Dubbo中服务消费者和服务提供者之间的请求和响应过程

服务提供者初始化完成之后，对外暴露Exporter。服务消费者初始化完成之后，得到的是Proxy代理，方法调用的时候就是调用代理。服务消费者经过初始化之后，得到的是一个动态代理类，InvokerInvocationHandler，包含MockClusterInvoker，MockClusterInvoker包含一个RegistryDirectory和FailoverClusterInvoker。Java动态代理，每一个动态代理类都必须要实现InvocationHandler这个接口，并且每一个代理类的实例都关联到了一个handler，当我们通过代理对象调用一个方法的时候，这个方法就会被转发为由实现了InvocationHandler这个接口的类的invoke方法来进行调用。
服务消费者发起调用请求
InvokerInvocationHandler实现了InvocationHandler接口，当我们调用helloService.sayHello();的时候，实际上会调用invoke()方法：

  //proxy是代理的真实对象
  //method调用真实对象的方法
  //args调用真实对象的方法的参数
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        if (method.getDeclaringClass() == Object.class) {
            return method.invoke(invoker, args);
        }
        //方法名sayHello
        String methodName = method.getName();
        //参数类型
        Class<?>[] parameterTypes = method.getParameterTypes();
        if (parameterTypes.length == 0) {
            if ("toString".equals(methodName)) {
                return invoker.toString();
            } else if ("$destroy".equals(methodName)) {
                invoker.destroy();
                return null;
            } else if ("hashCode".equals(methodName)) {
                return invoker.hashCode();
            }
        } else if (parameterTypes.length == 1 && "equals".equals(methodName)) {
            return invoker.equals(args[0]);
        }
        RpcInvocation rpcInvocation = new RpcInvocation(method, invoker.getInterface().getName(), protocolServiceKey, args);
        String serviceKey = invoker.getUrl().getServiceKey();
        rpcInvocation.setTargetServiceUniqueName(serviceKey);

        // invoker.getUrl() returns consumer url.
        RpcContext.setRpcContext(invoker.getUrl());

        if (consumerModel != null) {
            rpcInvocation.put(Constants.CONSUMER_MODEL, consumerModel);
            rpcInvocation.put(Constants.METHOD_MODEL, consumerModel.getMethodModel(method));
        }

        return invoker.invoke(rpcInvocation).recreate();
    }

先看下new RpcInvocation，Invocation是会话域，它持有调用过程中的变量，比如方法名，参数类型等。
接着是invoker.invoke()，这里invoker是MockClusterInvoker，进入MockClusterInvoker.invoker()：

    @Override
    public Result invoke(Invocation invocation) throws RpcException {
        if (invocation instanceof RpcInvocation) {
            ((RpcInvocation) invocation).setInvoker(this);
        }
        String mock = null;
        if (getUrl().hasMethodParameter(invocation.getMethodName())) {
            mock = getUrl().getParameter(in vocation.getMethodName() + "." + MOCK_KEY);
        }
        if (StringUtils.isBlank(mock)) {
            mock = getUrl().getParameter(MOCK_KEY);
        }

        if (StringUtils.isBlank(mock)) {
            throw new RpcException(new IllegalAccessException("mock can not be null. url :" + url));
        }
        mock = normalizeMock(URL.decode(mock));
        if (mock.startsWith(RETURN_PREFIX)) {
            mock = mock.substring(RETURN_PREFIX.length()).trim();
            try {
                Type[] returnTypes = RpcUtils.getReturnTypes(invocation);
                Object value = parseMockValue(mock, returnTypes);
                return AsyncRpcResult.newDefaultAsyncResult(value, invocation);
            } catch (Exception ew) {
                throw new RpcException("mock return invoke error. method :" + invocation.getMethodName()
                        + ", mock:" + mock + ", url: " + url, ew);
            }
        } else if (mock.startsWith(THROW_PREFIX)) {
            mock = mock.substring(THROW_PREFIX.length()).trim();
            if (StringUtils.isBlank(mock)) {
                throw new RpcException("mocked exception for service degradation.");
            } else { // user customized class
                Throwable t = getThrowable(mock);
                throw new RpcException(RpcException.BIZ_EXCEPTION, t);
            }
        } else { //impl mock
            try {
                Invoker<T> invoker = getInvoker(mock);
                return invoker.invoke(invocation);
            } catch (Throwable t) {
                throw new RpcException("Failed to create mock implementation class " + mock, t);
            }
        }
    }

result = this.invoker.invoke(invocation);这里invoker是FailoverClusterInvoker，会首先进入AbstractClusterInvoker的invoke方法：

