TCP粘包/拆包--利用FixedLengthFrameDecoder解决TCP粘包问题

前面我们介绍了利用LineBasedFrameDecoder和DelemiterBasedFrameDecoder解决TCP的粘包/拆包的问题，

现在我们继续介绍Netty的最后一种解码器--FixedLengthFrameDecoder。

利用FixedLengthFrameDecoder解码器，无论一次接收到多少数据报，它都会按照构造函数中设置的固定长度进行解码，如果是半包消息，FixedLengthFrameDecoder会缓存半包消息并等待下个包到达后进行拼包，直到读取到一个完整的包。

1. EchoServer.java

import io.netty.bootstrap.ServerBootstrap;
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.FixedLengthFrameDecoder;
import io.netty.handler.codec.string.StringDecoder;


public class EchoServer {
    private final static int port = 8080;

    public static void main(String[] args) {
        start();
    }

    private static void start() {
        final EchoServerHandler serverHandler = new EchoServerHandler();
        // 创建EventLoopGroup
        EventLoopGroup bossGroup = new NioEventLoopGroup();
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        // 创建EventLoopGroup
        ServerBootstrap b = new ServerBootstrap();
        b.group(bossGroup, workerGroup)
        .channel(NioServerSocketChannel.class)
        .option(ChannelOption.SO_BACKLOG, 100)
        .childHandler(new ChannelInitializer() {
            @Override
            protected void initChannel(SocketChannel socketChannel) throws Exception {
            socketChannel.pipeline().addLast(
                    new FixedLengthFrameDecoder(20));
            socketChannel.pipeline().addLast(new StringDecoder());
            socketChannel.pipeline().addLast(serverHandler);
            }
        });

        try {
            // 异步地绑定服务器;调用sync方法阻塞等待直到绑定完成
            ChannelFuture f = b.bind(port).sync();
            // 获取Channel的CloseFuture，并且阻塞当前线程直到它完成
            f.channel().closeFuture().sync();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            // 优雅的关闭EventLoopGroup，释放所有的资源
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }
}

2. EchoServerHandler.java

import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandler.Sharable;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;
@Sharable
public class EchoServerHandler extends ChannelInboundHandlerAdapter{
    private int counter = 0;
    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        String body = (String )msg;
        System.out.println(
                "This is " + ++counter +" times receive client [" + body +"]"
        );

        ByteBuf resp = Unpooled.copiedBuffer(body.getBytes());
        ctx.writeAndFlush(resp);
    }

    /**
     * 异常处理
     * @param ctx
     * @param cause
     * @throws Exception
     */
    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        //打印异常栈跟踪
        cause.printStackTrace();

        // 关闭该Channel
        ctx.close();
    }
}

3. EchoClient.java

import io.netty.bootstrap.Bootstrap;
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.FixedLengthFrameDecoder;
import io.netty.handler.codec.string.StringDecoder;

import java.net.InetSocketAddress;

public class EchoClient {
    private final static String HOST = "localhost";
    private final static int PORT = 8080;

    public static void start() {
        EventLoopGroup group = new NioEventLoopGroup();
        Bootstrap bootstrap = new Bootstrap();
        bootstrap.group(group)
                .channel(NioSocketChannel.class)
                .option(ChannelOption.TCP_NODELAY, true)
                .handler(new ChannelInitializer() {
                    @Override
                    protected void initChannel(SocketChannel socketChannel) throws Exception {
                        socketChannel.pipeline().addLast(new FixedLengthFrameDecoder(20));
                        socketChannel.pipeline().addLast(new StringDecoder());
                        socketChannel.pipeline().addLast(new EchoClientHandler());
                    }
                });
        try {
            ChannelFuture f = bootstrap.connect(HOST,PORT).sync();
            f.channel().closeFuture().sync();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }finally {
            group.shutdownGracefully();
        }
    }

    public static void main(String[] args) {
        start();
    }
}

4. EchoClientHandler.java

import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandler.Sharable;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;

@Sharable
public class EchoClientHandler extends ChannelInboundHandlerAdapter {

    private int counter;
    static final String ECHO_REQ = "Hi, Welcome to Netty World!";

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        String body = (String) msg;
        System.out.println(
                "This is "+ ++counter + " times receive server:[" + body +"]"
        );
    }

    @Override
    public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
        ctx.flush();
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        cause.printStackTrace();
        ctx.close();
    }

    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        for (int i = 0; i < 10; i++) {
            ctx.writeAndFlush(Unpooled.copiedBuffer(ECHO_REQ.getBytes()));
        }
    }
}

分别运行EchoServer和EchoClient，控制台的运行结果如下

服务端运行结果：

This is 1 times receive client [Hi, Welcome to Netty]
This is 2 times receive client [ World!Hi, Welcome t]
This is 3 times receive client [o Netty World!Hi, We]
This is 4 times receive client [lcome to Netty World]
This is 5 times receive client [!Hi, Welcome to Nett]
This is 6 times receive client [y World!Hi, Welcome ]
This is 7 times receive client [to Netty World!Hi, W]
This is 8 times receive client [elcome to Netty Worl]
This is 9 times receive client [d!Hi, Welcome to Net]
This is 10 times receive client [ty World!Hi, Welcome]
This is 11 times receive client [ to Netty World!Hi, ]
This is 12 times receive client [Welcome to Netty Wor]
This is 13 times receive client [ld!Hi, Welcome to Ne]

客户端运行结果：

This is 1 times receive server:[Hi, Welcome to Netty]
This is 2 times receive server:[ World!Hi, Welcome t]
This is 3 times receive server:[o Netty World!Hi, We]
This is 4 times receive server:[lcome to Netty World]
This is 5 times receive server:[!Hi, Welcome to Nett]
This is 6 times receive server:[y World!Hi, Welcome ]
This is 7 times receive server:[to Netty World!Hi, W]
This is 8 times receive server:[elcome to Netty Worl]
This is 9 times receive server:[d!Hi, Welcome to Net]
This is 10 times receive server:[ty World!Hi, Welcome]
This is 11 times receive server:[ to Netty World!Hi, ]
This is 12 times receive server:[Welcome to Netty Wor]
This is 13 times receive server:[ld!Hi, Welcome to Ne]

节选自《Netty权威指南第二版》， 代码部分有部分改动。