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SpringCloud Alibaba系列——12Dubbo的RPC通讯原理
老老老JR老北
发布于 2022-8-18 17:30
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作者 | 一起撸Java
来源 |今日头条
学习目标
Dubbo的rpc原理
第1章 消费端请求流程
下面我们来从大局分析消费端的整个请求流程，前面我们分析过消费端用代理对象调用方法时，最终会来到InvokerInvocationHandler中，那么我们就从这个类开始看起。先掉到该类的invoker方法，该类持有了MigrationInvoker类的引用。
1.1 InvokerInvocationHandler
@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable { if (method.getDeclaringClass() == Object.class) { return method.invoke(invoker, args); } String methodName = method.getName(); 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 = url.getServiceKey(); rpcInvocation.setTargetServiceUniqueName(serviceKey); // invoker.getUrl() returns consumer url. RpcServiceContext.setRpcContext(url); if (consumerModel != null) { rpcInvocation.put(Constants.CONSUMER_MODEL, consumerModel); rpcInvocation.put(Constants.METHOD_MODEL, consumerModel.getMethodModel(method)); } //掉到 MigrationInvoker return invoker.invoke(rpcInvocation).recreate();}1.2 MigrationInvoker
@Overridepublic Result invoke(Invocation invocation) throws RpcException { if (currentAvailableInvoker != null) { if (step == APPLICATION_FIRST) { // call ratio calculation based on random value if (ThreadLocalRandom.current().nextDouble(100) > promotion) { return invoker.invoke(invocation); } } //代码会走这里，走到MockClusterInvoker中 return currentAvailableInvoker.invoke(invocation); } switch (step) { case APPLICATION_FIRST: if (checkInvokerAvailable(serviceDiscoveryInvoker)) { currentAvailableInvoker = serviceDiscoveryInvoker; } else if (checkInvokerAvailable(invoker)) { currentAvailableInvoker = invoker; } else { currentAvailableInvoker = serviceDiscoveryInvoker; } break; case FORCE_APPLICATION: currentAvailableInvoker = serviceDiscoveryInvoker; break; case FORCE_INTERFACE: default: currentAvailableInvoker = invoker; } return currentAvailableInvoker.invoke(invocation);}1.3 MockClusterInvoker
在该invoker中进行了服务的降级逻辑的处理
@Overridepublic Result invoke(Invocation invocation) throws RpcException { Result result = null; //获取url中的mock参数 String value = getUrl().getMethodParameter(invocation.getMethodName(), MOCK_KEY, Boolean.FALSE.toString()).trim(); if (value.length() == 0 || "false".equalsIgnoreCase(value)) { //no mock //如果没mock则直接走后面调用逻辑 result = this.invoker.invoke(invocation); } else if (value.startsWith("force")) { //如果是force开头 if (logger.isWarnEnabled()) { logger.warn("force-mock: " + invocation.getMethodName() + " force-mock enabled, url : " + getUrl()); } //强制降级，调用mock的实现类逻辑 //force:direct mock result = doMockInvoke(invocation, null); } else { //fail-mock try { result = this.invoker.invoke(invocation); //fix:#4585 if(result.getException() != null && result.getException() instanceof RpcException){ RpcException rpcException= (RpcException)result.getException(); if(rpcException.isBiz()){ throw rpcException; }else { result = doMockInvoke(invocation, rpcException); } } } catch (RpcException e) { if (e.isBiz()) { throw e; } if (logger.isWarnEnabled()) { logger.warn("fail-mock: " + invocation.getMethodName() + " fail-mock enabled , url : " + getUrl(), e); } result = doMockInvoke(invocation, e); } } return result;}1.4 FilterChainNode
经过消费端过滤器处理
