Netty 学习笔记（七）、Netty 如何解决 TCP 粘包/拆包问题（二）

概述

上篇博客我简单介绍了粘包、拆包问题出现的原因以及 Netty 如何解决该问题，在博客最后我给出通过换行符分割数据包解决粘包问题的方案。然而在实际业务场景中，数据包可能不适用换行符分割这种方式，因为实际数据中可能包含换行符。本篇博客我就来简单补充两种更完整的解决方案：

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DelimiterBasedFrameDecoder

DelimiterBasedFrameDecoder 类可以自定义分割符，一般在代码中这样使用：

ByteBuf buf = Unpooled.copiedBuffer("$_".getBytes());
DelimiterBasedFrameDecoder decoder = new DelimiterBasedFrameDecoder(1024, buf);

上述代码就创建一种以 “$_” 为分隔符的缓冲池处理器，其中它的作用机制和 LineBasedFrameDecoder 类似，都是遍历 ByteBuf 中可读字节，根据关键字分割 TCP 包。其中1024表示最大长度，如果超过最大长度还没有出现分隔符就报错。

下面我们看一组具体示例，这里我们使用 Echo 服务器，其中我只列出核心代码：

Echo 服务端：

bootstrap.group(bossGroup, workerGroup)
        .channel(NioServerSocketChannel.class)
        .option(ChannelOption.SO_BACKLOG, 1024)
        .childHandler(new ChannelInitializer<SocketChannel>() {
            @Override
            protected void initChannel(SocketChannel socketChannel) throws Exception {
                ByteBuf buf = Unpooled.copiedBuffer("$_".getBytes());
                socketChannel.pipeline().addLast(
                        new DelimiterBasedFrameDecoder(1024, buf)
                );
                socketChannel.pipeline().addLast(new StringDecoder());
                socketChannel.pipeline().addLast(new EchoServerHandler());
            }
        });

@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
    String body = (String) msg;
    System.out.println("Time Server Receive message：[" + body + "] , and counter =  " + ++counter);
    String result = body + "$_";
    ByteBuf buf = Unpooled.copiedBuffer(result.getBytes());
    ctx.writeAndFlush(buf);
}

Echo 客户端：

bootstrap.group(group)
        .channel(NioSocketChannel.class)
        .option(ChannelOption.TCP_NODELAY, true)
        .handler(new ChannelInitializer<SocketChannel>() {
            @Override
            protected void initChannel(SocketChannel socketChannel) throws Exception {
                ByteBuf buf = Unpooled.copiedBuffer("$_".getBytes());
                socketChannel.pipeline().addLast(
                        new DelimiterBasedFrameDecoder(1024, buf)
                );
                socketChannel.pipeline().addLast(new StringDecoder());
                socketChannel.pipeline().addLast(new EchoClientHandler());
            }
        });

private byte[] bytes = "Hi Netty！$_".getBytes();
 
@Override
public void channelActive(ChannelHandlerContext ctx) throws Exception {
    ByteBuf buf = null;
    for (int i = 0; i < 10; i++) {
        buf = Unpooled.buffer(bytes.length);
        buf.writeBytes(bytes);
        ctx.writeAndFlush(buf);
    }
}

@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
    String message = (String) msg;
    System.out.println("Time Client Receive message：[" + message + "] , and counter =  " + ++counter);
}

执行结果：

服务端：
Time Server Receive message：[Hi Netty！] , and counter =  1
Time Server Receive message：[Hi Netty！] , and counter =  2
...
Time Server Receive message：[Hi Netty！] , and counter =  10

客户端:
Time Client Receive message：[Hi Netty！] , and counter =  1
Time Client Receive message：[Hi Netty！] , and counter =  2
...
Time Client Receive message：[Hi Netty！] , and counter =  10

从执行结果就可以看出，TCP 消息包在缓冲区被 DelimiterBasedFrameDecoder 按我们预设的关键词分割为多个数据包。

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FixedLengthFrameDecoder

FixedLengthFrameDecoder 类一般用作定长数据包，其中它在代码中一般这样使用：

FixedLengthFrameDecoder decoder = new FixedLengthFrameDecoder(20);

上述代码就创建了一个按20字节长度为一数据包的处理器，其中它的机制和 DelimiterBasedFrameDecoder 类似。

下面我们具体看示例，同样采用 Echo 服务器，只列出核心代码：

Echo 服务端：

bootstrap.group(bossGroup, workerGroup)
        .channel(NioServerSocketChannel.class)
        .option(ChannelOption.SO_BACKLOG, 1024)
        .childHandler(new ChannelInitializer<SocketChannel>() {
            @Override
            protected void initChannel(SocketChannel socketChannel) throws Exception {
                ByteBuf buf = Unpooled.copiedBuffer("$_".getBytes());
                socketChannel.pipeline().addLast(
                        new FixedLengthFrameDecoder(20)
                );
                socketChannel.pipeline().addLast(new StringDecoder());
                socketChannel.pipeline().addLast(new EchoServerHandler());
            }
        });
        
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
    String body = (String) msg;
    System.out.println("Time Server Receive message：[" + body + "] , and counter =  " + ++counter);
    ByteBuf buf = Unpooled.copiedBuffer(body.getBytes());
    ctx.writeAndFlush(buf);
}

Echo 客户端：

 bootstrap.group(group)
         .channel(NioSocketChannel.class)
         .option(ChannelOption.TCP_NODELAY, true)
         .handler(new ChannelInitializer<SocketChannel>() {
             @Override
             protected void initChannel(SocketChannel socketChannel) throws Exception {
                 ByteBuf buf = Unpooled.copiedBuffer("$_".getBytes());
                 socketChannel.pipeline().addLast(
                         new FixedLengthFrameDecoder(20)
                 );
                 socketChannel.pipeline().addLast(new StringDecoder());
                 socketChannel.pipeline().addLast(new EchoClientHandler());
             }
         });

private byte[] bytes = "Hello Netty !!!!!!!!".getBytes();         

@Override
public void channelActive(ChannelHandlerContext ctx) throws Exception {
    ByteBuf buf = null;
    for (int i = 0; i < 10; i++) {
        buf = Unpooled.buffer(bytes.length);
        buf.writeBytes(bytes);
        ctx.writeAndFlush(buf);
    }
}

@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
    String message = (String) msg;
    System.out.println("Time Client Receive message：[" + message + "] , and counter =  " + ++counter);
}

** 执行结果**：

服务端：
Time Server Receive message：[Hello Netty !!!!!!!!] , and counter =  1
Time Server Receive message：[Hello Netty !!!!!!!!] , and counter =  2
...
Time Server Receive message：[Hello Netty !!!!!!!!] , and counter =  10

客户端：
Time Client Receive message：[Hello Netty !!!!!!!!] , and counter =  1
Time Client Receive message：[Hello Netty !!!!!!!!] , and counter =  2
...
Time Client Receive message：[Hello Netty !!!!!!!!] , and counter =  10

上述示例中，我们每个数据包长度固定20字节，和 FixedLengthFrameDecoder 处理器配置长度相同，这样所有的数据包都可以完整分割，从执行结果来看也没有出现任何问题。

至此，Netty 常用的解决 TCP 粘包、拆包问题的 API 介绍完毕。

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参考

《Netty 权威指南》