1 Exchange 层概述

官方定义：

exchange 信息交换层：封装请求响应模式，同步转异步，以 Request, Response 为中心，扩展接口为 Exchanger, ExchangeChannel, ExchangeClient, ExchangeServer。

其中Exchanger是SPI扩展点，是该层的入口。其中客户端通过ExchangeClient.request发送请求，服务端通过ExchangeHandler的reply方法处理请求并返回结果。

为了理解上面官方的定义，下面将使用该层的类创建一个客户端和服务器端的应用。

2 实践例子

2.1 项目结构

由于是TCP框架，所以有服务端和客户端，两端的代码。

服务端代码：ExchangeServerTest 启动类，AlfServerExchangeHandler服务端的业务逻辑处理类(类似Netty的Handler作用)。

客户端代码：ExchangeClientTest 启动类，AlfClientExchangeHandler 客户端的业务逻辑处理类。

2.2 服务端代码

ExchangeServerTest类，使用Exchanger接口绑定(bind)端口8888，启动服务器。

注意URL中添加的codec属性，如果不添加程序会走telnet的实现，程序会报错。可以参考：可以参考AbstractEndpoint的getChannelCodec函数。

package org.example.dubbo.exchange;

import org.apache.dubbo.common.URL;
import org.apache.dubbo.common.URLBuilder;
import org.apache.dubbo.remoting.RemotingException;
import org.apache.dubbo.remoting.exchange.ExchangeServer;
import org.apache.dubbo.remoting.exchange.Exchanger;
import org.apache.dubbo.remoting.exchange.support.header.HeaderExchanger;

import java.io.IOException;

/** 服务端代码 */
public class ExchangeServerTest {
    public static void main(String[] args) throws RemotingException, IOException {
        //构建URL, dubbo中靠URL来传递参数
        URLBuilder urlBuilder = new URLBuilder();
        urlBuilder.setHost("localhost");
        urlBuilder.setPort(8888);
        //指定超时事件, 调试时防止超时
        urlBuilder.addParameter("codec", "exchange");
        URL url = urlBuilder.build();

        //Exchanger层入口类，可以通过SPI方式获取实现，这里为了简单直接new了HeaderExchanger
        Exchanger exchanger = new HeaderExchanger();
        //服务端调用bind方法
        ExchangeServer exchangeServer = exchanger.bind(url, new AlfServerExchangeHandler());

        System.out.println("服务器启动完成");
        //等待，防止程序提前结束
        System.in.read();
    }
}

AlfServerExchangeHandler类，主要关注reply方法，该方法处理客户端发来的请求，然后通过CompletableFuture异步返回给客户端。还记得官方说的“同步转异步”吗？这里是一个体现。

package org.example.dubbo.exchange;

import org.apache.dubbo.remoting.Channel;
import org.apache.dubbo.remoting.RemotingException;
import org.apache.dubbo.remoting.exchange.ExchangeChannel;
import org.apache.dubbo.remoting.exchange.ExchangeHandler;

import java.util.concurrent.CompletableFuture;

/** 服务端业务处理 */
public class AlfServerExchangeHandler implements ExchangeHandler {

    @Override
    public CompletableFuture<Object> reply(ExchangeChannel channel, Object request) throws RemotingException {
        System.out.println("reply AAA, request=" + request);
        CompletableFuture<Object> stringCompletableFuture = CompletableFuture.supplyAsync(() -> "服务器8888为你服务");
        return stringCompletableFuture;
    }

    @Override
    public void connected(Channel channel) throws RemotingException {
        System.out.println("connected AAA");
    }

    @Override
    public void disconnected(Channel channel) throws RemotingException {
        System.out.println("disconnected AAA");
    }

    @Override
    public void sent(Channel channel, Object message) throws RemotingException {
        System.out.println("sent AAA, message =" + message);
    }

    @Override
    public void received(Channel channel, Object message) throws RemotingException {
        System.out.println("received AAA");
    }