class FilterChainNode<T, TYPE extends Invoker<T>, FILTER extends BaseFilter> implements Invoker<T>{ TYPE originalInvoker; Invoker<T> nextNode; FILTER filter; public FilterChainNode(TYPE originalInvoker, Invoker<T> nextNode, FILTER filter) { this.originalInvoker = originalInvoker; this.nextNode = nextNode; this.filter = filter; } public TYPE getOriginalInvoker() { return originalInvoker; } @Override public Class<T> getInterface() { return originalInvoker.getInterface(); } @Override public URL getUrl() { return originalInvoker.getUrl(); } @Override public boolean isAvailable() { return originalInvoker.isAvailable(); } @Override public Result invoke(Invocation invocation) throws RpcException { Result asyncResult; try { asyncResult = filter.invoke(nextNode, invocation); } catch (Exception e) { if (filter instanceof ListenableFilter) { ListenableFilter listenableFilter = ((ListenableFilter) filter); try { Filter.Listener listener = listenableFilter.listener(invocation); if (listener != null) { listener.onError(e, originalInvoker, invocation); } } finally { listenableFilter.removeListener(invocation); } } else if (filter instanceof FILTER.Listener) { FILTER.Listener listener = (FILTER.Listener) filter; listener.onError(e, originalInvoker, invocation); } throw e; } finally { } return asyncResult.whenCompleteWithContext((r, t) -> { if (filter instanceof ListenableFilter) { ListenableFilter listenableFilter = ((ListenableFilter) filter); Filter.Listener listener = listenableFilter.listener(invocation); try { if (listener != null) { if (t == null) { listener.onResponse(r, originalInvoker, invocation); } else { listener.onError(t, originalInvoker, invocation); } } } finally { listenableFilter.removeListener(invocation); } } else if (filter instanceof FILTER.Listener) { FILTER.Listener listener = (FILTER.Listener) filter; if (t == null) { listener.onResponse(r, originalInvoker, invocation); } else { listener.onError(t, originalInvoker, invocation); } } }); } @Override public void destroy() { originalInvoker.destroy(); } @Override public String toString() { return originalInvoker.toString(); }}
从断点的栈帧来看，这里经过了3个filter，分别是，ConsumerContextFilter、FutureFilter、MonitorFilter。
1.5 AbstractClusterInvoker
然后代码走到了FailoverClusterInvoker的父类的invoke方法，里面定义了核心的流程
@Overridepublic Result invoke(final Invocation invocation) throws RpcException { //判断是否销毁 checkWhetherDestroyed(); // binding attachments into invocation. // Map<String, Object> contextAttachments = RpcContext.getClientAttachment().getObjectAttachments(); // if (contextAttachments != null && contextAttachments.size() != 0) { // ((RpcInvocation)invocation).addObjectAttachmentsIfAbsent(contextAttachments); // } //获取服务列表 List<Invoker<T>> invokers = list(invocation); //spi 获取负载均衡类实例 LoadBalance loadbalance = initLoadBalance(invokers, invocation); RpcUtils.attachInvocationIdIfAsync(getUrl(), invocation); return doInvoke(invocation, invokers, loadbalance);}在父类中定义了核心的流程，比如，获取服务列表，获取负载均衡器，然后掉钩子方法doInvoke，这里是一个典型的模板设计模式，不同的子类实例doInvoke掉到的是不同的子类逻辑，该方法是一个抽象的钩子方法。
1.6 FailoverClusterInvoker
doInvoke方法勾到了FailoverClusterInvoker类中，这个是默认的集群容错，在该类中定义了如何重试的策略，如果出现了RpcException那么就会重试。再一次调用。
public Result doInvoke(Invocation invocation, final List<Invoker<T>> invokers, LoadBalance loadbalance) throws RpcException { List<Invoker<T>> copyInvokers = invokers; //invokers校验 checkInvokers(copyInvokers, invocation); String methodName = RpcUtils.getMethodName(invocation); //计算调用次数 int len = calculateInvokeTimes(methodName); // retry loop. RpcException le = null; // last exception. //记录已经调用过了的服务列表 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. //如果掉完一次后，服务列表更新了，再次获取服务列表 if (i > 0) { checkWhetherDestroyed(); copyInvokers = list(invocation); // check again checkInvokers(copyInvokers, invocation); } //根据负载均衡算法，选择一个服务调用 Invoker<T> invoker = select(loadbalance, invocation, copyInvokers, invoked); //记录已经调用过的invoker invoked.add(invoker); RpcContext.getServiceContext().setInvokers((List) invoked); try { //具体的服务调用逻辑 Result result = invokeWithContext(invoker, 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);}1.7 AbstractInvoker
接着来到了DubboInvoker的父类逻辑
