ResponseFuture future = currentClient.request(inv, timeout);
    // For compatibility
    FutureAdapter<Object> futureAdapter = new FutureAdapter<>(future);
    RpcContext.getContext().setFuture(futureAdapter);
    Result result;
    if (isAsyncFuture) { 
        // register resultCallback, sometimes we need the asyn result being processed by the filter chain.
        result = new AsyncRpcResult(futureAdapter, futureAdapter.getResultFuture(), false);
    } else { 
    
    AsyncRpcResult的getRpcResult实现：
    public Result getRpcResult() { 
        Result result;
        try { 
            result = resultFuture.get();
        } catch (Exception e) { 
            // This should never happen;
            logger.error("", e);
            result = new RpcResult();
        }
        return result;
    }
    // 即调用了DefaultFuture的get()方法来获取结果，get中会通过DefaultFuture的done，调用done.await进行等待，这里是实现的关键，具体看下面的分析。
    
    // 代码2
    // currentClient.request底层最终调用HeaderExchangeChannel的request方法：通过DefaultFuture.newFuture(channel, req, timeout)创建DefaultFuture实例future并返回。
    
     public ResponseFuture request(Object request, int timeout) throws RemotingException { 
        if (closed) { 
            throw new RemotingException(this.getLocalAddress(), null, "Failed to send request " + request + ", cause: The channel " + this + " is closed!");
        }
        // create request.
        Request req = new Request();
        req.setVersion(Version.getProtocolVersion());
        req.setTwoWay(true);
        req.setData(request);
        DefaultFuture future = DefaultFuture.newFuture(channel, req, timeout);
        try { 
            channel.send(req);
        } catch (RemotingException e) { 
            future.cancel();
            throw e;
        }
        return future;
    }
    // 其中Request如下：
    public Request() { 
        mId = newId();
    }
    private static long newId() { 
        // getAndIncrement() When it grows to MAX_VALUE, it will grow to MIN_VALUE, and the negative can be used as ID
        return INVOKE_ID.getAndIncrement();
    }
    private static final AtomicLong INVOKE_ID = new AtomicLong(0);
    // 这里是关键：INVOKE_ID是静态递增的AtomicLong，即客户端的每次请求都每个请求都是有一个递增唯一的id的，这个id用于在客户端唯一确定一个请求。
    
    // 代码3
    DefaultFuture future = DefaultFuture.newFuture(channel, req, timeout)的实现如下：
    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(Constants.TIMEOUT_KEY, Constants.DEFAULT_TIMEOUT);
        // put into waiting map.
        FUTURES.put(id, this);
        CHANNELS.put(id, channel);
    }
    // 其中FUTURES.put(id, this);的FUTURES：
    private static final Map<Long, DefaultFuture> FUTURES = new ConcurrentHashMap<>();
    // 即为静态常量，存放请求的id和DefaultFuture。

*  客户端接收到服务端的RPC调用响应，从底层到顶层依次是NettyClient获取NettyServer的响应，NettyClient将响应向上传递给HeaderExchangeHandler的received方法：

// 代码1
    // NettyClient将底层的netty bootstrap交给构造函数传进来的handler处理，这个handler就是HeaderExchangeHandler：
    public NettyClient(final URL url, final ChannelHandler handler) throws RemotingException { 
        super(url, wrapChannelHandler(url, handler));
    }
    
    @Override
    protected void doOpen() throws Throwable { 
        NettyHelper.setNettyLoggerFactory();
        bootstrap = new ClientBootstrap(channelFactory);
        // config
        // @see org.jboss.netty.channel.socket.SocketChannelConfig
        bootstrap.setOption("keepAlive", true);
        bootstrap.setOption("tcpNoDelay", true);
        bootstrap.setOption("connectTimeoutMillis", getTimeout());
        final NettyHandler nettyHandler = new NettyHandler(getUrl(), this);
        bootstrap.setPipelineFactory(new ChannelPipelineFactory() { 
            @Override
            public ChannelPipeline getPipeline() { 
                NettyCodecAdapter adapter = new NettyCodecAdapter(getCodec(), getUrl(), NettyClient.this);
                ChannelPipeline pipeline = Channels.pipeline();
                pipeline.addLast("decoder", adapter.getDecoder());
                pipeline.addLast("encoder", adapter.getEncoder());
                pipeline.addLast("handler", nettyHandler);
                return pipeline;
            }
        });
    }
    