    @Override
    public void caught(Channel channel, Throwable exception) throws RemotingException {
        System.out.println("caught AAA");
        exception.printStackTrace();
    }

    @Override
    public String telnet(Channel channel, String message) throws RemotingException {
        System.out.println("telnet AAA, message = " + message);
        return null;
    }
}

2.3 客户端代码

ExchangeClientTest客户端代码入口，使用Exchanger接口连接(connect)函数来连接本机的8888端口。

注意一下CompletableFuture<Object> completableFuture = exchangeClient.request("你是谁?", null); 这句代码，通过是哦也能够CompletableFuture，把获取相应结果异步了。

package org.example.dubbo.exchange;

import org.apache.dubbo.common.URL;
import org.apache.dubbo.common.URLBuilder;

import org.apache.dubbo.remoting.RemotingException;
import org.apache.dubbo.remoting.exchange.*;
import org.apache.dubbo.remoting.exchange.support.header.HeaderExchanger;

import java.io.IOException;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;

/** 客户端*/
public class ExchangeClientTest {
    public static void main(String[] args) throws RemotingException, IOException, ExecutionException, InterruptedException {
        //构建URL, dubbo中靠URL来传递参数
        URLBuilder urlBuilder = new URLBuilder();
        urlBuilder.setHost("localhost");
        urlBuilder.setPort(8888);
        //指定超时事件, 调试时防止超时
        urlBuilder.addParameter("timeout", 1000 * 200);
        //指定编码器，可以参考AbstractEndpoint的getChannelCodec函数
        urlBuilder.addParameter("codec", "exchange");
        URL url = urlBuilder.build();
        //Exchanger层入口类，可以通过SPI方式获取实现，这里为了简单直接new了HeaderExchanger
        Exchanger exchanger = new HeaderExchanger();

        //客户端调用connect方法连接服务端
        ExchangeClient exchangeClient = exchanger.connect(url, new AlfClientExchangeHandler());
        //发送消息
        CompletableFuture<Object> completableFuture = exchangeClient.request("你是谁?", null);
        System.out.println("客户端发送消息----------");
        Object o = completableFuture.get();
        System.out.println("客户端接收到消息----------" + o);
        //等待，防止程序提前结束
        System.in.read();
    }
}

AlfClientExchangeHandler类，业务处理器。主要是展示一下发送消息和获取响应时被掉用的方法。

package org.example.dubbo.exchange;

import org.apache.dubbo.remoting.Channel;
import org.apache.dubbo.remoting.RemotingException;
import org.apache.dubbo.remoting.exchange.ExchangeChannel;
import org.apache.dubbo.remoting.exchange.ExchangeHandler;

import java.util.concurrent.CompletableFuture;

/** 客户端处理器*/
public class AlfClientExchangeHandler implements ExchangeHandler {

    @Override
    public CompletableFuture<Object> reply(ExchangeChannel channel, Object request) throws RemotingException {
        //本例中是客户端发送的请求，所以该方法不会被调用的
        System.out.println("reply BBB, request=" + request);
        CompletableFuture<Object> stringCompletableFuture = CompletableFuture.supplyAsync(() -> "客户端返回数据");
        return stringCompletableFuture;
    }

    @Override
    public void connected(Channel channel) throws RemotingException {
        System.out.println("connected BBB");
    }

    @Override
    public void disconnected(Channel channel) throws RemotingException {
        System.out.println("disconnected BBB");
    }

    @Override
    public void sent(Channel channel, Object message) throws RemotingException {
        System.out.println("sent BBB, message =" + message);
    }

    @Override
    public void received(Channel channel, Object message) throws RemotingException {
        System.out.println("received BBB");
    }

    @Override
    public void caught(Channel channel, Throwable exception) throws RemotingException {
        System.out.println("caught BBB");
    }

    @Override
    public String telnet(Channel channel, String message) throws RemotingException {
        System.out.println("telnet BBB, message = " + message);
        return null;
    }
}