@Overridepublic 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 (isDestroyed()) { 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"); } RpcInvocation invocation = (RpcInvocation) inv; //调用 invocation初始化 // prepare rpc invocation prepareInvocation(invocation); //具体的rpc调用流程 // do invoke rpc invocation and return async result AsyncRpcResult asyncResult = doInvokeAndReturn(invocation); //阻塞拿返回结果 // wait rpc result if sync waitForResultIfSync(asyncResult, invocation); return asyncResult;}在doInvokeAndReturn方法中是走了远程rpc调用，然后在waitForResultIfSync里面如果是同步调用就掉了CompletableFuture的get方法完成了调用阻塞，这里就是异步调用然后用get的方式阻塞，异步调用框架的同步阻塞改造。我们看看waitForResultIfSync(asyncResult, invocation);方法。
private void waitForResultIfSync(AsyncRpcResult asyncResult, RpcInvocation invocation) { //如果非同步调用，要直接返回asyncResult if (InvokeMode.SYNC != invocation.getInvokeMode()) { return; } try { /* * NOTICE! * must call {@link java.util.concurrent.CompletableFuture#get(long, TimeUnit)}because * {@link java.util.concurrent.CompletableFuture#get()} was proved to have serious performance drop. */ //同步调用，其实是在这里阻塞拿返回结果 asyncResult.get(Integer.MAX_VALUE, TimeUnit.MILLISECONDS); } catch (InterruptedException e) { throw new RpcException("Interrupted unexpectedly while waiting for remote result to return! method: " + invocation.getMethodName() + ", provider: " + getUrl() + ", cause: " + e.getMessage(), e); } catch (ExecutionException e) { Throwable rootCause = e.getCause(); if (rootCause instanceof TimeoutException) { throw new RpcException(RpcException.TIMEOUT_EXCEPTION, "Invoke remote method timeout. method: " + invocation.getMethodName() + ", provider: " + getUrl() + ", cause: " + e.getMessage(), e); } else if (rootCause instanceof RemotingException) { throw new RpcException(RpcException.NETWORK_EXCEPTION, "Failed to invoke remote method: " + invocation.getMethodName() + ", provider: " + getUrl() + ", cause: " + e.getMessage(), e); } else { throw new RpcException(RpcException.UNKNOWN_EXCEPTION, "Fail to invoke remote method: " + invocation.getMethodName() + ", provider: " + getUrl() + ", cause: " + e.getMessage(), e); } } catch (Throwable e) { throw new RpcException(e.getMessage(), e); }}1.8 DubboInvoker
接着我们看看DubboInvoker的远程调用逻辑，这个是父类的方法，同样是模板设计模式doInvoke是钩子方法，勾到了子类DubboInvoker的逻辑
private AsyncRpcResult doInvokeAndReturn(RpcInvocation invocation) { AsyncRpcResult asyncResult; try { //调用核心代码 asyncResult = (AsyncRpcResult) doInvoke(invocation); } catch (InvocationTargetException e) { Throwable te = e.getTargetException(); if (te != null) { // if biz exception if (te instanceof RpcException) { ((RpcException) te).setCode(RpcException.BIZ_EXCEPTION); } asyncResult = AsyncRpcResult.newDefaultAsyncResult(null, te, invocation); } else { asyncResult = AsyncRpcResult.newDefaultAsyncResult(null, e, invocation); } } catch (RpcException e) { // if biz exception if (e.isBiz()) { asyncResult = AsyncRpcResult.newDefaultAsyncResult(null, e, invocation); } else { throw e; } } catch (Throwable e) { asyncResult = AsyncRpcResult.newDefaultAsyncResult(null, e, invocation); } // set server context RpcContext.getServiceContext().setFuture(new FutureAdapter<> (asyncResult.getResponseFuture())); return asyncResult;}我们看看DubboInvoker中的doInvoke逻辑
@Overrideprotected 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; if (clients.length == 1) { currentClient = clients[0]; } else { currentClient = clients[index.getAndIncrement() % clients.length]; } try { //如果是单工通讯 //@Method(name = "doKill",isReturn = false) 这样配置就是单工通讯 boolean isOneway = RpcUtils.isOneway(getUrl(), invocation); int timeout = calculateTimeout(invocation, methodName); invocation.put(TIMEOUT_KEY, timeout); //单工就不需要等待返回结果 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); //request具体的rpc远程调用 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); }}我们以双工调用为例来分析调用过程。