    @Override
    public Result invoke(final Invocation invocation) throws RpcException {
        //检查是否被销毁
        checkWhetherDestroyed();

        // binding attachments into invocation.
        Map<String, Object> contextAttachments = RpcContext.getContext().getObjectAttachments();
        if (contextAttachments != null && contextAttachments.size() != 0) {
            ((RpcInvocation) invocation).addObjectAttachments(contextAttachments);
        }

        //根据invocation中的参数来获取所有的invoker列表
        List<Invoker<T>> invokers = list(invocation);
        //我们没有配置负载均衡的参数，默认使用random
        //这里得到的是RandomLoadBalance
        LoadBalance loadbalance = initLoadBalance(invokers, invocation);
        //如果是异步操作默认添加invocation id
        RpcUtils.attachInvocationIdIfAsync(getUrl(), invocation);
        //这里是子类实现，FailoverClusterInvoker中，执行调用
        return doInvoke(invocation, invokers, loadbalance);
    }

FailoverClusterInvoker.doInvoke()：

    public Result doInvoke(Invocation invocation, final List<Invoker<T>> invokers, LoadBalance loadbalance) throws RpcException {
        List<Invoker<T>> copyInvokers = invokers;
        //检查invokers是否为空
        checkInvokers(copyInvokers, invocation);
        String methodName = RpcUtils.getMethodName(invocation);
        //重试次数
        int len = getUrl().getMethodParameter(methodName, RETRIES_KEY, DEFAULT_RETRIES) + 1;
        if (len <= 0) {
            len = 1;
        }
        // retry loop.
        RpcException le = null; // last exception.
         //已经调用过的invoker
        List<Invoker<T>> invoked = new ArrayList<Invoker<T>>(copyInvokers.size()); // invoked invokers.
        Set<String> providers = new HashSet<String>(len);
        for (int i = 0; i < len; i++) {
            //Reselect before retry to avoid a change of candidate `invokers`.
            //NOTE: if `invokers` changed, then `invoked` also lose accuracy.
            //重试时，进行重新选择，避免重试时invoker列表已发生变化.
            //注意：如果列表发生了变化，那么invoked判断会失效，因为invoker示例已经改变
            if (i > 0) {
                checkWhetherDestroyed();
                copyInvokers = list(invocation);
                // check again
                //重新检查一下
                checkInvokers(copyInvokers, invocation);
            }
            
             //使用负载均衡选择invoker.（负载均衡咱先不做解释）
            Invoker<T> invoker = select(loadbalance, invocation, copyInvokers, invoked);
            invoked.add(invoker);
             //添加到以调用过的列表中
            RpcContext.getContext().setInvokers((List) invoked);
            try {
                 //开始调用，返回结果
                Result result = invoker.invoke(invocation);
                if (le != null && logger.isWarnEnabled()) {
                    logger.warn("Although retry the method " + methodName
                            + " in the service " + getInterface().getName()
                            + " was successful by the provider " + invoker.getUrl().getAddress()
                            + ", but there have been failed providers " + providers
                            + " (" + providers.size() + "/" + copyInvokers.size()
                            + ") from the registry " + directory.getUrl().getAddress()
                            + " on the consumer " + NetUtils.getLocalHost()
                            + " using the dubbo version " + Version.getVersion() + ". Last error is: "
                            + le.getMessage(), le);
                }
                return result;
            } catch (RpcException e) {
                if (e.isBiz()) { // biz exception.
                    throw e;
                }
                le = e;
            } catch (Throwable e) {
                le = new RpcException(e.getMessage(), e);
            } finally {
                providers.add(invoker.getUrl().getAddress());
            }
        }
        throw new RpcException(le.getCode(), "Failed to invoke the method "
                + methodName + " in the service " + getInterface().getName()
                + ". Tried " + len + " times of the providers " + providers
                + " (" + providers.size() + "/" + copyInvokers.size()
                + ") from the registry " + directory.getUrl().getAddress()
                + " on the consumer " + NetUtils.getLocalHost() + " using the dubbo version "
                + Version.getVersion() + ". Last error is: "
                + le.getMessage(), le.getCause() != null ? le.getCause() : le);
    }

