一、引言

传统的 Java BIO（Blocking I/O）模型在高并发场景下效率低下。NIO、Netty 与异步 IO 的出现，彻底改变了这一格局。

二、BIO vs NIO vs AIO

三、NIO 原理与示例

NIO 以 Channel、Buffer、Selector 为核心：

Selector selector = Selector.open();
ServerSocketChannel server = ServerSocketChannel.open();
server.bind(new InetSocketAddress(8080));
server.configureBlocking(false);
server.register(selector, SelectionKey.OP_ACCEPT);

while (true) {
    selector.select();
    Set<SelectionKey> keys = selector.selectedKeys();
    for (SelectionKey key : keys) {
        if (key.isAcceptable()) {
            SocketChannel client = server.accept();
            client.configureBlocking(false);
            client.register(selector, SelectionKey.OP_READ);
        } else if (key.isReadable()) {
            SocketChannel channel = (SocketChannel) key.channel();
            ByteBuffer buffer = ByteBuffer.allocate(256);
            int read = channel.read(buffer);
            if (read > 0) {
                System.out.println("接收数据：" + new String(buffer.array()).trim());
            }
        }
    }
}

该示例展示了单线程管理多连接的能力。

四、Netty 框架

Netty 是对 NIO 的高层封装，提供高性能、低延迟的网络通信框架。

4.1 核心组件

4.2 示例代码

public class NettyServer {
    public static void main(String[] args) throws Exception {
        EventLoopGroup bossGroup = new NioEventLoopGroup(1);
        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) {
                     ch.pipeline().addLast(new SimpleChannelInboundHandler<String>() {
                         @Override
                         protected void channelRead0(ChannelHandlerContext ctx, String msg) {
                             System.out.println("收到消息: " + msg);
                             ctx.writeAndFlush("Echo: " + msg);
                         }
                     });
                 }
             });

            ChannelFuture f = b.bind(8080).sync();
            f.channel().closeFuture().sync();
        } finally {
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }
}

五、异步 IO（AIO）机制

Java 7 引入了 AsynchronousSocketChannel 实现真正异步 IO：

AsynchronousServerSocketChannel server = AsynchronousServerSocketChannel.open()
    .bind(new InetSocketAddress(8080));

server.accept(null, new CompletionHandler<AsynchronousSocketChannel, Void>() {
    @Override
    public void completed(AsynchronousSocketChannel client, Void att) {
        ByteBuffer buffer = ByteBuffer.allocate(1024);
        client.read(buffer, buffer, new CompletionHandler<Integer, ByteBuffer>() {
            public void completed(Integer result, ByteBuffer buf) {
                buf.flip();
                System.out.println("接收: " + new String(buf.array()).trim());
            }
            public void failed(Throwable exc, ByteBuffer buf) {}
        });
    }
    public void failed(Throwable exc, Void att) {}
});

六、总结

理解 NIO 与 Netty 的核心思想，是迈向高性能网络编程的必经之路。