CompletableFuture appResponseFuture = currentClient.request(inv, timeout, executor).thenApply(obj ->(AppResponse) obj);这行代码就是调用后端逻辑代码，我们看看request方法。
1.9 HeaderExchangeChannel
@Overridepublic 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. //创建了Request对象，并生了一个流水id 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;}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();}DefaultFuture future = DefaultFuture.newFuture(channel, req, timeout, executor);该方法创建了一个CompletableFuture对象，并把对象缓存了起来，缓存的key就是刚刚生成Request对象的id，这个id会传递给消费端，然后消费端响应的时候又会传递回来的，然后会从缓存中根据传递回来的id拿到对应的CompletableFuture对象然后把阻塞的地方唤醒。
private DefaultFuture(Channel channel, Request request, int timeout) { this.channel = channel; this.request = request; //这里是生成请求流水，请求和响应的流水id this.id = request.getId(); this.timeout = timeout > 0 ? timeout : channel.getUrl().getPositiveParameter(TIMEOUT_KEY, DEFAULT_TIMEOUT); // put into waiting map. //通过id来找对应请求的future对象 FUTURES.put(id, this); CHANNELS.put(id, channel);}1.10 NettyChannel
@Overridepublic void send(Object message, boolean sent) throws RemotingException { // whether the channel is closed super.send(message, sent); boolean success = true; int timeout = 0; try { //netty通讯，把请求发送到服务端 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"); }}从上述描述可以看出，客户端的请求流程其实会有一个get方法的阻塞动作，然后把请求通过netty通讯的方式发送给服务端，然后有一个Request对象请求到了服务端，Request对象中有一个流水id，建立了一个流水id和CompletableFuture对象的映射关系，到时候会从这个映射关系中获取到CompletableFuture对象的。
第2章 服务端接收流程
服务的接收，前面分析过Netty的服务端有一个核心的处理handler，这个类就是NettyServerHandler，当客户端请求过来时，请求就会掉到channelRead方法，我们来看看这个方法：
@Overridepublic void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception { NettyChannel channel = NettyChannel.getOrAddChannel(ctx.channel(), url, handler); handler.received(channel, msg);}前面我们在服务的暴露流程那节课中我们分析了，这个handler是一个层级关系的handler，我们来分析一下这个层级调用。
2.1 MultiMessageHandler
这个是对不同类型的消息进行处理
@Overridepublic void received(Channel channel, Object message) throws RemotingException { if (message instanceof MultiMessage) { MultiMessage list = (MultiMessage) message; for (Object obj : list) { try { handler.received(channel, obj); } catch (ExecutionException e) { logger.error("MultiMessageHandler received fail.", e); handler.caught(channel, e); } } } else { handler.received(channel, message); }}2.2 HeartbeatHandler
这个是心跳的handler，设置读消息的时间戳用于在发送心跳的时候获取到最后读的时候跟当前时间对比，如果超过了心跳时间间隔设置就发送心跳消息。然后如果收到的请求是一个心跳请求则发送应答消息给对端系统。
@Overridepublic void received(Channel channel, Object message) throws RemotingException { //设置读数据的时间戳 setReadTimestamp(channel); //如果是心跳请求 if (isHeartbeatRequest(message)) { Request req = (Request) message; if (req.isTwoWay()) { //应答消息 Response res = new Response(req.getId(), req.getVersion()); res.setEvent(HEARTBEAT_EVENT); //发送应答给对方 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; } //如果是收到的心跳应答消息 if (isHeartbeatResponse(message)) { if (logger.isDebugEnabled()) { logger.debug("Receive heartbeat response in thread " + Thread.currentThread().getName()); } return; } handler.received(channel, message);}2.3 AllChannelHandler
这个是框架线程池隔离类，主要目的是把io线程转换成框架线程池，这样做的目的可以释放io线程，让io线程去处理其他用户的请求，提高整个系统的吞吐量。
@Overridepublic 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); }}可以看到，这里开启了线程池，我们都知道线程池也有线程和队列大小的，如果超过了这个大小是会抛出
RejectedExecutionException的，可以看到代码是有在catch里面判断该异常的，如果有该异常出现则会发送应答消息给客户端的。告诉客户端服务端的线程池已经满了。