Result result = invoker.invoke(invocation);调用并返回结果，会首先进入InvokerWrapper，然后进入ListenerInvokerWrapper的invoke方法，接着进入AbstractInvoker的invoke：

  @Override
    public Result invoke(Invocation inv) throws RpcException {
        // if invoker is destroyed due to address refresh from registry, let's allow the current invoke to proceed
        if (destroyed.get()) {
            logger.warn("Invoker for service " + this + " on consumer " + NetUtils.getLocalHost() + " is destroyed, "
                    + ", dubbo version is " + Version.getVersion() + ", this invoker should not be used any longer");
        }
        //转成RpcInvocation
        RpcInvocation invocation = (RpcInvocation) inv;
        invocation.setInvoker(this);
        if (CollectionUtils.isNotEmptyMap(attachment)) {
            invocation.addObjectAttachmentsIfAbsent(attachment);
        }

        Map<String, Object> contextAttachments = RpcContext.getContext().getObjectAttachments();
        if (CollectionUtils.isNotEmptyMap(contextAttachments)) {
            /**
             * invocation.addAttachmentsIfAbsent(context){@link RpcInvocation#addAttachmentsIfAbsent(Map)}should not be used here,
             * because the {@link RpcContext#setAttachment(String, String)} is passed in the Filter when the call is triggered
             * by the built-in retry mechanism of the Dubbo. The attachment to update RpcContext will no longer work, which is
             * a mistake in most cases (for example, through Filter to RpcContext output traceId and spanId and other information).
             */
            invocation.addObjectAttachments(contextAttachments);
        }

        invocation.setInvokeMode(RpcUtils.getInvokeMode(url, invocation));
        //异步的话，需要添加id
        RpcUtils.attachInvocationIdIfAsync(getUrl(), invocation);

        AsyncRpcResult asyncResult;
        try {
            asyncResult = (AsyncRpcResult) doInvoke(invocation);
        } catch (InvocationTargetException e) { // biz exception
            Throwable te = e.getTargetException();
            if (te == null) {
                asyncResult = AsyncRpcResult.newDefaultAsyncResult(null, e, invocation);
            } else {
                if (te instanceof RpcException) {
                    ((RpcException) te).setCode(RpcException.BIZ_EXCEPTION);
                }
                asyncResult = AsyncRpcResult.newDefaultAsyncResult(null, te, invocation);
            }
        } catch (RpcException e) {
            if (e.isBiz()) {
                asyncResult = AsyncRpcResult.newDefaultAsyncResult(null, e, invocation);
            } else {
                throw e;
            }
        } catch (Throwable e) {
            asyncResult = AsyncRpcResult.newDefaultAsyncResult(null, e, invocation);
        }
        RpcContext.getContext().setFuture(new FutureAdapter(asyncResult.getResponseFuture()));
        return asyncResult;
    }

DubboInvoker.doInvoke()：

    @Override
    protected Result doInvoke(final Invocation invocation) throws Throwable {
        RpcInvocation inv = (RpcInvocation) invocation;
        final String methodName = RpcUtils.getMethodName(invocation);
        inv.setAttachment(PATH_KEY, getUrl().getPath());
        inv.setAttachment(VERSION_KEY, version);

