netty 实现代理服务器

Netty是一款基于NIO的网络编程框架，提供了高效、稳定、灵活的网络编程能力。使用Netty实现代理服务器可以简化开发过程，提高性能和可维护性。

以下是使用Netty实现代理服务器的示例代码：

import io.netty.bootstrap.Bootstrap;
import io.netty.buffer.ByteBuf;
import io.netty.channel.*;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioSocketChannel;
import io.netty.handler.codec.http.*;
import io.netty.handler.logging.LogLevel;
import io.netty.handler.logging.LoggingHandler;
import io.netty.handler.stream.ChunkedWriteHandler;

public class ProxyServer {
    public static void main(String[] args) throws Exception {
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        try {
            Bootstrap bootstrap = new Bootstrap();
            bootstrap.group(workerGroup)
                    .channel(NioSocketChannel.class)
                    .handler(new LoggingHandler(LogLevel.INFO))
                    .option(ChannelOption.AUTO_READ, false)
                    .handler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        protected void initChannel(SocketChannel ch) throws Exception {
                            ch.pipeline().addLast(new HttpClientCodec());
                            ch.pipeline().addLast(new HttpObjectAggregator(65536));
                            ch.pipeline().addLast(new ChunkedWriteHandler());
                            ch.pipeline().addLast(new ProxyServerHandler());
                        }
                    });

            ChannelFuture future = bootstrap.connect("www.example.com", 80).sync();
            future.channel().closeFuture().sync();
        } finally {
            workerGroup.shutdownGracefully();
        }
    }

    private static class ProxyServerHandler extends ChannelInboundHandlerAdapter {
        private Channel remoteChannel;

        @Override
        public void channelActive(ChannelHandlerContext ctx) throws Exception {
            remoteChannel = ctx.channel();
            ctx.read();
        }

        @Override
        public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
            if (msg instanceof HttpRequest) {
                HttpRequest request = (HttpRequest) msg;
                String host = request.headers().get("Host");
                ChannelFuture future = new Bootstrap()
                        .group(ctx.channel().eventLoop())
                        .channel(ctx.channel().getClass())
                        .handler(new LoggingHandler(LogLevel.INFO))
                        .option(ChannelOption.AUTO_READ, false)
                        .option(ChannelOption.CONNECT_TIMEOUT_MILLIS, 10000)
                        .handler(new ChannelInitializer<SocketChannel>() {
                            @Override
                            protected void initChannel(SocketChannel ch) throws Exception {
                                ch.pipeline().addLast(new HttpResponseDecoder());
                                ch.pipeline().addLast(new HttpObjectAggregator(65536));
                                ch.pipeline().addLast(new ChunkedWriteHandler());
                                ch.pipeline().addLast(new ChannelInboundHandlerAdapter() {
                                    @Override
                                    public void channelActive(ChannelHandlerContext ctx) throws Exception {
                                        ctx.writeAndFlush(request);
                                        ctx.read();
                                    }

                                    @Override
                                    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
                                        if (msg instanceof HttpResponse) {
                                            HttpResponse response = (HttpResponse) msg;
                                            response.headers().remove("Transfer-Encoding");
                                            response.headers().remove("Content-Length");
                                            remoteChannel.writeAndFlush(response);
                                            remoteChannel.writeAndFlush(new ChunkedNioStream((ByteBuf) msg));
                                        } else if (msg instanceof HttpContent) {
                                            remoteChannel.writeAndFlush(new ChunkedNioStream((ByteBuf) msg));
                                            if (msg instanceof LastHttpContent) {
                                                remoteChannel.writeAndFlush(LastHttpContent.EMPTY_LAST_CONTENT)
                                                        .addListener(ChannelFutureListener.CLOSE);
                                            }
                                        }
                                    }

                                    @Override
                                    public void channelInactive(ChannelHandlerContext ctx) throws Exception {
                                        remoteChannel.writeAndFlush(LastHttpContent.EMPTY_LAST_CONTENT)
                                                .addListener(ChannelFutureListener.CLOSE);
                                    }

                                    @Override
                                    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
                                        cause.printStackTrace();
                                        remoteChannel.writeAndFlush(LastHttpContent.EMPTY_LAST_CONTENT)
                                                .addListener(ChannelFutureListener.CLOSE);
                                    }
                                });
                            }
                        })
                        .connect(host, 80);
                remoteChannel.config().setAutoRead(false);
                future.addListener((ChannelFutureListener) future1 -> {
                    if (future1.isSuccess()) {
                        remoteChannel.config().setAutoRead(true);
                        ctx.channel().config().setAutoRead(true);
                    } else {
                        remoteChannel.writeAndFlush(LastHttpContent.EMPTY_LAST_CONTENT)
                                .addListener(ChannelFutureListener.CLOSE);
                        ctx.writeAndFlush(LastHttpContent.EMPTY_LAST_CONTENT)
                                .addListener(ChannelFutureListener.CLOSE);
                    }
                });
            } else if (msg instanceof HttpContent) {
                remoteChannel.writeAndFlush(new ChunkedNioStream((ByteBuf) msg));
                if (msg instanceof LastHttpContent) {
                    remoteChannel.writeAndFlush(LastHttpContent.EMPTY_LAST_CONTENT);
                }
            }
        }

        @Override
        public void channelInactive(ChannelHandlerContext ctx) throws Exception {
            if (remoteChannel != null) {
                remoteChannel.writeAndFlush(LastHttpContent.EMPTY_LAST_CONTENT)
                        .addListener(ChannelFutureListener.CLOSE);
            }
        }

        @Override
        public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
            cause.printStackTrace();
            if (remoteChannel != null) {
                remoteChannel.writeAndFlush(LastHttpContent.EMPTY_LAST_CONTENT)
                        .addListener(ChannelFutureListener.CLOSE);
            }
            ctx.close();
        }
    }
}

以上代码中，代理服务器连接到目标服务器的IP地址和端口号是硬编码的，你需要根据实际情况进行修改。在启动代理服务器之后，当客户端发送HTTP请求时，会在一个新的线程中处理请求，解析请求并连接到目标服务器，将请求转发给目标服务器。接收到目标服务器的响应后，将响应转发给客户端。