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dubbo在服务消费时调用的方法栈比较深，所以得一边看一边记，还是比较费力的。在dubbo服务发现中，我们看到通过ReferenceConfig#get()返回的是要调用接口的代理对象，因此通过接口的代理对象调用方法时是调用InvocationHandler(InvokerInvocationHandler)#invoke()方法，此时会调用注入的Invoker#invoke()方法，而InvokerInvocationHandler对象的invoker默认是DubboInvoker实现的，因此DubboInvoker#invoke()方法被调用，最终调用子类DubboInvoker#doInvoke()方法。

// DubboInvoker=>AbstractInvoker.javapublic Result invoke(Invocation inv) throws RpcException {//设置一系列参数......try {//调用子类方法return doInvoke(invocation);} catch (InvocationTargetException e) { // biz exception......}}

这里是调用远程服务有三种，异步无返回值、异步有返回值、同步阻塞获取返回，以同步方式往下分析。

// DubboInvokerprotected Result doInvoke(final Invocation invocation) throws Throwable {RpcInvocation inv = (RpcInvocation) invocation;final String methodName = RpcUtils.getMethodName(invocation);//接口路径inv.setAttachment(Constants.PATH_KEY, getUrl().getPath());inv.setAttachment(Constants.VERSION_KEY, version);//获取已经建立连接的客户端ExchangeClient currentClient;if (clients.length == 1) {currentClient = clients[0];} else {currentClient = clients[index.getAndIncrement() % clients.length];}try {//先从invocation中获取，再从URL中获取boolean isAsync = RpcUtils.isAsync(getUrl(), invocation);boolean isOneway = RpcUtils.isOneway(getUrl(), invocation);int timeout = getUrl().getMethodParameter(methodName, Constants.TIMEOUT_KEY, Constants.DEFAULT_TIMEOUT);//单向调用，直接调用不获取返回值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) {//异步调用有返回值需要手动获取ResponseFuture future = currentClient.request(inv, timeout);RpcContext.getContext().setFuture(new FutureAdapter<Object>(future));return new RpcResult();} else {//同步调用，get()会阻塞地获取返回值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);}}

DubboProtocol#getSharedClient()生成netty客户端时，会使用ReferenceCountExchangeClient装饰客户端ExchangeClient，然后被保存到数组中，因此这里获取的currentClient为ReferenceCountExchangeClient，调用注入的ExchangeClient(HeaderExchangeClient)对象client的request()方法。

// ReferenceCountExchangeClientpublic ResponseFuture request(Object request, int timeout) throws RemotingException {return client.request(request, timeout);}

这里会继续调用内部的ExchangeChannel对象channel的request()方法，那这里的channel怎么来得呢。构造HeaderExchangeClient对象时new出来的，同时构造方法中的client为NettyClient实例对象。

// HeaderExchangeClient.javapublic ResponseFuture request(Object request, int timeout) throws RemotingException {return channel.request(request, timeout);}public HeaderExchangeClient(Client client, boolean needHeartbeat) {....this.client = client;this.channel = new HeaderExchangeChannel(client);....}

那继续往下看HeaderExchangeChannel#request()方法，好像离发送请求数据越来越近了，这里会先构造Request和DefaultFuture，再调用内部注入的channel(NettyClient)的send()方法。

// HeaderExchangeChannelpublic 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("2.0.0");req.setTwoWay(true);req.setData(request);DefaultFuture future = new DefaultFuture(channel, req, timeout);try {channel.send(req);} catch (RemotingException e) {future.cancel();throw e;}return future;}

NettyClient的继承关系是NettyClient=>AbstractClient=>AbstractPeer，此时内部AbstractClient#send()方法被调用。这里就是先getChannel()获取通道再调用send()，再分析这个channel怎么来的，在doConnect()时如果连接服务端成功则能获取Channel，默认为NettyChannel。

// AbstractClientpublic void send(Object message, boolean sent) throws RemotingException {if (send_reconnect && !isConnected()) {connect();}//getChannel()由子类NettyClientChannel channel = getChannel();//TODO Can the value returned by getChannel() be null? need improvement.if (channel == null || !channel.isConnected()) {throw new RemotingException(this, "message can not send, because channel is closed . url:" + getUrl());}channel.send(message, sent);}