        ExchangeClient currentClient;
        //在初始化的时候，引用服务的过程中会保存一个连接到服务端的Client
        if (clients.length == 1) {
            currentClient = clients[0];
        } else {
            currentClient = clients[index.getAndIncrement() % clients.length];
        }
        try {
            //单向标志
            boolean isOneway = RpcUtils.isOneway(getUrl(), invocation);
            int timeout = calculateTimeout(invocation, methodName);
            invocation.put(TIMEOUT_KEY, timeout);
            //单向的，反送完不管结果
            if (isOneway) {
                boolean isSent = getUrl().getMethodParameter(methodName, Constants.SENT_KEY, false);
                currentClient.send(inv, isSent);
                return AsyncRpcResult.newDefaultAsyncResult(invocation);
            } else {
                ExecutorService executor = getCallbackExecutor(getUrl(), inv);
                CompletableFuture<AppResponse> appResponseFuture =
                        currentClient.request(inv, timeout, executor).thenApply(obj -> (AppResponse) obj);
                // save for 2.6.x compatibility, for example, TraceFilter in Zipkin uses com.alibaba.xxx.FutureAdapter
                FutureContext.getContext().setCompatibleFuture(appResponseFuture);
                AsyncRpcResult result = new AsyncRpcResult(appResponseFuture, inv);
                result.setExecutor(executor);
                return result;
            }
        } 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);
        }
    }

我们这里使用的是同步调用，看(Result) currentClient.request(inv, timeout).get();方法，这里的client是ReferenceCountExchangeClient，直接调用HeaderExchangeClient的request方法：

    @Override
    public CompletableFuture<Object> request(Object request, int timeout) throws RemotingException {
        return channel.request(request, timeout);
    }

进入HeaderExchangeChannel的request方法：

    @Override
    public CompletableFuture<Object> request(Object request, int timeout, ExecutorService executor) 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, executor);
        try {
            channel.send(req);
        } catch (RemotingException e) {
            future.cancel();
            throw e;
        }
        return future;
    }

channel.send(req)，首先会调用AbstractPeer的send方法：

   @Override
    public void send(Object message) throws RemotingException {
        send(message, url.getParameter(Constants.SENT_KEY, false));
    }

AbstractClient执行发送：

    @Override
    public void send(Object message, boolean sent) throws RemotingException {
        //重连
        if (needReconnect && !isConnected()) {
            connect();
        }
        //先获取Channel，是在NettyClient中实现的
        Channel channel = getChannel();
        //TODO Can the value returned by getChannel() be null? need improvement.
        //TODO getChannel返回的状态是否包含null需要改进
        if (channel == null || !channel.isConnected()) {
            throw new RemotingException(this, "message can not send, because channel is closed . url:" + getUrl());
        }
        channel.send(message, sent);
    }

channel.send(message, sent);首先经过AbstractChannel的send方法处理，只是判断是否关闭了，然后是NettyChannel的send来继续处理，这里就把消息发送到服务端了：

  @Override
    public void send(Object message, boolean sent) throws RemotingException {
        // whether the channel is closed
        super.send(message, sent);

        boolean success = true;
        int timeout = 0;
        try {
            //交给netty处理
            ChannelFuture future = channel.writeAndFlush(message);
            if (sent) {
                // wait timeout ms
                timeout = getUrl().getPositiveParameter(TIMEOUT_KEY, DEFAULT_TIMEOUT);
                success = future.await(timeout);
            }
            Throwable cause = future.cause();
            if (cause != null) {
                throw cause;
            }
        } catch (Throwable e) {
            removeChannelIfDisconnected(channel);
            throw new RemotingException(this, "Failed to send message " + PayloadDropper.getRequestWithoutData(message) + " to " + getRemoteAddress() + ", cause: " + e.getMessage(), e);
        }
        if (!success) {
            throw new RemotingException(this, "Failed to send message " + PayloadDropper.getRequestWithoutData(message) + " to " + getRemoteAddress()
                    + "in timeout(" + timeout + "ms) limit");
        }
    }

服务提供者处理并响应请求
服务端已经打开端口并监听请求的到来，当服务消费者发送调用请求的时候，经过Netty的处理后会到dubbo中的codec相关方法中先进行解码，入口是NettyCodecAdapter.messageReceived()，关于这个方法的代码在dubbo编解码的那篇文章中已经分析过，不再重复。经过解码之后，会进入到NettyHandler.messageReceived()方法：

    @Override
    public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) throws Exception {
        //获取channel
        NettyChannel channel = NettyChannel.getOrAddChannel(ctx.getChannel(), url, handler);
        try {
            //这里handler是NettyServer
            handler.received(channel, e.getMessage());
        } finally {
            NettyChannel.removeChannelIfDisconnected(ctx.getChannel());
        }
    }

