NettyProxy

package com.sdww8591.proxy;

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

public class NettyProxy {

    private static final Integer LISTEN = 8777;

    private static final String HOST = "localhost";

    private static final Integer PORT = 80;

    @Getter
    private static EventLoopGroup workerEventLoopGroup = new NioEventLoopGroup(10);

    @SneakyThrows
    public static void startProxy() {
        ServerBootstrap serverBootstrap = new ServerBootstrap();
        EventLoopGroup acceptEventLoopGroup = new NioEventLoopGroup(1);
        serverBootstrap.group(acceptEventLoopGroup, workerEventLoopGroup)
                .channel(NioServerSocketChannel.class)
                .localAddress(LISTEN)
                .childHandler(new ChannelInitializer<SocketChannel>() {
                    @Override
                    protected void initChannel(SocketChannel ch) throws Exception {
                        ch.pipeline().addLast(new ProxyChannelHandler(HOST, PORT));
                    }
                });
        ChannelFuture future = serverBootstrap.bind().sync();
        future.channel().closeFuture().sync();
    }

    public static void main(String[] args) {
        startProxy();
    }
}

ProxyChannelHandler

package com.sdww8591.proxy;

import io.netty.bootstrap.Bootstrap;
import io.netty.channel.*;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioSocketChannel;

import java.util.concurrent.TimeUnit;

public class ProxyChannelHandler extends ChannelInboundHandlerAdapter {

    private String host;

    private Integer port;

    private Channel targetChannel;

    public ProxyChannelHandler(String host, Integer port) {
        this.host = host;
        this.port = port;
    }

    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        ChannelFuture future = buildTargetChannel(ctx.channel());
        targetChannel = future.channel();
        future.get(30, TimeUnit.SECONDS);
    }

    private ChannelFuture buildTargetChannel(Channel srcChannel) {
        Bootstrap bootstrap = new Bootstrap();
        return bootstrap.group(NettyProxy.getWorkerEventLoopGroup())
                .channel(NioSocketChannel.class)
                .remoteAddress(host, port)
                .handler(new ChannelInitializer<SocketChannel>() {
                    @Override
                    protected void initChannel(SocketChannel ch) throws Exception {
                        ch.pipeline().addLast(new ChannelInboundHandlerAdapter() {
                            @Override
                            public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
                                srcChannel.writeAndFlush(msg);
                            }

                            @Override
                            public void channelInactive(ChannelHandlerContext ctx) throws Exception {
                                srcChannel.pipeline().fireChannelInactive();
                            }
                        });
                    }
                }).connect();
    }

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        targetChannel.writeAndFlush(msg);
    }

    @Override
    public void channelInactive(ChannelHandlerContext ctx) throws Exception {
        targetChannel.pipeline().fireChannelInactive();
    }
}