NettyChannel#send()中就会把请求发送给服务端了。

// NettyChannelpublic void send(Object message, boolean sent) throws RemotingException {super.send(message, sent);boolean success = true;int timeout = 0;try {ChannelFuture future = channel.write(message);if (sent) {timeout = getUrl().getPositiveParameter(Constants.TIMEOUT_KEY, Constants.DEFAULT_TIMEOUT);success = future.await(timeout);}Throwable cause = future.getCause();if (cause != null) {throw cause;}} catch (Throwable e) {throw new RemotingException(this, "Failed to send message " + message + " to " + getRemoteAddress() + ", cause: " + e.getMessage(), e);}if (!success) {throw new RemotingException(this, "Failed to send message " + message + " to " + getRemoteAddress()+ "in timeout(" + timeout + "ms) limit");}}

至此服务消费者向服务提供者发送调用服务的请求完成，最终返回的在HeaderExchangeChannel#request()构建的DefaultFuture。还记得DubboInvoker#doInvoke()吗，这里的get()获取服务端响应的代码如下，就是不断轮询判断服务端是否响应，超时则抛出异常。

// DefaultFuturepublic 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();}

总结下发送请求的过程：首先获取已经建立连接的netty客户端，然后构建Request和DefaultFuture，通过netty通道将请求发送给netty服务端之后，DefaultFuture#get()会超时等待，默认超时时间1S。

接下来看服务端怎么处理调用请求的。服务提供者收到服务消费者的调用请求后，首先在DubboCodec#decodeBody()对编码后的请求字节流数据进行解码，得到调用远程服务的Request对象。

    protected Object decodeBody(Channel channel, InputStream is, byte[] header) throws IOException {byte flag = header[2], proto = (byte) (flag & SERIALIZATION_MASK);Serialization s = CodecSupport.getSerialization(channel.getUrl(), proto);// get request id.long id = Bytes.bytes2long(header, 4);if ((flag & FLAG_REQUEST) == 0) {......} else {// decode request.Request req = new Request(id);req.setVersion("2.0.0");req.setTwoWay((flag & FLAG_TWOWAY) != 0);if ((flag & FLAG_EVENT) != 0) {req.setEvent(Request.HEARTBEAT_EVENT);}try {Object data;if (req.isHeartbeat()) {data = decodeHeartbeatData(channel, deserialize(s, channel.getUrl(), is));} else if (req.isEvent()) {data = decodeEventData(channel, deserialize(s, channel.getUrl(), is));} else {DecodeableRpcInvocation inv;if (channel.getUrl().getParameter(Constants.DECODE_IN_IO_THREAD_KEY,Constants.DEFAULT_DECODE_IN_IO_THREAD)) {inv = new DecodeableRpcInvocation(channel, req, is, proto);inv.decode();} else {inv = new DecodeableRpcInvocation(channel, req,new UnsafeByteArrayInputStream(readMessageData(is)), proto);}data = inv;}req.setData(data);} catch (Throwable t) {if (log.isWarnEnabled()) {log.warn("Decode request failed: " + t.getMessage(), t);}// bad requestreq.setBroken(true);req.setData(t);}return req;}}

在创建netty服务器的时候会调用NettyServer#doOpen()创建NettyHandler，当前服务端收到请求数据时钩子函数messageReceived会被调用。

//NettyHandler 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());}}

服务端处理请求是分层的，消费者调用提供者的请求在AllChannelHandler#received()中被处理的，此时请求会通过线程池调度线程执行。

// AllChannelHandlerpublic void received(Channel channel, Object message) throws RemotingException {ExecutorService cexecutor = getExecutorService();try {cexecutor.execute(new ChannelEventRunnable(channel, handler, ChannelState.RECEIVED, message));} catch (Throwable t) {......}}