接着会进入AbstractPeer的received方法

    @Override
    public void received(Channel ch, Object msg) throws RemotingException {
        if (closed) {
            return;
        }
        handler.received(ch, msg);
    }

进入MultiMessageHandler的received方法：

    @Override
    public void received(Channel channel, Object message) throws RemotingException {
        if (message instanceof MultiMessage) {
            MultiMessage list = (MultiMessage) message;
            for (Object obj : list) {
                handler.received(channel, obj);
            }
        } else {
            handler.received(channel, message);
        }
    }

进入HeartbeatHandler的received方法：

    @Override
    public void received(Channel channel, Object message) throws RemotingException {
        setReadTimestamp(channel);
        //心跳请求处理
        if (isHeartbeatRequest(message)) {
            Request req = (Request) message;
            if (req.isTwoWay()) {
                Response res = new Response(req.getId(), req.getVersion());
                res.setEvent(HEARTBEAT_EVENT);
                channel.send(res);
                if (logger.isInfoEnabled()) {
                    int heartbeat = channel.getUrl().getParameter(Constants.HEARTBEAT_KEY, 0);
                    if (logger.isDebugEnabled()) {
                        logger.debug("Received heartbeat from remote channel " + channel.getRemoteAddress()
                                + ", cause: The channel has no data-transmission exceeds a heartbeat period"
                                + (heartbeat > 0 ? ": " + heartbeat + "ms" : ""));
                    }
                }
            }
            return;
        }
        //心跳回应消息处理
        if (isHeartbeatResponse(message)) {
            if (logger.isDebugEnabled()) {
                logger.debug("Receive heartbeat response in thread " + Thread.currentThread().getName());
            }
            return;
        }
        //这里是AllChannelHandler
        handler.received(channel, message);
    }

继续进入AllChannelHandler的received方法：

    @Override
    public void received(Channel channel, Object message) throws RemotingException {
       //获取线程池执行
        ExecutorService executor = getPreferredExecutorService(message);
        try {
            //handler是DecodeHandler
            executor.execute(new ChannelEventRunnable(channel, handler, ChannelState.RECEIVED, message));
        } catch (Throwable t) {
            if(message instanceof Request && t instanceof RejectedExecutionException){
                sendFeedback(channel, (Request) message, t);
                return;
            }
            throw new ExecutionException(message, channel, getClass() + " error when process received event .", t);
        }
    }

这里会去启动新线程执行ChannelEventRunnable的run方法，接着去调用DecodeHandler的received方法：

    @Override
    public void received(Channel channel, Object message) throws RemotingException {
        if (message instanceof Decodeable) {
            decode(message);
        }

        //解码请求类型
        if (message instanceof Request) {
            decode(((Request) message).getData());
        }

        //解码响应类型
        if (message instanceof Response) {
            decode(((Response) message).getResult());
        }
        
        //解码之后到HeaderExchangeHandler中处理
        handler.received(channel, message);
    }

解码之后到HeaderExchangeHandler的received方法：

  @Override
    public void received(Channel channel, Object message) throws RemotingException {
        final ExchangeChannel exchangeChannel = HeaderExchangeChannel.getOrAddChannel(channel);
        if (message instanceof Request) {
            // handle request.
            //request类型的消息
            Request request = (Request) message;
            if (request.isEvent()) {
                //判断心跳还是正常请求
                //    处理心跳
                handlerEvent(channel, request);
            } else {
                if (request.isTwoWay()) {
                    //正常的请求
                    //需要返回
                    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);
        }
    }

先看下处理请求，并构造响应信息：

 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 {
            CompletionStage<Object> future = handler.reply(channel, msg);
            future.whenComplete((appResult, t) -> {
                try {
                    if (t == null) {
                        res.setStatus(Response.OK);
                        res.setResult(appResult);
                    } 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);
                }
            });
        } catch (Throwable e) {
            res.setStatus(Response.SERVICE_ERROR);
            res.setErrorMessage(StringUtils.toString(e));
            channel.send(res);
        }
    }

