使用包定长FixedLengthFrameDecoder解决半包粘包

四、使用包定长FixedLengthFrameDecoder解决半包粘包

4.1 试验

由于客户端发给服务器端的是hello server,im a client字符串，该字符串占用24字节，所以在服务器端channelpipeline里面添加一个长度为24的定长解码器和二进制转换为string的解码器：

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然后修改NettyServerHandler的channelRead如下：

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由于服务器发给客户端的是hello client ,im server字符串，该字符串占用23字节，所以在客户端端channelpipeline里面添加一个长度为23的定长解码器和二进制转换为string的解码器：

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然后修改NettyClientHandler的channelRead如下：

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然后重新启动服务器客户端，结果如下：
服务器端结果：

----Server Started----
--- accepted client---
0receive client info: hello server,im a client
send info to client:hello client ,im server
1receive client info: hello server,im a client
send info to client:hello client ,im server
2receive client info: hello server,im a client
send info to client:hello client ,im server
3receive client info: hello server,im a client
send info to client:hello client ,im server
4receive client info: hello server,im a client
send info to client:hello client ,im server
5receive client info: hello server,im a client
send info to client:hello client ,im server
6receive client info: hello server,im a client
send info to client:hello client ,im server
7receive client info: hello server,im a client
send info to client:hello client ,im server
8receive client info: hello server,im a client
send info to client:hello client ,im server
9receive client info: hello server,im a client
send info to client:hello client ,im server

客户端结果：

--- client already connected----
0receive from server:hello client ,im server
1receive from server:hello client ,im server
2receive from server:hello client ,im server
3receive from server:hello client ,im server
4receive from server:hello client ,im server
5receive from server:hello client ,im server
6receive from server:hello client ,im server
7receive from server:hello client ,im server
8receive from server:hello client ,im server
9receive from server:hello client ,im server

可知使用FixedLengthFrameDecoder已经解决了半包粘包问题。

4.2 FixedLengthFrameDecoder的原理

顾名思义是使用包定长方式来解决粘包半包问题，假设服务端接受到下面四个包分片：

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那么使用FixedLengthFrameDecoder(3)会将接受buffer里面的上面数据解码为下面固定长度为3的3个包

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FixedLengthFrameDecoder是继承自 ByteToMessageDecoder类的,当服务器接受buffer数据就绪后会调用ByteToMessageDecoder的channelRead方法进行读取，下面我们从这个函数开始讲解：

public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        //4.2.1
        if (msg instanceof ByteBuf) {
            CodecOutputList out = CodecOutputList.newInstance();
            try {
                ...
                //4.2.2
                callDecode(ctx, cumulation, out);
            } catch (DecoderException e) {
                throw e;
            } catch (Throwable t) {
                throw new DecoderException(t);
            } finally {
                ...
            }
        } else {
            //4.2.3
            ctx.fireChannelRead(msg);
        }
}

如上代码4.2.2具体是方法callDecode进行数据读取的，其代码如下：

protected void callDecode(ChannelHandlerContext ctx, ByteBuf in, List