Netty 之解码器 LengthFieldBasedFrameDecoder

https://www.jianshu.com/p/c90ec659397c

public class MyProtocolDecoder extends LengthFieldBasedFrameDecoder {

    private static final int HEADER_SIZE = 6;

    /**
     *
     * @param maxFrameLength  帧的最大长度
     * @param lengthFieldOffset length字段偏移的地址
     * @param lengthFieldLength length字段所占的字节长
     * @param lengthAdjustment 修改帧数据长度字段中定义的值，可以为负数 因为有时候我们习惯把头部记入长度,若为负数,则说明要推后多少个字段
     * @param initialBytesToStrip 解析时候跳过多少个长度
     * @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
     */

    public MyProtocolDecoder(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 {
        //在这里调用父类的方法,实现指得到想要的部分,我在这里全部都要,也可以只要body部分
        in = (ByteBuf) super.decode(ctx,in);  

        if(in == null){
            return null;
        }
        if(in.readableBytes()<HEADER_SIZE){
            throw new Exception("字节数不足");
        }
        //读取type字段
        byte type = in.readByte();
        //读取flag字段
        byte flag = in.readByte();
        //读取length字段
        int length = in.readInt();
        
        if(in.readableBytes()!=length){
            throw new Exception("标记的长度不符合实际长度");
        }
        //读取body
        byte []bytes = new byte[in.readableBytes()];
        in.readBytes(bytes);

        return new MyProtocolBean(type,flag,length,new String(bytes,"UTF-8"));

    }
}