    // 代码2
    // HeaderExchangeHandler的received实现：对于服务端的响应调用handleResponse方法处理
    @Override
    public void received(Channel channel, Object message) throws RemotingException { 
        channel.setAttribute(KEY_READ_TIMESTAMP, System.currentTimeMillis());
        final ExchangeChannel exchangeChannel = HeaderExchangeChannel.getOrAddChannel(channel);
        try { 
            if (message instanceof Request) { 
                // handle request.
                Request request = (Request) message;
                if (request.isEvent()) { 
                    handlerEvent(channel, request);
                } else { 
                    if (request.isTwoWay()) { 
                        handleRequest(exchangeChannel, request);
                    } else { 
                        handler.received(exchangeChannel, request.getData());
                    }
                }
            } else if (message instanceof Response) { 
                handleResponse(channel, (Response) message);
            } else if (message instanceof String) { 
                if (isClientSide(channel)) { 
                    Exception e = new Exception("Dubbo client can not supported string message: " + message + " in channel: " + channel + ", url: " + channel.getUrl());
                    logger.error(e.getMessage(), e);
                } else { 
                    String echo = handler.telnet(channel, (String) message);
                    if (echo != null && echo.length() > 0) { 
                        channel.send(echo);
                    }
                }
            } else { 
                handler.received(exchangeChannel, message);
            }
        } finally { 
            HeaderExchangeChannel.removeChannelIfDisconnected(channel);
        }
    }
    
    // handleResponse的实现：静态方法，通过局部变量，即参数传入的方式保证线程安全，调用DefaultFuture.received方法。
    static void handleResponse(Channel channel, Response response) throws RemotingException { 
        if (response != null && !response.isHeartbeat()) { 
            DefaultFuture.received(channel, response);
        }
    }
    
    DefaultFuture.received的实现：
    public static void received(Channel channel, Response response) { 
        try { 
            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());
        }
    }
    
    // response将客户端的request的id原样返回了，客户端接收结果线程从FUTURES中移除该请求的id和DefaultFuture实例future，调用future的doReceived处理：调用done的signal通知在done中等待的线程。
    private void doReceived(Response res) { 
        lock.lock();
        try { 
            response = res;
            if (done != null) { 
                done.signal();
            }
        } finally { 
            lock.unlock();
        }
        if (callback != null) { 
            invokeCallback(callback);
        }
    }
    
    // 由上面的分析可知，客户端请求时，调用了DefaultFuture的get()方法在请求线程异步来获取结果，get的实现如下：在done调用await等待结果，从而通过await和signal实现线程之间的通信，客户端请求线程得到通知最终获取到了结果。
    @Override
    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()) { 
                    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));
            }
        }
        return returnFromResponse();
    }

*  服务端从Netty Server接收请求，然后向上传给HeaderExchangeHandler处理：

void handleRequest(final ExchangeChannel channel, Request req) throws RemotingException { 
        Response res = new Response(req.getId(), req.getVersion());
        if (req.isBroken()) { 
            Object data = req.getData();
    
