Netty分布式固定长度解码器实现原理剖析

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);
        }
    }
}