进入DubboProtocol中的ExchangeHandlerAdapter的replay方法：

 public CompletableFuture<Object> reply(ExchangeChannel channel, Object message) throws RemotingException {

            if (!(message instanceof Invocation)) {
                throw new RemotingException(channel, "Unsupported request: "
                        + (message == null ? null : (message.getClass().getName() + ": " + message))
                        + ", channel: consumer: " + channel.getRemoteAddress() + " --> provider: " + channel.getLocalAddress());
            }

            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.getObjectAttachments().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.getContext().setRemoteAddress(channel.getRemoteAddress());
            Result result = invoker.invoke(inv);
            return result.thenApply(Function.identity());
        }

先看下getInvoker获取Invoker：

Invoker<?> getInvoker(Channel channel, Invocation inv) throws RemotingException {
        boolean isCallBackServiceInvoke = false;
        boolean isStubServiceInvoke = false;
        int port = channel.getLocalAddress().getPort();
        String path = (String) inv.getObjectAttachments().get(PATH_KEY);

        //如果是客户端的回调服务
        // if it's callback service on client side
        isStubServiceInvoke = Boolean.TRUE.toString().equals(inv.getObjectAttachments().get(STUB_EVENT_KEY));
        if (isStubServiceInvoke) {
            port = channel.getRemoteAddress().getPort();
        }

        //callback
        isCallBackServiceInvoke = isClientSide(channel) && !isStubServiceInvoke;
        if (isCallBackServiceInvoke) {
            path += "." + inv.getObjectAttachments().get(CALLBACK_SERVICE_KEY);
            inv.getObjectAttachments().put(IS_CALLBACK_SERVICE_INVOKE, Boolean.TRUE.toString());
        }

        String serviceKey = serviceKey(
                port,
                path,
                (String) inv.getObjectAttachments().get(VERSION_KEY),
                (String) inv.getObjectAttachments().get(GROUP_KEY)
        );
        //从之前缓存的exporterMap中查找Exporter
        DubboExporter<?> exporter = (DubboExporter<?>) exporterMap.get(serviceKey);

        if (exporter == null) {
            throw new RemotingException(channel, "Not found exported service: " + serviceKey + " in " + exporterMap.keySet() + ", may be version or group mismatch " +
                    ", channel: consumer: " + channel.getRemoteAddress() + " --> provider: " + channel.getLocalAddress() + ", message:" + getInvocationWithoutData(inv));
        }

        //得到Invoker，返回
        return exporter.getInvoker();
    }

再看执行调用invoker.invoke(inv);，会先进入InvokerWrapper：

    @Override
    public Result invoke(Invocation invocation) throws RpcException {
        return invoker.invoke(invocation);
    }

接着进入AbstractProxyInvoker：

   @Override
    public Result invoke(Invocation invocation) throws RpcException {
        try {
            Object value = doInvoke(proxy, invocation.getMethodName(), invocation.getParameterTypes(), invocation.getArguments());
            CompletableFuture<Object> future = wrapWithFuture(value);
            CompletableFuture<AppResponse> appResponseFuture = future.handle((obj, t) -> {
                AppResponse result = new AppResponse();
                if (t != null) {
                    if (t instanceof CompletionException) {
                        result.setException(t.getCause());
                    } else {
                        result.setException(t);
                    }
                } else {
                    result.setValue(obj);
                }
                return result;
            });
            return new AsyncRpcResult(appResponseFuture, invocation);
        } catch (InvocationTargetException e) {
            if (RpcContext.getContext().isAsyncStarted() && !RpcContext.getContext().stopAsync()) {
                logger.error("Provider async started, but got an exception from the original method, cannot write the exception back to consumer because an async result may have returned the new thread.", e);
            }
            return AsyncRpcResult.newDefaultAsyncResult(null, e.getTargetException(), invocation);
        } catch (Throwable e) {
            throw new RpcException("Failed to invoke remote proxy method " + invocation.getMethodName() + " to " + getUrl() + ", cause: " + e.getMessage(), e);
        }
    }