protected void sendFeedback(Channel channel, Request request, Throwable t) throws RemotingException { if (request.isTwoWay()) { String msg = "Server side(" + url.getIp() + "," + url.getPort() + ") thread pool is exhausted, detail msg:" + t.getMessage(); Response response = new Response(request.getId(), request.getVersion()); response.setStatus(Response.SERVER_THREADPOOL_EXHAUSTED_ERROR); response.setErrorMessage(msg); channel.send(response); return; }}我们再看看线程类ChannelEventRunnable
@Overridepublic void run() { //如果是接受请求 if (state == ChannelState.RECEIVED) { try { handler.received(channel, message); } catch (Exception e) { logger.warn("ChannelEventRunnable handle " + state + " operation error, channel is " + channel + ", message is " + message, e); } } else { switch (state) { case CONNECTED: try { handler.connected(channel); } catch (Exception e) { logger.warn("ChannelEventRunnable handle " + state + " operation error, channel is " + channel, e); } break; case DISCONNECTED: try { handler.disconnected(channel); } catch (Exception e) { logger.warn("ChannelEventRunnable handle " + state + " operation error, channel is " + channel, e); } break; case SENT: try { handler.sent(channel, message); } catch (Exception e) { logger.warn("ChannelEventRunnable handle " + state + " operation error, channel is " + channel + ", message is " + message, e); } break; case CAUGHT: try { handler.caught(channel, exception); } catch (Exception e) { logger.warn("ChannelEventRunnable handle " + state + " operation error, channel is " + channel + ", message is: " + message + ", exception is " + exception, e); } break; default: logger.warn("unknown state: " + state + ", message is " + message); } }}2.4 DecodeHandler
该handler主要是对请求过来的数据进行编解码序列化的，把网络传输过来的数据编程业务数据。默认采用的是用Hessian2做的序列化。序列化过程就是用流的方式编解码这里就不做过多的分析。
@Overridepublic void received(Channel channel, Object message) throws RemotingException { if (message instanceof Decodeable) { decode(message); } //如果是请求消息 if (message instanceof Request) { decode(((Request) message).getData()); } //如果是响应消息 if (message instanceof Response) { decode(((Response) message).getResult()); } handler.received(channel, message);}2.5 HeaderExchangeHandler
接着来到了消息交换层的handler逻辑，在之类对接收到消息调用了被代理方法，然后根据被代理方法的返回值回调了调用方的逻辑。
@Overridepublic void received(Channel channel, Object message) throws RemotingException { final ExchangeChannel exchangeChannel = HeaderExchangeChannel.getOrAddChannel(channel); //服务端接收到的请求消息 if (message instanceof Request) { // handle request. Request request = (Request) message; if (request.isEvent()) { handlerEvent(channel, request); } else { //如果是双工通讯 if (request.isTwoWay()) { //核心代码 handleRequest(exchangeChannel, request); } else { handler.received(exchangeChannel, request.getData()); } } } else if (message instanceof Response) { //如果是客户端接收到的响应消息 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()); 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); }}我们可以看到核心代码就是handleRequest(exchangeChannel, request);，我们看看该方法
void handleRequest(final ExchangeChannel channel, Request req) throws RemotingException { //这里注意，id是从request对象中获取到的id，这个id响应数据时是需要返回给客户端的 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; } //获取到编解码后的数据 // find handler by message class. Object msg = req.getData(); try { //调用DubboProtocol中的ExchangeHandlerAdapter中的reply方法进行后续的filter调用 CompletionStage<Object> future = handler.reply(channel, msg); //future完成后回调客户端，响应数据回去 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)); } //回调客户端响应数据 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); }}我们可以看到创建了一个Response对象，对象的id就是Request对象传递过来的id，
Response res = new Response(req.getId(), req.getVersion());然后调用了这行代码CompletionStage future = handler.reply(channel, msg);
前面我们分析过，这个reply方法会调到DubboProtocol中的一个内部类，我们看看这个reply方法。
