netty源码：（38）ByteToMessageDecoder类

ByteToMessageDecoder是一个解码器,是一个ChannelInboundHandlerAdapter,它用来将ByteBuf中的字节流解析成另外的消息格式。它的核心方法是decode,

decode方法的in参数表示接收字节的来源，out参数表示节码之后输出的目的地。
比如，StringDecoder继承了ByteToMessageDecoder,它的decode方法代码如下：

ByteToMessageDecoder类不能被标记为@ChannelHandler.Sharable,因为ByteToMessageDecoder通常包含状态信息，共享的话会造成多线程问题。

自定义的类可以继承ByteToMessageDecoder，重写decode方法，示例：

package cn.edu.tju;

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

import java.nio.charset.Charset;
import java.util.List;

public class MyLongDecoder extends ByteToMessageDecoder {
    @Override
    protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {

        int readIndex = in.readerIndex();
        int writerIndex = in.writerIndex();

        int readableBytes = in.readableBytes();
        String str = (String) in.readCharSequence(writerIndex, Charset.defaultCharset());
        long result = Long.parseLong(str);
        out.add(result);
    }
}

使用自定义的Decoder:

package cn.edu.tju;

import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.*;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;
import io.netty.handler.codec.string.StringDecoder;

import java.net.InetSocketAddress;

public class NettyTcpServer5 {
    public static void main(String[] args) {
        EventLoopGroup bossGroup = new NioEventLoopGroup(1);
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        EventLoopGroup businessGroup = new NioEventLoopGroup();

        try {
            ServerBootstrap  serverBootstrap = new ServerBootstrap();
            serverBootstrap.group(bossGroup, workerGroup);
            serverBootstrap.channel(NioServerSocketChannel.class);
            MyLongDecoder myLongDecoder = new MyLongDecoder();

            serverBootstrap.childHandler(new ChannelInitializer<SocketChannel>() {
                @Override
                protected void initChannel(SocketChannel ch) throws Exception {
                    ChannelPipeline pipeline = ch.pipeline();
                    pipeline.addLast(businessGroup, myLongDecoder);
                    pipeline.addLast(businessGroup,new MyLongInboundHandler2());


                }
            });

            ChannelFuture channelFuture = serverBootstrap.bind(new InetSocketAddress(8899))
                    .sync();
            channelFuture.channel().closeFuture().sync();
        } catch (Exception ex){
            System.out.println(ex.getMessage());
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }



    }
}

package cn.edu.tju;

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

public class MyLongInboundHandler2 extends ChannelInboundHandlerAdapter {
    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        long result = (long)msg;
        System.out.println(msg);
    }
}