java - Netty 4.0 ByteBuf keep() 使用

Question

我正在尝试了解基于https://github.com/traccar/traccar解码aTrack协议(protocol)实现，他们使用Netty作为他们的平台实现，但我不明白ByteBuf对象的retain方法的用途是什么。他们在这个方法中使用了它:

protected Object decode(
        Channel channel, SocketAddress remoteAddress, Object msg) throws Exception {

    ByteBuf buf = (ByteBuf) msg;

    if (buf.getUnsignedShort(buf.readerIndex()) == 0xfe02) {
        if (channel != null) {
            channel.writeAndFlush(new NetworkMessage(buf.retain(), remoteAddress)); // keep-alive message
        }
        return null;
    } else if (buf.getByte(buf.readerIndex()) == '$') {
        return decodeInfo(channel, remoteAddress, buf.toString(StandardCharsets.US_ASCII).trim());
    } else if (buf.getByte(buf.readerIndex() + 2) == ',') {
        return decodeText(channel, remoteAddress, buf.toString(StandardCharsets.US_ASCII).trim());
    } else {
        return decodeBinary(channel, remoteAddress, buf);
    }
}

有人可以解释一下 ByteBuf keep() 方法是如何工作的吗？

谢谢。

Answer 1

来自Netty in action书:

In the case of encoders and decoders,once a message has been encoded or decoded, it will automatically be released by a call to ReferenceCountUtil.release(message). If you need to keep a reference for later use you can call ReferenceCountUtil.retain(message). This increments the reference count, preventing the message from being released.

作为关于引用计数是什么的进一步说明，这将会有所帮助:

Reference counting is a technique for optimizing memory use and performance by releasing the resources held by an object when it is no longer referenced by other objects. A ReferenceCounted implementation instance will normally start out with an active reference count of 1. As long as the reference count is greater than 0, the object is guaranteed not to be released. When the number of active references decreases to 0, the instance will be released. Note that an object that has been released should no longer be available for use.