Netty基础教程：构建简易Netty客户端与服务器

### 简介
Java NIO技术虽然有效解决了阻塞I/O的问题，但其复杂的API和配置使得许多开发人员望而却步。Netty作为一款高性能的异步事件驱动网络应用框架，不仅简化了网络编程，还提供了丰富的功能和良好的性能。接下来，我们将通过一个简单的例子来展示如何使用Netty实现基本的客户端和服务器。

### 服务器端实现
首先，我们来看一下服务器端的实现。服务器的主要任务是监听客户端的连接请求，并处理接收到的数据。

```java
package com.example.netty.server;

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;

public class SimpleEchoServer {
private final int port;

public SimpleEchoServer(int port) {
this.port = port;
}

public void start() throws Exception {
EventLoopGroup bossGroup = new NioEventLoopGroup();
EventLoopGroup workerGroup = new NioEventLoopGroup();
try {
ServerBootstrap b = new ServerBootstrap();
b.group(bossGroup, workerGroup)
.channel(NioServerSocketChannel.class)
.localAddress(port)
.childHandler(new ChannelInitializer() {
@Override
protected void initChannel(SocketChannel ch) throws Exception {
ch.pipeline().addLast(new SimpleEchoServerHandler());
}
});
ChannelFuture f = b.bind().sync();
f.channel().closeFuture().sync();
} finally {
workerGroup.shutdownGracefully();
bossGroup.shutdownGracefully();
}
}

public static void main(String[] args) throws Exception {
int port = 65535;
new SimpleEchoServer(port).start();
}
}
```

#### 处理客户端连接
接下来是处理客户端连接的具体逻辑。

```java
package com.example.netty.server;

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

public class SimpleEchoServerHandler extends ChannelInboundHandlerAdapter {
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) {
ByteBuf in = (ByteBuf) msg;
System.out.println("Server received: " + in.toString(io.netty.util.CharsetUtil.UTF_8));
ctx.write(in);
}

@Override
public void channelReadComplete(ChannelHandlerContext ctx) {
ctx.writeAndFlush(Unpooled.EMPTY_BUFFER).addListener(ChannelFutureListener.CLOSE);
}

@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
cause.printStackTrace();
ctx.close();
}
}
```

### 客户端实现
客户端负责发起连接请求，并发送数据到服务器。

```java
package com.example.netty.client;

import io.netty.bootstrap.Bootstrap;
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.NioSocketChannel;

public class SimpleEchoClient {
private final String host;
private final int port;

public SimpleEchoClient(String host, int port) {
this.host = host;
this.port = port;
}

public void start() throws Exception {
EventLoopGroup workerGroup = new NioEventLoopGroup();
try {
Bootstrap b = new Bootstrap();
b.group(workerGroup)
.channel(NioSocketChannel.class)
.remoteAddress(host, port)
.handler(new ChannelInitializer() {
@Override
protected void initChannel(SocketChannel ch) {
ch.pipeline().addLast(new SimpleEchoClientHandler());
}
});
ChannelFuture f = b.connect().sync();
f.channel().closeFuture().sync();
} finally {
workerGroup.shutdownGracefully();
}
}

public static void main(String[] args) throws Exception {
String host = "127.0.0.1";
int port = 65535;
new SimpleEchoClient(host, port).start();
}
}
```

#### 客户端数据处理
客户端在连接成功后，会发送一条消息并接收服务器的响应。

```java
package com.example.netty.client;

import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.SimpleChannelInboundHandler;
import io.netty.util.CharsetUtil;

public class SimpleEchoClientHandler extends SimpleChannelInboundHandler {
@Override
public void channelActive(ChannelHandlerContext ctx) {
ctx.writeAndFlush(Unpooled.copiedBuffer("Hello, Netty!", CharsetUtil.UTF_8));
}

@Override
protected void channelRead0(ChannelHandlerContext ctx, ByteBuf msg) {
System.out.println("Client received: " + msg.toString(CharsetUtil.UTF_8));
}

@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
cause.printStackTrace();
ctx.close();
}
}
```

### 结果展示
按照上述步骤先启动服务器，再启动客户端，你将看到客户端发送的消息被服务器接收并回显。