说明

io.netty.handler.codec.ByteToMessageDecoder是一个解码器，从字节数据转换为其它类型的数据。
ByteToMessageDecoder内部有一个累加器，将收到的字节读出来累积到一个ByteBuf中。
ByteToMessageDecoder是个抽象类型，其中抽象函数decode(ChannelHandlerContext ctx, ByteBuf in, List out)必须由子类来实现。怎么将字节流转换为其它类型的数据由子类在这个函数中实现。所以，从本质上说，ByteToMessageDecoder并不知道怎么转换数据，必须由子类来实现转换的逻辑。它也不知道一条完整的报文应该包含多少字节。
ByteToMessageDecoder还会将decode(ChannelHandlerContext ctx, ByteBuf in, List out)转换以后的数据（放在参数out中）通过ChannelHandlerContext的函数fireChannelRead(Object msg)进行触发，传递给ChannelPipeline中后续的ChannelHandler。

示例

服务端代码片段

package com.thb.power.server;

import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelOption;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.nio.NioServerSocketChannel;
import io.netty.handler.logging.LogLevel;
import io.netty.handler.logging.LoggingHandler;

/**
 * 服务端的主函数
 * @author thb
 *
 */
public class MainStation {
	
	static final int PORT = Integer.parseInt(System.getProperty("port", "22335"));

	public static void main(String[] args) throws Exception {
		// 配置服务器
		EventLoopGroup bossGroup = new NioEventLoopGroup();
		EventLoopGroup workerGroup = new NioEventLoopGroup();
		try {
			ServerBootstrap b =  new ServerBootstrap();
			b.group(bossGroup, workerGroup)
			 .channel(NioServerSocketChannel.class)
			 .option(ChannelOption.SO_BACKLOG, 100)
			 .handler(new LoggingHandler(LogLevel.INFO))
			 .childHandler(new MainStationInitializer());
			
			// 启动服务端
			ChannelFuture f = b.bind(PORT).sync();
			
			// 等待直到server socket关闭
			f.channel().closeFuture().sync();
		} finally {
			// 关闭所有event loops以便终止所有的线程
			bossGroup.shutdownGracefully();
			workerGroup.shutdownGracefully();
		}
	}

}

package com.thb.power.server;

import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.socket.SocketChannel;
import io.netty.handler.logging.LogLevel;
import io.netty.handler.logging.LoggingHandler;

public class MainStationInitializer extends ChannelInitializer<SocketChannel> {

	 @Override
	 public void initChannel(SocketChannel ch) throws Exception {
		 ChannelPipeline p = ch.pipeline();		
		 
		 p.addLast(new LoggingHandler(LogLevel.INFO));
		 p.addLast(new ServerHandler());		
	 }
}

package com.thb.power.server;

import java.util.List;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.handler.codec.ByteToMessageDecoder;

public class ServerHandler extends ByteToMessageDecoder {

	@Override
	protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) {
		System.out.println("ServerHandler.decode->readableBytes: " + in.readableBytes());
		if (in.readableBytes() < 12) {
			return;
		}
		
		out.add(in.readBytes(12));
	}
}