固定长度解码器

• For example, if you received the following four fragmented packets:

• <pre>

• +---+----+------+----+

• | A | BC | DEFG | HI |

• +---+----+------+----+

• </pre>

• A FixedLengthFrameDecoderwill decode them into the following three packets with the fixed length:

• <pre>

• +-----+-----+-----+

• | ABC | DEF | GHI |

• +-----+-----+-----+

解码

protected final void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
    // 解码
    Object decoded = decode(ctx, in);
    // 如果解码成功,那么将返回结果添加到out中
    if (decoded != null) {
        out.add(decoded);
    }
}

protected Object decode(
        @SuppressWarnings("UnusedParameters") ChannelHandlerContext ctx, ByteBuf in) throws Exception {
    // 如果没有到达规定的定长,那么返回null
    // 在ByteToMessageDecode那边会认为没有读取任何数据,直接结束循环,等待下次读取事件的到来
    if (in.readableBytes() < frameLength) {
        return null;
    } else {
        // 可见数据满足要求, 截取定长的数据返回.
        return in.readRetainedSlice(frameLength);
    }
}

readRetainedSlice

// 这里实际上是生成源Buffer的只读缓冲区,且retain
public ByteBuf readRetainedSlice(int length) {
    ByteBuf slice = retainedSlice(readerIndex, length);
    readerIndex += length;
    return slice;
}