            String msg;
            if (data == null) msg = null;
            else if (data instanceof Throwable) msg = StringUtils.toString((Throwable) data);
            else msg = data.toString();
            res.setErrorMessage("Fail to decode request due to: " + msg);
            res.setStatus(Response.BAD_REQUEST);
    
            channel.send(res);
            return;
        }
        // find handler by message class.
        Object msg = req.getData();
        try { 
            // handle data.
            CompletableFuture<Object> future = handler.reply(channel, msg);
            if (future.isDone()) { 
                res.setStatus(Response.OK);
                res.setResult(future.get());
                channel.send(res);
                return;
            }
            future.whenComplete((result, t) -> { 
                try { 
                    if (t == null) { 
                        res.setStatus(Response.OK);
                        res.setResult(result);
                    } else { 
                        res.setStatus(Response.SERVICE_ERROR);
                        res.setErrorMessage(StringUtils.toString(t));
                    }
                    channel.send(res);
                } catch (RemotingException e) { 
                    logger.warn("Send result to consumer failed, channel is " + channel + ", msg is " + e);
                } finally { 
                    // HeaderExchangeChannel.removeChannelIfDisconnected(channel);
                }
            });
        } catch (Throwable e) { 
            res.setStatus(Response.SERVICE_ERROR);
            res.setErrorMessage(StringUtils.toString(e));
            channel.send(res);
        }
    }
    
    // 构造response，获取客户端请求req的id，进行回传：Response res = new Response(req.getId(), req.getVersion());
    
    // handle data.
    CompletableFuture<Object> future = handler.reply(channel, msg);
    if (future.isDone()) { 
        res.setStatus(Response.OK);
        res.setResult(future.get());
        channel.send(res);
        return;
    }
    // 调用handler.reply，最终调用本地的Service，进行方法调用，即我们在配置文件中指定的dubbo:service的ref参数对应的bean。
    // handler是DubboProtocol中的requestHandler：
    private ExchangeHandler requestHandler = new ExchangeHandlerAdapter() { 
        @Override
        public CompletableFuture<Object> reply(ExchangeChannel channel, Object message) throws RemotingException { 
            if (message instanceof Invocation) { 
                Invocation inv = (Invocation) message;
                Invoker<?> invoker = getInvoker(channel, inv);
                // need to consider backward-compatibility if it's a callback
                if (Boolean.TRUE.toString().equals(inv.getAttachments().get(IS_CALLBACK_SERVICE_INVOKE))) { 
                    String methodsStr = invoker.getUrl().getParameters().get("methods");
                    boolean hasMethod = false;
                    if (methodsStr == null || !methodsStr.contains(",")) { 
                        hasMethod = inv.getMethodName().equals(methodsStr);
                    } else { 
                        String[] methods = methodsStr.split(",");
                        for (String method : methods) { 
                            if (inv.getMethodName().equals(method)) { 
                                hasMethod = true;
                                break;
                            }
                        }
                    }
                    if (!hasMethod) { 
                        logger.warn(new IllegalStateException("The methodName " + inv.getMethodName()
                                + " not found in callback service interface ,invoke will be ignored."
                                + " please update the api interface. url is:"
                                + invoker.getUrl()) + " ,invocation is :" + inv);
                        return null;
                    }
                }
                RpcContext rpcContext = RpcContext.getContext();
                boolean supportServerAsync = invoker.getUrl().getMethodParameter(inv.getMethodName(), Constants.ASYNC_KEY, false);
                if (supportServerAsync) { 
                    CompletableFuture<Object> future = new CompletableFuture<>();
                    rpcContext.setAsyncContext(new AsyncContextImpl(future));
                }
                rpcContext.setRemoteAddress(channel.getRemoteAddress());
                Result result = invoker.invoke(inv);
    
                if (result instanceof AsyncRpcResult) { 
                    return ((AsyncRpcResult) result).getResultFuture().thenApply(r -> (Object) r);
                } else { 
                    return CompletableFuture.completedFuture(result);
                }
            }
            throw new RemotingException(channel, "Unsupported request: "
                    + (message == null ? null : (message.getClass().getName() + ": " + message))
                    + ", channel: consumer: " + channel.getRemoteAddress() + " --> provider: " + channel.getLocalAddress());
        }
     ...
    }
    // 核心为：invoker在ServiceConfig的export时，封装了实际Server的ref，invoke最终交给ref进行方法调用。
    Invoker<?> invoker = getInvoker(channel, inv);
    Result result = invoker.invoke(inv);