这里会用DecodeHandler对请求解码后到HeaderExchangeHandler#received()中继续处理。

// ChannelEventRunnablepublic void run() {switch (state) {......case RECEIVED:try {handler.received(channel, message);} catch (Exception e) {logger.warn("ChannelEventRunnable handle " + state + " operation error, channel is " + channel+ ", message is " + message, e);}break;......}}

发现收到的消息是请求Request时，调用handleRequest()处理。

// HeaderExchangeHandlerpublic void received(Channel channel, Object message) throws RemotingException {channel.setAttribute(KEY_READ_TIMESTAMP, System.currentTimeMillis());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()) {Response response = handleRequest(exchangeChannel, request);channel.send(response);} 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);}}

在handleRequest()中会调用DubboProtocol中的属性requestHandler引用的匿名内部类对象中的reply()方法。

// HeaderExchangeHandlerResponse handleRequest(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);return res;}// find handler by message class.//获取请求数据，即客户端创建的RpcInvocation对象Object msg = req.getData();try {// handle data.//调用接口方法Object result = handler.reply(channel, msg);res.setStatus(Response.OK);res.setResult(result);} catch (Throwable e) {res.setStatus(Response.SERVICE_ERROR);res.setErrorMessage(StringUtils.toString(e));}return res;}

这里其实就是获取Invoker，再调用Invoker#invoke()。

// DubboProtocol.javaprivate ExchangeHandler requestHandler = new ExchangeHandlerAdapter() {@Overridepublic Object reply(ExchangeChannel channel, Object message) throws RemotingException {if (message instanceof Invocation) {Invocation inv = (Invocation) message;Invoker<?> invoker = getInvoker(channel, inv);......return invoker.invoke(inv);}......}};

在DubboProtocol#export()暴露接口服务时会将serviceKey对应的DubboExporter添加到exporterMap(ConcurrentHashMap<String, Exporter<?>>)中，此时就可以通过serviceKey获取到对应的DubboExporter，再通过DubboExporter获取Invoker，Invoker是在ServiceConfig中的doExportUrlsFor1Protocol()中生成的。

    Invoker<?> getInvoker(Channel channel, Invocation inv) throws RemotingException {......String serviceKey = serviceKey(port, path, inv.getAttachments().get(Constants.VERSION_KEY), inv.getAttachments().get(Constants.GROUP_KEY));DubboExporter<?> exporter = (DubboExporter<?>) exporterMap.get(serviceKey);......return exporter.getInvoker();}

找到Invoker以后就会调用invoke()，最终调用子类的doInvoke()方法，wrapper是动态生成的，但是逻辑就是调用接口实现类对象中的方法。

    public Result invoke(Invocation invocation) throws RpcException {try {//调用子类的doInvoke()方法return new RpcResult(doInvoke(proxy, invocation.getMethodName(), invocation.getParameterTypes(), invocation.getArguments()));} catch (InvocationTargetException e) {return new RpcResult(e.getTargetException());} catch (Throwable e) {throw new RpcException("Failed to invoke remote proxy method " + invocation.getMethodName() + " to " + getUrl() + ", cause: " + e.getMessage(), e);}}public <T> Invoker<T> getInvoker(T proxy, Class<T> type, URL url) {// TODO Wrapper cannot handle this scenario correctly: the classname contains '$'final Wrapper wrapper = Wrapper.getWrapper(proxy.getClass().getName().indexOf('$') < 0 ? proxy.getClass() : type);return new AbstractProxyInvoker<T>(proxy, type, url) {@Overrideprotected Object doInvoke(T proxy, String methodName,Class<?>[] parameterTypes,Object[] arguments) throws Throwable {return wrapper.invokeMethod(proxy, methodName, parameterTypes, arguments);}};}

调用方法获取到返回值以后就会在HeaderExchangeHandler#received()中将响应发给服务消费者，服务消费者再以类似的过程对响应数据进行解码，返回到应用层。
总结下服务提供者处理调用请求的过程：首先对请求字节流数据进行解码，得到请求Request，然后到保存导出模块的map根据serviceKey获取到代理对象，最终通过代理对象调用接口实现类对象的方法，将返回值发送给服务提供者