public class FixedLengthFrameDecoder extends ByteToMessageDecoder {

    //长度大小

    private final int frameLength;

    public FixedLengthFrameDecoder(int frameLength) {

        if (frameLength <= 0) {

            throw new IllegalArgumentException(

                    "frameLength must be a positive integer: " + frameLength);

        }

        //保存当前frameLength

        this.frameLength = frameLength;

    }

    @Override

    protected final void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {

        //通过ByteBuf去解码.解码到对象之后添加到out上

        Object decoded = decode(ctx, in);

        if (decoded != null) {

            //将解析到byteBuf添加到对象里面

            out.add(decoded);

        }

    }

    protected Object decode(

            @SuppressWarnings("UnusedParameters") ChannelHandlerContext ctx, ByteBuf in) throws Exception {

        //字节是否小于这个固定长度

        if (in.readableBytes() < frameLength) {

            return null;

        } else {

            //当前累加器中截取这个长度的数值

            return in.readRetainedSlice(frameLength);

        }

    }

}