Dubbo源码分析----处理请求

Dubbo默认是使用Netty进行通信的，那么Netty会配置一个Handler，来处理一些事件，所以这个Handler是核心，主要找一下Dubbo初始化Netty的时候设置的Handler

回顾一下网络通信相关，在DubboProtocol中，会调用createServer方法返回一个Server对象，这是一个切入点

    private ExchangeServer createServer(URL url) {
        //....
        ExchangeServer server;
        try {
            server = Exchangers.bind(url, requestHandler);
        } catch (RemotingException e) {
            throw new RpcException("Fail to start server(url: " + url + ") " + e.getMessage(), e);
        }
        //....
        return server;
    }

bind方法内部对requestHandler进行了层层的装饰，但是这里也不是我们要找的地方，在NettyServer的doOpen才是我们要找的(具体流程可以看下网络通信的那篇文章)

    protected void doOpen() throws Throwable {
        //....
        bootstrap.setPipelineFactory(new ChannelPipelineFactory() {
            public ChannelPipeline getPipeline() {
                NettyCodecAdapter adapter = new NettyCodecAdapter(getCodec() ,getUrl(), NettyServer.this);
                ChannelPipeline pipeline = Channels.pipeline();
                /*int idleTimeout = getIdleTimeout();
                if (idleTimeout > 10000) {
                    pipeline.addLast("timer", new IdleStateHandler(timer, idleTimeout / 1000, 0, 0));
                }*/
                pipeline.addLast("decoder", adapter.getDecoder());
                pipeline.addLast("encoder", adapter.getEncoder());
                pipeline.addLast("handler", nettyHandler);
                return pipeline;
            }
        });
        // ....
    }

可以看到，这个NettyHandler是处理请求的入口，看下NettyHandler的结构图，其中最下面的handler是requestHandler

当Netty接收到请求的时候，会调用NettyServer的messageReceived方法，请求依次经过图中红色框框的Handler，不同的Handler有不同的功能，只需要看下几个关键的Handler

AllChannelHandler

这个Handler在讲Dubbo的Dispatcher的ThreadPool的时候讲过，就是将请求交给线程池处理

    public void received(Channel channel, Object message) throws RemotingException {
        ExecutorService cexecutor = getExecutorService();
        try {
            cexecutor.execute(new ChannelEventRunnable(channel, handler, ChannelState.RECEIVED, message));
        } catch (Throwable t) {
            throw new ExecutionException(message, channel, getClass() + " error when process received event .", t);
        }
    }

那么看下ChannelEventRunnable的run方法

    public void run() {
        switch (state) {
            case CONNECTED:
                    handler.connected(channel);
                break;
            case DISCONNECTED:
                    handler.disconnected(channel);
                break;
            case SENT:
                    handler.sent(channel,message);
                break;
            case RECEIVED:
                    handler.received(channel, message);
                break;
            case CAUGHT:
                    handler.caught(channel, exception);
                break;
            default:
                logger.warn("unknown state: " + state + ", message is " + message);
        }
    }

在线程中将请求交由下一个handler处理，从上面的图中可以知道AllChannelHandler下两个handler是DecodeHandler和HeaderExchangeHandler，这两个Handler做了一些特殊处理，然后将请求转发给requestHandler

    private ExchangeHandler requestHandler = new ExchangeHandlerAdapter() {

        public Object reply(ExchangeChannel channel, Object message) throws RemotingException {
            if (message instanceof Invocation) {
                Invocation inv = (Invocation) message;
                Invoker<?> invoker = getInvoker(channel, inv);// 通过参数信息获取本地Invoker
               //....
                RpcContext.getContext().setRemoteAddress(channel.getRemoteAddress());
                return invoker.invoke(inv);//调用
            }
            throw new RemotingException(channel, "Unsupported request: " + message == null ? null : (message.getClass().getName() + ": " + message) + ", channel: consumer: " + channel.getRemoteAddress() + " --> provider: " + channel.getLocalAddress());
        }

        //....
    };

getInvoker方法如下

    Invoker<?> getInvoker(Channel channel, Invocation inv) throws RemotingException{
        boolean isCallBackServiceInvoke = false;
        boolean isStubServiceInvoke = false;
        int port = channel.getLocalAddress().getPort();
        String path = inv.getAttachments().get(Constants.PATH_KEY);//xxx.xxx.xxx.xxxService
        //如果是客户端的回调服务.
        isStubServiceInvoke = Boolean.TRUE.toString().equals(inv.getAttachments().get(Constants.STUB_EVENT_KEY));
        if (isStubServiceInvoke){
            port = channel.getRemoteAddress().getPort();
        }
        //callback
        isCallBackServiceInvoke = isClientSide(channel) && !isStubServiceInvoke;
        if(isCallBackServiceInvoke){
            path = inv.getAttachments().get(Constants.PATH_KEY)+"."+inv.getAttachments().get(Constants.CALLBACK_SERVICE_KEY);
            inv.getAttachments().put(IS_CALLBACK_SERVICE_INVOKE, Boolean.TRUE.toString());
        }
        String serviceKey = serviceKey(port, path, inv.getAttachments().get(Constants.VERSION_KEY), inv.getAttachments().get(Constants.GROUP_KEY));// port+serviceName+version+group区分的key

        // 暴露服务的时候会将Exporter放到map中
        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:" + inv);

        return exporter.getInvoker();
    }

getInvoker也是比较简单的，这时候得到的Invoker相当于是本地服务的一个代理对象，invoke之后就会调用到真实的方法