Netty--TCP粘包和拆包

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TCP协议以流的方式进行数据传输，它无法理解其上层协议的数据意义，而是根据TCP缓冲区的大小对数据进行拆分或组装，即上层一个完整的包可能被拆为几个TCP包来发送，或上层几个包会被组合为一个TCP包发送，这就是TCP的粘包和拆包问题。TCP协议按照自己的工作方式工作，包的拆分和组装问题就需要上层应用来解决，比如每次发送定长的数据包，数据包中包含数据的大小或在包结尾处增加标记等方法。

Netty提供了多种解码器如LineBasedFrameDecoder和StringDecoder等来解决这个问题，只需要在pipeline中加入这些解码器即可，如：

--

new ChannelInitializer<SocketChannel>() {
    protected void initChannel(SocketChannel socketChannel) throws Exception {
        socketChannel.pipeline().addLast(new LineBasedFrameDecoder(1024));
        socketChannel.pipeline().addLast(new StringDecoder());
        socketChannel.pipeline().addLast(new TimeClientHandler());
    }
}

--

StringDecoder 作用是将接受到的对象转换为字符串，然后继续调用后面的handler。

LineBasedFrameDecoder 以换号符为结束标记的解码器，它依次遍历ByteBuf中的可读字节，如果碰到 或 则设置为结束。参数为最大长度，如果读到最大长度还是没有换号则会抛出异常： Unexpected exception from downstream : frame length (16) exceeds the allowed maximum (10)

DelimiterBasedFrameDecoder 是以分隔符作为结束标志的解码器，并且解码时会自动去掉分隔符

FixedLengthFrameDecoder 是用于定长消息的解码器。

下面是一个简单的例子，演示几种decoder的用法：

EchoServer

package com.luangeng.netty.echo;

import io.netty.bootstrap.ServerBootstrap;
import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelOption;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;
import io.netty.handler.codec.DelimiterBasedFrameDecoder;
import io.netty.handler.codec.FixedLengthFrameDecoder;
import io.netty.handler.codec.LineBasedFrameDecoder;
import io.netty.handler.codec.string.StringDecoder;
import io.netty.handler.logging.LogLevel;
import io.netty.handler.logging.LoggingHandler;

/**
 * Created by LG on 2017/11/20.
 */
public class EchoServer {
    public static void main(String[] args) {
        new EchoServer().bind(8080);
    }

    public void bind(int port) {
        //配置服务端的线程组，一个用于服务端接收客户端连接，另一个进行SocketChannel的网络读写
        EventLoopGroup bossGroup = new NioEventLoopGroup();
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        try {
            //ServerBootstrap启动NIO服务端的辅助启动类
            ServerBootstrap bootstrap = new ServerBootstrap();
            bootstrap.group(bossGroup, workerGroup)
                    .channel(NioServerSocketChannel.class)
                    .option(ChannelOption.SO_BACKLOG, 100)
                    .handler(new LoggingHandler(LogLevel.INFO))
                    .childHandler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        protected void initChannel(SocketChannel ch) throws Exception {
                            ByteBuf delimiter = Unpooled.copiedBuffer("$".getBytes());
                            //ch.pipeline().addLast(new LineBasedFrameDecoder(1024));
                            ch.pipeline().addLast(new FixedLengthFrameDecoder(25));
                            //ch.pipeline().addLast(new DelimiterBasedFrameDecoder(1024, delimiter));
                            ch.pipeline().addLast(new StringDecoder());
                            ch.pipeline().addLast(new EchoServerHandler());
                        }
                    });
            //绑定端口，sync为同步阻塞方法，等待绑定成功,ChannelFuture用于异步操作的通知回调
            ChannelFuture future = bootstrap.bind(port).sync();
            System.out.println("server started");
            //等待服务端监听端口关闭
            future.channel().closeFuture().sync();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            System.out.println("server shuting down");
            //释放线程资源
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }

}

---

EchoServerHandler

package com.luangeng.netty.echo;

import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;

/**
 * Created by LG on 2017/11/20.
 */
public class EchoServerHandler extends ChannelInboundHandlerAdapter {
    int count = 0;

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        String body = (String)msg;
        System.out.println("The server received("+count+++"): " + body);

        body = body + "$";
        ByteBuf response = Unpooled.copiedBuffer(body.getBytes());
        ctx.writeAndFlush(response);//异步发送
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        System.out.println("Unexpected exception from downstream : " + cause.getMessage());
        ctx.close();
    }