这里的doInvoke是在JavassistProxyFactory中的AbstractProxyInvoker实例：

    @Override
    public <T> Invoker<T> getInvoker(T proxy, Class<T> type, URL url) {
        // TODO Wrapper cannot handle this scenario correctly: the classname contains '$'
        // TODO Wrapper类不能正确处理带$的类名
        final Wrapper wrapper = Wrapper.getWrapper(proxy.getClass().getName().indexOf('$') < 0 ? proxy.getClass() : type);
        return new AbstractProxyInvoker<T>(proxy, type, url) {
            @Override
            protected Object doInvoke(T proxy, String methodName,
                                      Class<?>[] parameterTypes,
                                      Object[] arguments) throws Throwable {
                //这里就调用了具体的方法
                return wrapper.invokeMethod(proxy, methodName, parameterTypes, arguments);
            }
        };
    }

消息处理完后返回到HeaderExchangeHandler的received方法：

public void received(Channel channel, Object message) throws RemotingException {
    channel.setAttribute(KEY_READ_TIMESTAMP, System.currentTimeMillis());
    ExchangeChannel exchangeChannel = HeaderExchangeChannel.getOrAddChannel(channel);
    try {
        //request类型的消息
        if (message instanceof Request) {
            Request request = (Request) message;
            if (request.isEvent()) {//判断心跳还是正常请求
                //    处理心跳
                handlerEvent(channel, request);
            } else {//正常的请求
                //需要返回
                if (request.isTwoWay()) {
                    //处理请求，并构造响应信息，这在上面已经解析过了
                    Response response = handleRequest(exchangeChannel, request);
                    //NettyChannel，发送响应信息
                    channel.send(response);
                } else {//不需要返回的处理
                    handler.received(exchangeChannel, request.getData());
                }
            }
        } else if (message instanceof Response) {//response类型的消息
            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());
            } else {//telnet类型
                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);
    }
}

解析完请求，构造完响应消息，就开始发送响应了,channel.send(response);，先经过AbstractPeer:

public void send(Object message) throws RemotingException {
    //NettyChannel
    send(message, url.getParameter(Constants.SENT_KEY, false));
}

进入NettyChannel中，进行响应消息的发送：

  @Override
    public void send(Object message, boolean sent) throws RemotingException {
        // whether the channel is closed
        super.send(message, sent);

        boolean success = true;
        int timeout = 0;
        try {
            ChannelFuture future = channel.writeAndFlush(message);
            if (sent) {
                // wait timeout ms
                timeout = getUrl().getPositiveParameter(TIMEOUT_KEY, DEFAULT_TIMEOUT);
                success = future.await(timeout);
            }
            Throwable cause = future.cause();
            if (cause != null) {
                throw cause;
            }
        } catch (Throwable e) {
            removeChannelIfDisconnected(channel);
            throw new RemotingException(this, "Failed to send message " + PayloadDropper.getRequestWithoutData(message) + " to " + getRemoteAddress() + ", cause: " + e.getMessage(), e);
        }
        if (!success) {
            throw new RemotingException(this, "Failed to send message " + PayloadDropper.getRequestWithoutData(message) + " to " + getRemoteAddress()
                    + "in timeout(" + timeout + "ms) limit");
        }
    }

消费者接受到服务端返回的响应后的处理
服务提供者端接收到消费者端的请求并处理之后，返回给消费者端，消费者这边接受响应的入口跟提供者差不多，也是NettyCodecAdapter.messageReceived()，经过解码，到NettyHandler.messageReceived()处理：

    @Override
    public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) throws Exception {
        NettyChannel channel = NettyChannel.getOrAddChannel(ctx.getChannel(), url, handler);
        try {
            handler.received(channel, e.getMessage());
        } finally {
            NettyChannel.removeChannelIfDisconnected(ctx.getChannel());
        }
    }

先经过AbstractPeer的received方法：

    @Override
    public void received(Channel ch, Object msg) throws RemotingException {
        if (closed) {
            return;
        }
        handler.received(ch, msg);
    }

进入MultiMessageHandler：

    @Override
    public void received(Channel channel, Object message) throws RemotingException {
        if (message instanceof MultiMessage) {
            MultiMessage list = (MultiMessage) message;
            for (Object obj : list) {
                handler.received(channel, obj);
            }
        } else {
            handler.received(channel, message);
        }
    }