@Overridepublic 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; //获取到invoker，这个invoker就是FilterChainNode 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.getServiceContext().setRemoteAddress(channel.getRemoteAddress()); Result result = invoker.invoke(inv); return result.thenApply(Function.identity());}这个方法简单来讲就是掉了生产方的FilterChainNode逻辑，最后掉到了被代理方法，为什么能掉到被代理方法前面分析过，这里就不再赘述了。我们来看看调用栈帧
调用完成以后就要回调调用方了
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)); } //回调客户端响应数据 channel.send(res); } catch (RemotingException e) { logger.warn("Send result to consumer failed, channel is " + channel + ", msg is " + e); }});从上面的分析来看，生产方经过了一系列handler的处理，最终掉到了被代理逻辑，然后响应到了客户端了。
第3章 客户端接收响应
服务端处理完成后，服务端会把Response对象响应给客户端，客户端接收到响应以后接着处理，前面的课程我们分析了，netty客户端的核心handler是，NettyClientHandler，一样响应过来后会掉到channelRead方法。
在客户端接收到了Response对象后，一样的要经过服务的一些列handler的逻辑，这里我就不再赘述了。
这里我们只着重分析一下HeaderExchangeHandler
3.1 HeaderExchangeHandler
@Overridepublic void received(Channel channel, Object message) throws RemotingException { final ExchangeChannel exchangeChannel = HeaderExchangeChannel.getOrAddChannel(channel); //服务端接收到的请求消息 if (message instanceof Request) { // handle request. Request request = (Request) message; if (request.isEvent()) { handlerEvent(channel, request); } else { //如果是双工通讯 if (request.isTwoWay()) { //核心代码 handleRequest(exchangeChannel, request); } else { handler.received(exchangeChannel, request.getData()); } } } else if (message instanceof Response) { //如果是客户端接收到的响应消息 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()); 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); }}客户端接收到Response响应后，走的逻辑就是 handleResponse(channel, (Response) message);逻辑了，我们看看这个方法。
static void handleResponse(Channel channel, Response response) throws RemotingException { if (response != null && !response.isHeartbeat()) { DefaultFuture.received(channel, response); }}public static void received(Channel channel, Response response, boolean timeout) { try { //根据返回的id，从map中找到该请求对应的future对象 DefaultFuture future = FUTURES.remove(response.getId()); if (future != null) { Timeout t = future.timeoutCheckTask; if (!timeout) { // decrease Time t.cancel(); } //收到服务端的回调后，唤醒get方法 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 status is " + response.getStatus() + (channel == null ? "" : ", channel: " + channel.getLocalAddress() + " -> " + channel.getRemoteAddress()) + ", please check provider side for detailed result."); } } finally { CHANNELS.remove(response.getId()); }}private void doReceived(Response res) { if (res == null) { throw new IllegalStateException("response cannot be null"); } if (res.getStatus() == Response.OK) { //收到返回结果后，这里是去唤醒get方法的。唤醒必须是接收到返回结果后才去唤醒 this.complete(res.getResult()); } else if (res.getStatus() == Response.CLIENT_TIMEOUT || res.getStatus() == Response.SERVER_TIMEOUT) { this.completeExceptionally(new TimeoutException(res.getStatus() == Response.SERVER_TIMEOUT, channel, res.getErrorMessage())); } else { this.completeExceptionally(new RemotingException(channel, res.getErrorMessage())); } // the result is returning, but the caller thread may still waiting // to avoid endless waiting for whatever reason, notify caller thread to return. if (executor != null && executor instanceof ThreadlessExecutor) { ThreadlessExecutor threadlessExecutor = (ThreadlessExecutor) executor; if (threadlessExecutor.isWaiting()) { threadlessExecutor.notifyReturn(new IllegalStateException("The result has returned, but the biz thread is still waiting" + " which is not an expected state, interrupt the thread manually by returning an exception.")); } }}前面我们分析客户端的请求流程时，我们看到了一个get方法的阻塞，其实上述逻辑最终掉了this.complete(res.getResult());方法，这个方法就会唤醒get方法然后把返回结果传递给get方法，这样就完成了同步调用流程了。
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已于2022-8-18 17:30:14修改
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