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2020
10-09

Netty粘包拆包问题解决方案

TCP黏包拆包

TCP是一个流协议,就是没有界限的一长串二进制数据。TCP作为传输层协议并不不了解上层业务数据的具体含义,它会根据TCP缓冲区的实际情况进行数据包的划分,所以在业务上认为是一个完整的包,可能会被TCP拆分成多个包进行发送,也有可能把多个小的包封装成一个大的数据包发送,这就是所谓的TCP粘包和拆包问题。
怎么解决?

  • • 消息定长度,传输的数据大小固定长度,例如每段的长度固定为100字节,如果不够空位补空格
  • • 在数据包尾部添加特殊分隔符,比如下划线,中划线等
  • • 将消息分为消息头和消息体,消息头中包含表示信息的总长度

Netty提供了多个解码器,可以进行分包的操作,分别是:

  • • LineBasedFrameDecoder (回车换行分包)
  • • DelimiterBasedFrameDecoder(特殊分隔符分包)
  • • FixedLengthFrameDecoder(固定长度报文来分包)
  • • LengthFieldBasedFrameDecoder(自定义长度来分包)

制造粘包和拆包问题

为了验证我们的解码器能够解决这种粘包和拆包带来的问题,首先我们就制造一个这样的问题,以此用来做对比。
服务端:

public static void main(String[] args) {
    EventLoopGroup bossGroup = new NioEventLoopGroup();
    EventLoopGroup workerGroup = new NioEventLoopGroup();
    ServerBootstrap bootstrap = new ServerBootstrap();
    bootstrap.group(bossGroup, workerGroup)
        .channel(NioServerSocketChannel.class)
        .childHandler(new ChannelInitializer<SocketChannel>() { 
          @Override
          public void initChannel(SocketChannel ch) throws Exception {
            ch.pipeline().addLast("decoder", new StringDecoder());
            ch.pipeline().addLast("encoder", new StringEncoder());
            ch.pipeline().addLast(new ChannelInboundHandlerAdapter() {
              @Override
              public void channelRead(ChannelHandlerContext ctx, Object msg) {
                System.err.println("server:" + msg.toString());
                ctx.writeAndFlush(msg.toString() + "你好" );
              }
            });
          }
        })
        .option(ChannelOption.SO_BACKLOG, 128)
        .childOption(ChannelOption.SO_KEEPALIVE, true);
    try {
      ChannelFuture f = bootstrap.bind(2222).sync();
       f.channel().closeFuture().sync();
    } catch (InterruptedException e) {
      e.printStackTrace();
    } finally {
      workerGroup.shutdownGracefully();
      bossGroup.shutdownGracefully();
    }
  }

客户端我们发送一个比较长的字符串,如果服务端收到的消息是一条,那么就是对的,如果是多条,那么就有问题了。

public static void main(String[] args) {
    EventLoopGroup workerGroup = new NioEventLoopGroup();
    Channel channel = null;
    try {
      Bootstrap b = new Bootstrap();
      b.group(workerGroup);
      b.channel(NioSocketChannel.class);
      b.option(ChannelOption.SO_KEEPALIVE, true);
      b.handler(new ChannelInitializer<SocketChannel>() {
        @Override
        public void initChannel(SocketChannel ch) throws Exception {
          ch.pipeline().addLast("decoder", new StringDecoder());
          ch.pipeline().addLast("encoder", new StringEncoder());
          ch.pipeline().addLast(new ChannelInboundHandlerAdapter() {
            @Override
            public void channelRead(ChannelHandlerContext ctx, Object msg) {
              System.err.println("client:" + msg.toString());
            }
          });
        }
      });
      ChannelFuture f = b.connect("127.0.0.1", 2222).sync();
      channel = f.channel();
      StringBuilder msg = new StringBuilder();
      for (int i = 0; i < 100; i++) {
        msg.append("hello yinjihuan");
      }
      channel.writeAndFlush(msg);
    } catch(Exception e) {
      e.printStackTrace();
    }
  }

首先启动服务端,然后再启动客户端,通过控制台可以看到服务接收的数据分成了2次,这就是我们要解决的问题。

server:hello yinjihuanhello....
server:o yinjihuanhello...

