netty4.x使用小记

Hello World

引入netty包

    io.netty
    netty-all
    4.1.33.Final

实现handler

import io.netty.buffer.ByteBuf;

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

/**
 * Handles a server-side channel.
 */
public class DiscardServerHandler extends ChannelInboundHandlerAdapter { 

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) { 
        ByteBuf in = (ByteBuf) msg;
        try {
            while (in.isReadable()) {
                System.out.print((char) in.readByte());
                System.out.flush();
            }
        } finally {
            ReferenceCountUtil.release(msg);
        }
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
        // Close the connection when an exception is raised.
        cause.printStackTrace();
        ctx.close();
    }
}

写main方法

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;
    
/**
 * Discards any incoming data.
 */
public class DiscardServer {
    
    private int port;
    
    public DiscardServer(int port) {
        this.port = port;
    }
    
    public void run() 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() { 
                 @Override
                 public void initChannel(SocketChannel ch) throws Exception {
                     ch.pipeline().addLast(new DiscardServerHandler());
                 }
             })
             .option(ChannelOption.SO_BACKLOG, 128)
             .childOption(ChannelOption.SO_KEEPALIVE, true); 
    
            // Bind and start to accept incoming connections.
            ChannelFuture f = b.bind(port).sync();
    
            // Wait until the server socket is closed.
            // In this example, this does not happen, but you can do that to gracefully
            // shut down your server.
            f.channel().closeFuture().sync();
        } finally {
            workerGroup.shutdownGracefully();
            bossGroup.shutdownGracefully();
        }
    }
    
    public static void main(String[] args) throws Exception {
        int port = 5566;
        if (args.length > 0) {
            port = Integer.parseInt(args[0]);
        }

        new DiscardServer(port).run();
    }
}

• 这里有两个NioEventLoopGroup：bossGroup不断地接收新连接，workerGroup处理连接发送过来的数据。

验证

打开命令行，输入

telnet localhost 5566

输入任意字符，控制台将会把这些字符打印出来

知识点

1. @Sharable注解
   netty官方对@Sharable的描述如下

If a ChannelHandler is annotated with the @Sharable annotation, it means you can create an instance of the handler just once and add it to one or more ChannelPipelines multiple times without a race condition.

If this annotation is not specified, you have to create a new handler instance every time you add it to a pipeline because it has unshared state such as member variables.

This annotation is provided for documentation purpose, just like the JCIP annotations.

参考资料

• netty官方文档：https://netty.io/wiki/user-guide-for-4.x.html