Netty中，通讯的双方建立连接后，会把数据按照ByteBuf的方式进行传输，例如http协议中，就是通过HttpRequestDecoder对ByteBuf数据流进行处理，转换成http的对象。基于这个思路，我自定义一种通讯协议：Server和客户端直接传输java对象。

实现的原理是通过Encoder把java对象转换成ByteBuf流进行传输，通过Decoder把ByteBuf转换成java对象进行处理，处理逻辑如下图所示：

传输的java bean为Person：

package com.guowl.testobjcoder;

import java.io.Serializable;

// 必须实现Serializable接口
public class Person implements Serializable{
    private static final long   serialVersionUID    = 1L;
    private String  name;
    private String  sex;
    private int     age;

    public String toString() {
        return "name:" + name + " sex:" + sex + " age:" + age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getSex() {
        return sex;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

Server端类：Server PersonDecoder BusinessHandler

1、Server：启动netty服务

package com.guowl.testobjcoder;

import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelOption;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;

public class Server {
    public void start(int port) throws Exception {
        EventLoopGroup bossGroup = new NioEventLoopGroup();
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        try {
            ServerBootstrap b = new ServerBootstrap();
            b.group(bossGroup, workerGroup).channel(NioServerSocketChannel.class)
                    .childHandler(new ChannelInitializer<SocketChannel>() {
                                @Override
                                public void initChannel(SocketChannel ch) throws Exception {
                                    ch.pipeline().addLast(new PersonDecoder());
                                    ch.pipeline().addLast(new BusinessHandler());
                                }
                            }).option(ChannelOption.SO_BACKLOG, 128)
                    .childOption(ChannelOption.SO_KEEPALIVE, true);

            ChannelFuture f = b.bind(port).sync();

            f.channel().closeFuture().sync();
        } finally {
            workerGroup.shutdownGracefully();
            bossGroup.shutdownGracefully();
        }
    }

    public static void main(String[] args) throws Exception {
        Server server = new Server();
        server.start(8000);
    }
}

2、PersonDecoder：把ByteBuf流转换成Person对象，其中ByteBufToBytes是读取ButeBuf的工具类，上一篇文章中提到过，在此不在详述。ByteObjConverter是byte和obj的互相转换的工具。

package com.guowl.testobjcoder;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.handler.codec.ByteToMessageDecoder;

import java.util.List;

import com.guowl.utils.ByteBufToBytes;
import com.guowl.utils.ByteObjConverter;

public class PersonDecoder extends ByteToMessageDecoder {
    @Override
    protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
        ByteBufToBytes read = new ByteBufToBytes();
        Object obj = ByteObjConverter.ByteToObject(read.read(in));
        out.add(obj);
    }

}

3、BusinessHandler 读取Person信息，并打印

package com.guowl.testobjcoder;

import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class BusinessHandler extends ChannelInboundHandlerAdapter {
    private Logger  logger  = LoggerFactory.getLogger(BusinessHandler.class);

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        Person person = (Person) msg;
        logger.info("BusinessHandler read msg from client :" + person);
    }

    @Override
    public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
        ctx.flush();
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {

    }
}

Client端的类：Client ClientInitHandler PersonEncoder

1、Client 建立与Server的连接

package com.guowl.testobjcoder;

import io.netty.bootstrap.Bootstrap;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelOption;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioSocketChannel;

public class Client {
    public void connect(String host, int port) throws Exception {
        EventLoopGroup workerGroup = new NioEventLoopGroup();

        try {
            Bootstrap b = new Bootstrap();
            b.group(workerGroup);
            b.channel(NioSocketChannel.class);
            b.option(ChannelOption.SO_KEEPALIVE, true);
            b.handler(new ChannelInitializer<SocketChannel>() {
                @Override
                public void initChannel(SocketChannel ch) throws Exception {
                    ch.pipeline().addLast(new PersonEncoder());
                    ch.pipeline().addLast(new ClientInitHandler());
                }
            });

            ChannelFuture f = b.connect(host, port).sync();
            f.channel().closeFuture().sync();
        } finally {
            workerGroup.shutdownGracefully();
        }

    }

    public static void main(String[] args) throws Exception {
        Client client = new Client();
        client.connect("127.0.0.1", 8000);
    }
}

2、ClientInitHandler 向Server发送Person对象

package com.guowl.testobjcoder;

import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class ClientInitHandler extends ChannelInboundHandlerAdapter {
    private static Logger   logger  = LoggerFactory.getLogger(ClientInitHandler.class);
    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        logger.info("HelloClientIntHandler.channelActive");
        Person person = new Person();
        person.setName("guowl");
        person.setSex("man");
        person.setAge(30);
        ctx.write(person);
        ctx.flush();
    }
}

3、PersonEncoder 把Person对象转换成ByteBuf进行传送

package com.guowl.testobjcoder;

import com.guowl.utils.ByteObjConverter;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.handler.codec.MessageToByteEncoder;

public class PersonEncoder extends MessageToByteEncoder<Person> {

    @Override
    protected void encode(ChannelHandlerContext ctx, Person msg, ByteBuf out) throws Exception {
        byte[] datas = ByteObjConverter.ObjectToByte(msg);
        out.writeBytes(datas);
        ctx.flush();
    }
}

工具类：ByteObjConverter

package com.guowl.utils;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;

public class ByteObjConverter {
    public static Object ByteToObject(byte[] bytes) {
        Object obj = null;
        ByteArrayInputStream bi = new ByteArrayInputStream(bytes);
        ObjectInputStream oi = null;
        try {
            oi = new ObjectInputStream(bi);
            obj = oi.readObject();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                bi.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
            try {
                oi.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return obj;
    }

    public static byte[] ObjectToByte(Object obj) {
        byte[] bytes = null;
        ByteArrayOutputStream bo = new ByteArrayOutputStream();
        ObjectOutputStream oo = null;
        try {
            oo = new ObjectOutputStream(bo);
            oo.writeObject(obj);
            bytes = bo.toByteArray();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                bo.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
            try {
                oo.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return (bytes);
    }
}

通过上述代码，实现了Server端与Client端直接使用person对象进行通信的目的。基于此，可以构建更为复杂的场景：Server端同时支撑多种协议，不同的协议采用不同的Decoder进行解析，解析结果保持统一，这样业务处理类可以保持接口一致。下一节将编写这样一个案例。

本例中需要注意的事项是：

1、Person对象必须实现Serializable接口，否则不能进行序列化。

2、PersonDecoder读取ByteBuf数据的时候，并没有对多次流式数据进行处理，而是简单的一次性接收，如果数据量大的情况下，可能会出现数据不完整，这个问题会在后续的学习中解决。