LineBasedFrameDecoder

用LineBasedFrameDecoder 来解决需要在发送的数据结尾加上回车换行符,这样LineBasedFrameDecoder 才知道这段数据有没有读取完整。

改造服务端代码,只需加上LineBasedFrameDecoder 解码器即可,构造函数的参数是数据包的最大长度。

 public void initChannel(SocketChannel ch) throws Exception {
   ch.pipeline().addLast(new LineBasedFrameDecoder(10240));
   ch.pipeline().addLast("decoder", new StringDecoder());
   ch.pipeline().addLast("encoder", new StringEncoder());
   ch.pipeline().addLast(new ChannelInboundHandlerAdapter() {
      @Override
      public void channelRead(ChannelHandlerContext ctx, Object msg) {
        System.err.println("server:" + msg.toString());
        ctx.writeAndFlush(msg.toString() + "你好");
      }
   });
}

改造客户端发送代码,再数据后面加上回车换行符

ChannelFuture f = b.connect("127.0.0.1", 2222).sync();
channel = f.channel();
StringBuilder msg = new StringBuilder();
for (int i = 0; i < 100; i++) {
  msg.append("hello yinjihuan");
}
channel.writeAndFlush(msg + System.getProperty("line.separator"));

DelimiterBasedFrameDecoder

DelimiterBasedFrameDecoder和LineBasedFrameDecoder差不多,DelimiterBasedFrameDecoder可以自己定义需要分割的符号,比如下划线,中划线等等。
改造服务端代码,只需加上DelimiterBasedFrameDecoder解码器即可,构造函数的参数是数据包的最大长度。我们用下划线来分割。

public void initChannel(SocketChannel ch) throws Exception {
   ch.pipeline().addLast(new DelimiterBasedFrameDecoder(10240, Unpooled.copiedBuffer("_".getBytes())));
   ch.pipeline().addLast("decoder", new StringDecoder());
   ch.pipeline().addLast("encoder", new StringEncoder());
   ch.pipeline().addLast(new ChannelInboundHandlerAdapter() {
      @Override
      public void channelRead(ChannelHandlerContext ctx, Object msg) {
        System.err.println("server:" + msg.toString());
        ctx.writeAndFlush(msg.toString() + "你好");
      }
   });
}

改造客户端发送代码,再数据后面加上下划线

ChannelFuture f = b.connect("127.0.0.1", 2222).sync();
channel = f.channel();
StringBuilder msg = new StringBuilder();
for (int i = 0; i < 100; i++) {
  msg.append("hello yinjihuan");
}
channel.writeAndFlush(msg + "_");

FixedLengthFrameDecoder

FixedLengthFrameDecoder是按固定的数据长度来进行解码的,也就是说你客户端发送的每条消息的长度是固定的,下面我们看看怎么使用。

服务端还是一样,增加FixedLengthFrameDecoder解码器即可。

 public void initChannel(SocketChannel ch) throws Exception {
   ch.pipeline().addLast(new FixedLengthFrameDecoder(1500));
   ch.pipeline().addLast("decoder", new StringDecoder());
   ch.pipeline().addLast("encoder", new StringEncoder());
   ch.pipeline().addLast(new ChannelInboundHandlerAdapter() {
      @Override
      public void channelRead(ChannelHandlerContext ctx, Object msg) {
        System.err.println("server:" + msg.toString());
        ctx.writeAndFlush(msg.toString() + "你好");
      }
   });
}

客户端,msg输出的长度就是1500

ChannelFuture f = b.connect("127.0.0.1", 2222).sync();
channel = f.channel();
StringBuilder msg = new StringBuilder();
for (int i = 0; i < 100; i++) {
  msg.append("hello yinjihuan");
}
System.out.println(msg.length());
channel.writeAndFlush(msg);

服务端代码:

public void initChannel(SocketChannel ch) throws Exception {
   ch.pipeline().addLast("frameDecoder", new LengthFieldBasedFrameDecoder(Integer.MAX_VALUE, 0, 4, 0, 4));
   ch.pipeline().addLast("frameEncoder", new LengthFieldPrepender(4));
   ch.pipeline().addLast("decoder", new StringDecoder());
   ch.pipeline().addLast("encoder", new StringEncoder());
   ch.pipeline().addLast(new ChannelInboundHandlerAdapter() {
      @Override
      public void channelRead(ChannelHandlerContext ctx, Object msg) {
        System.err.println("server:" + msg.toString());
        ctx.writeAndFlush(msg.toString() + "你好");
      }
   });
}

客户端,直接发送就行

ChannelFuture f = b.connect("127.0.0.1", 2222).sync();
channel = f.channel();![](https://s4.51cto.com/images/blog/202008/04/fb05cdb6bd8458bd1006a127ff9d12dc.png?x-oss-process=image/watermark,size_16,text_QDUxQ1RP5Y2a5a6i,color_FFFFFF,t_100,g_se,x_10,y_10,shadow_90,type_ZmFuZ3poZW5naGVpdGk=)
StringBuilder msg = new StringBuilder();
for (int i = 0; i < 100; i++) {
  msg.append("hello yinjihuan");
}
channel.writeAndFlush(msg);

源码参考:https://github.com/yinjihuan/netty-im

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