java - encode()什么都没读，但对一条消息进行了解码

Question

对于套接字服务器应用程序，我创建了一个PacketFragmenter，该程序读取数据包的长度(在数据包的第二个字节中)，然后将数据包发送回管道。

这是我写的代码:

public class PacketFragmenter extends ByteToMessageDecoder {

    @Override
    protected void decode(ChannelHandlerContext ctx, ByteBuf in,
            List<Object> out) throws Exception {
        //I read a byte just to make the reader index go to second byte
        in.readByte();

        //in the second byte i get the content's length
        int length = in.readByte();

        //if my content is smaller than the readableBytes, there's a problem, so i return
        if (in.readableBytes() < length) {
            return;
        }

        //If everything is good, i reset the reader index to be able to write the whole packet in the out buffer (because i need the first byte in next handler, same for the size)
        in.resetReaderIndex();
        //I send my packet to the next handler
        out.add(in.readBytes(length +2));
        //and i reset the rederIndex to be able to read another packet
        in.resetReaderIndex();
    }

}

我在测试中得到了这个堆栈:

io.netty.handler.codec.DecoderException: PacketFragmenter.decode() did not read anything but decoded a message.
    at io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:334)
    at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:229)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:339)
    at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:324)
    at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:847)
    at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:131)
    at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:511)
    at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:468)
    at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:382)
    at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:354)
    at io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:111)
    at io.netty.util.concurrent.DefaultThreadFactory$DefaultRunnableDecorator.run(DefaultThreadFactory.java:137)
    at java.lang.Thread.run(Thread.java:745)

但一切都按预期进行，我连续得到两个数据包，但它们被很好地拼接了，下一个处理程序正在执行他的工作。

所以我不知道我应该处理这个异常还是忽略它？或者也许我可以做一个简单的事情来解决它，我根本不是一个网络专家(一个星期前开始)，所以应该很容易解决。但是我在netty的用户指南上什么也没找到。

Answer 1

我会在Supamiu的答案中添加更多信息。
在许多情况下，将LengthFieldBasedFrameDecoder用作父类是正确的方法，但是没有解释为什么引发异常的原因。

如果产生消息，则还需要从ByteBuf中读取内容。添加此检查是为了捕获由用户解码器错误产生的无限循环。

因此，如果您从原始方法中删除最后一行in.resetReaderIndex();，则异常将消失:

public class PacketFragmenter extends ByteToMessageDecoder {

    @Override
    protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
        in.skipBytes(1);
        //in the second byte i get the content's length
        int length = in.readByte();

        //if my content is smaller than the readableBytes, there's a problem, so i return
        if (in.readableBytes() < length) {
            return;
        }

        //If everything is good, i reset the reader index to be able to write the whole packet in the out buffer (because i need the first byte in next handler, same for the size)
        in.resetReaderIndex();
        //I send my packet to the next handler
        out.add(in.readBytes(length + 2));
    }
}