CustomDecoder.java

package com.lyncc.netty.codec.lengthFieldBasedFrame;

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

public class CustomDecoder extends LengthFieldBasedFrameDecoder {
    
    //判断传送客户端传送过来的数据是否按照协议传输，头部信息的大小应该是 byte+byte+int = 1+1+4 = 6
    private static final int HEADER_SIZE = 6;
    
    private byte type;
    
    private byte flag;
    
    private int length;
    
    private String body;

    /**
     * 
     * @param maxFrameLength 解码时，处理每个帧数据的最大长度
     * @param lengthFieldOffset 该帧数据中，存放该帧数据的长度的数据的起始位置
     * @param lengthFieldLength 记录该帧数据长度的字段本身的长度
     * @param lengthAdjustment 修改帧数据长度字段中定义的值，可以为负数
     * @param initialBytesToStrip 解析的时候需要跳过的字节数
     * @param failFast 为true，当frame长度超过maxFrameLength时立即报TooLongFrameException异常，为false，读取完整个帧再报异常
     */
    public CustomDecoder(int maxFrameLength, int lengthFieldOffset, int lengthFieldLength,
            int lengthAdjustment, int initialBytesToStrip, boolean failFast) {
        super(maxFrameLength, lengthFieldOffset, lengthFieldLength,
                lengthAdjustment, initialBytesToStrip, failFast);
    }
    
    @Override
    protected Object decode(ChannelHandlerContext ctx, ByteBuf in) throws Exception {
        if (in == null) {
            return null;
        }
        if (in.readableBytes() < HEADER_SIZE) {
            throw new Exception("可读信息段比头部信息都小，你在逗我？");
        }
        
        //注意在读的过程中，readIndex的指针也在移动
        type = in.readByte();
        
        flag = in.readByte();
        
        length = in.readInt();
        
        if (in.readableBytes() < length) {
            throw new Exception("body字段你告诉我长度是"+length+",但是真实情况是没有这么多，你又逗我？");
        }
        ByteBuf buf = in.readBytes(length);
        byte[] req = new byte[buf.readableBytes()];
        buf.readBytes(req);
        body = new String(req, "UTF-8");
        
        CustomMsg customMsg = new CustomMsg(type,flag,length,body);
        return customMsg;
    }

}