TCP粘包产生原因

1. 应用程序write写入的字节数大小大于套接字发送缓冲区的大小。
2. 进行MSS大小的TCP分段。
3. 以太网帧的payload大于MTU进行IP分片。

用于解决TCP粘包问题的编码器

1. LineBasedFrameDecoder -- 基于** 行 **的解码器。遍历ByteBuf中的可读字节，判断是否有"\n"或"\r\n"，如有则以此结束位置。可配置单行最大长度，如达到最大长度，仍没有发现换行符，则抛出异常。(见代码片段1)

// 代码片段1
public class TimeServer {
    private class ChildChanneHandler extends ChannelInitiallizer<SocketChannel> {
        @override
        protected void initChannel(SocketChannel arg0) throws Execption { 
            arg0.pipeline().addLast ( new LineBasedFrameDecoder(1024));
            arg0.pipeline().addLast ( new StringDecoder());
            arg0.pipeline().addLast ( new TimeClientHandler());
        }
    }
}

2. StringDecoder -- 基于** 字符串 **的解码器。将接收到的byte[]转换成字符串。LineBasedFrameDecoder+StringDecoder的组合即为按行切换的文本解码器。
3. DelimiterBasedFrameDecoder -- 基于** 分隔符 **作为码流结束标示的消息解码器。(见代码片段2)

// 代码片段2
public class EchoServer {
    private class ChildChanneHandler extends ChannelInitiallizer<SocketChannel> {
        @override
        protected void initChannel(SocketChannel arg0) throws Execption { 
            ByteBuf delimiter = Unpooled.copiedBuffer("$_".getBytes()))
            arg0.pipeline().addLast ( new DelimiterBasedFrameDecoder(1024,delimiter));
            // arg0.pipeline().addLast ( new LineBasedFrameDecoder(1024));
            arg0.pipeline().addLast ( new StringDecoder());
            arg0.pipeline().addLast ( new TimeClientHandler());
        }
    }
}

4. FixedLengthFrameDecoder -- ** 固定长度 **解码器.(见代码片段3)

// 代码片段3
public class EchoServer {
   private class ChildChanneHandler extends ChannelInitiallizer<SocketChannel> {
       @override
       protected void initChannel(SocketChannel arg0) throws Execption { 
           ByteBuf delimiter = Unpooled.copiedBuffer("$_".getBytes()))
           // arg0.pipeline().addLast ( new DelimiterBasedFrameDecoder(1024,delimiter));
           // arg0.pipeline().addLast ( new LineBasedFrameDecoder(1024));
           arg0.pipeline().addLast ( new FixedLengthFrameDecoder(20));
           arg0.pipeline().addLast ( new StringDecoder());
           arg0.pipeline().addLast ( new TimeClientHandler());
       }
   }
}