}

---

EchoClient

package com.luangeng.netty.echo;

import io.netty.bootstrap.Bootstrap;
import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelOption;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioSocketChannel;
import io.netty.handler.codec.DelimiterBasedFrameDecoder;
import io.netty.handler.codec.FixedLengthFrameDecoder;
import io.netty.handler.codec.string.StringDecoder;

/**
 * Created by LG on 2017/11/20.
 */
public class EchoClient {

    public static void main(String[] args) {
        new EchoClient().connect("127.0.0.1", 8080);
    }

    public void connect(String host, int port) {
        //配置客户端NIO线程组
        EventLoopGroup group = new NioEventLoopGroup();
        try {
            Bootstrap bootstrap = new Bootstrap();
            bootstrap.group(group).channel(NioSocketChannel.class)
                    .option(ChannelOption.TCP_NODELAY, true)
                    .handler(new ChannelInitializer<SocketChannel>() {
                        protected void initChannel(SocketChannel ch) throws Exception {
                            ByteBuf delimiter = Unpooled.copiedBuffer("$".getBytes());
                            //ch.pipeline().addLast(new LineBasedFrameDecoder(1024));
                            ch.pipeline().addLast(new FixedLengthFrameDecoder(25));
                            //ch.pipeline().addLast(new DelimiterBasedFrameDecoder(1024, delimiter));
                            ch.pipeline().addLast(new StringDecoder());
                            ch.pipeline().addLast(new EchoClientHandler());
                        }
                    });
            //发起异步连接操作，同步等待连接成功
            ChannelFuture future = bootstrap.connect(host, port).sync();
            System.out.println("client started");
            //等待客户端链路关闭
            future.channel().closeFuture().sync();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            System.out.println("client shuting down");
            //释放NIO线程组
            group.shutdownGracefully();
        }
    }
}

---

EchoClientHandler

package com.luangeng.netty.echo;

import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;

/**
 * Created by LG on 2017/11/20.
 */
public class EchoServerHandler extends ChannelInboundHandlerAdapter {
    int count = 0;

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        String body = (String)msg;
        System.out.println("The server received("+count+++"): " + body);

        body = body + "";
        ByteBuf response = Unpooled.copiedBuffer(body.getBytes());
        ctx.writeAndFlush(response);//异步发送
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        System.out.println("Unexpected exception from downstream : " + cause.getMessage());
        ctx.close();
    }

}

---

执行结果：

Server：

2017-11-20 22:21:35,508 INFO [io.netty.handler.logging.LoggingHandler] - [id: 0x9b011f21] REGISTERED
2017-11-20 22:21:35,510 INFO [io.netty.handler.logging.LoggingHandler] - [id: 0x9b011f21] BIND: 0.0.0.0/0.0.0.0:8080
server started
2017-11-20 22:21:35,512 INFO [io.netty.handler.logging.LoggingHandler] - [id: 0x9b011f21, L:/0:0:0:0:0:0:0:0:8080] ACTIVE
2017-11-20 22:21:39,919 INFO [io.netty.handler.logging.LoggingHandler] - [id: 0x9b011f21, L:/0:0:0:0:0:0:0:0:8080] READ: [id: 0x09644a50, L:/127.0.0.1:8080 - R:/127.0.0.1:52039]
2017-11-20 22:21:39,920 INFO [io.netty.handler.logging.LoggingHandler] - [id: 0x9b011f21, L:/0:0:0:0:0:0:0:0:8080] READ COMPLETE
The server received(0): Hi,my name is luangeng
The server received(1): Hi,my name is luangeng
The server received(2): Hi,my name is luangeng
The server received(3): Hi,my name is luangeng
The server received(4): Hi,my name is luangeng
The server received(5): Hi,my name is luangeng
The server received(6): Hi,my name is luangeng
The server received(7): Hi,my name is luangeng
The server received(8): Hi,my name is luangeng
The server received(9): Hi,my name is luangeng

Client：

client started
0 client get: Hi,my name is luangeng
1 client get: Hi,my name is luangeng
2 client get: Hi,my name is luangeng
3 client get: Hi,my name is luangeng
4 client get: Hi,my name is luangeng
5 client get: Hi,my name is luangeng
6 client get: Hi,my name is luangeng
7 client get: Hi,my name is luangeng
8 client get: Hi,my name is luangeng
9 client get: Hi,my name is luangeng

end