进入HeartbeatHandler，根据不同类型进行处理：

@Override
    public void received(Channel channel, Object message) throws RemotingException {
        setReadTimestamp(channel);
        if (isHeartbeatRequest(message)) {
            Request req = (Request) message;
            if (req.isTwoWay()) {
                Response res = new Response(req.getId(), req.getVersion());
                res.setEvent(HEARTBEAT_EVENT);
                channel.send(res);
                if (logger.isInfoEnabled()) {
                    int heartbeat = channel.getUrl().getParameter(Constants.HEARTBEAT_KEY, 0);
                    if (logger.isDebugEnabled()) {
                        logger.debug("Received heartbeat from remote channel " + channel.getRemoteAddress()
                                + ", cause: The channel has no data-transmission exceeds a heartbeat period"
                                + (heartbeat > 0 ? ": " + heartbeat + "ms" : ""));
                    }
                }
            }
            return;
        }
        if (isHeartbeatResponse(message)) {
            if (logger.isDebugEnabled()) {
                logger.debug("Receive heartbeat response in thread " + Thread.currentThread().getName());
            }
            return;
        }
        handler.received(channel, message);
    }

进入AllChannelHandler：

    @Override
    public void received(Channel channel, Object message) throws RemotingException {
        ExecutorService executor = getPreferredExecutorService(message);
        try {
            executor.execute(new ChannelEventRunnable(channel, handler, ChannelState.RECEIVED, message));
        } catch (Throwable t) {
            if(message instanceof Request && t instanceof RejectedExecutionException){
                sendFeedback(channel, (Request) message, t);
                return;
            }
            throw new ExecutionException(message, channel, getClass() + " error when process received event .", t);
        }
    }

然后在新线程，ChannelEventRunnable的run方法中进入DecodeHandler：

    @Override
    public void received(Channel channel, Object message) throws RemotingException {
        if (message instanceof Decodeable) {
            decode(message);
        }

        if (message instanceof Request) {
            decode(((Request) message).getData());
        }

        if (message instanceof Response) {
            decode(((Response) message).getResult());
        }

        handler.received(channel, message);
    }

进入处理response的decode方法，进行解码response：

    private void decode(Object message) {
        if (message instanceof Decodeable) {
            try {
                ((Decodeable) message).decode();
                if (log.isDebugEnabled()) {
                    log.debug("Decode decodeable message " + message.getClass().getName());
                }
            } catch (Throwable e) {
                if (log.isWarnEnabled()) {
                    log.warn("Call Decodeable.decode failed: " + e.getMessage(), e);
                }
            } // ~ end of catch
        } // ~ end of if
    } // ~ end of method decode

接着会进入HeaderExchangerHandler.received () 方法：

    @Override
    public void received(Channel channel, Object message) throws RemotingException {
        final ExchangeChannel exchangeChannel = HeaderExchangeChannel.getOrAddChannel(channel);
        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);
        }
    }

handleResponse方法：

    static void handleResponse(Channel channel, Response response) throws RemotingException {
        if (response != null && !response.isHeartbeat()) {
            DefaultFuture.received(channel, response);
        }
    }

这一步设置response到消费者请求的Future中，以供消费者通过DefaultFuture.get()取得提供者的响应，此为同步转异步重要一步，且请求超时也由DefaultFuture控制。
然后就是return (Result) currentClient.request(inv, timeout).get();在DubboInvoker中，这里继续执行，然后执行Filter，最后返回到InvokerInvocationHandler.invoker()方法中，方法得到调用结果，结束！
注意：
消费者端的DubboInvoker发起请求后，后续的逻辑是异步的或是指定超时时间内阻塞的，直到得到响应结果后，继续执行DubboInvoker中逻辑。
对于异步请求时，消费者得到Future，其余逻辑均是异步的。
消费者还可以通过设置async、sent、return来调整处理逻辑，async指异步还是同步请求，sent指是否等待请求消息发出即阻塞等待是否成功发出请求、return指是否忽略返回值即但方向通信，一般异步时使用以减少Future对象的创建和管理成本。

参考博客：https://blog.csdn.net/chijunmei7041/article/details/100854806