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在实现TCP长连接功能中，客户端断线重连是一个常见问题。当我们使用Netty实现断线重连时，是否考虑过以下几个关键问题呢？这些都是笔者在实践中遇到的挑战，也是思考的起点。

服务端及公共依赖

首先，让我们看一下服务端的基本配置。以下是项目的主要依赖项：

       
        
            
     
      org.springframework.boot
             
     
      spring-boot-starter
             
     
      2.4.1
         
        
        
            
     
      io.netty
             
     
      netty-all
             
     
      4.1.56.Final
         
        
        
            
     
      org.jboss.marshalling
             
     
      jboss-marshalling-serial
             
     
      2.0.10.Final
         
    
   

服务端业务处理

服务端主要负责处理客户端连接和数据传输。以下是服务端的核心业务处理类：

@ChannelHandler.Sharablepublic class SimpleServerHandler extends ChannelInboundHandlerAdapter {    private static final InternalLogger log = InternalLoggerFactory.getInstance(SimpleServerHandler.class);    private static final ChannelGroup channels = new DefaultChannelGroup(GlobalEventExecutor.INSTANCE);    @Override    public void channelActive(ChannelHandlerContext ctx) throws Exception {        channels.add(ctx.channel());        log.info("客户端连接成功：地址：{}", ctx.channel().remoteAddress());        log.info("当前共有 {} 个客户端连接", channels.size());    }    @Override    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {        log.info("server channelRead: {}", msg);        ctx.channel().writeAndFlush("hello netty");    }    @Override    public void channelInactive(ChannelHandlerContext ctx) throws Exception {        log.info("channelInactive：客户端关闭");    }    @Override    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {        if (cause instanceof java.io.IOException) {            log.warn("exceptionCaught：客户端关闭");        } else {            cause.printStackTrace();        }    }}

服务端心跳检查

为了确保客户端在线状态，我们在服务端启用了心跳机制。以下是心跳处理类：

public class ServerHeartbeatHandler extends ChannelInboundHandlerAdapter {    private static final InternalLogger log = InternalLoggerFactory.getInstance(ServerHeartbeatHandler.class);    @Override    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {        log.info("server channelRead: {}", msg);        if (msg.equals("ping")) {            ctx.channel().writeAndFlush("pong");        } else {            ctx.fireChannelRead(msg);        }    }    @Override    public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception {        if (evt instanceof IdleStateEvent) {            IdleStateEvent idleStateEvent = (IdleStateEvent) evt;            if (idleStateEvent.state() == IdleState.READER_IDLE) {                log.info("超过心跳时间，关闭与服务端的连接：{}", ctx.channel().remoteAddress());                ctx.channel().close();            }        } else {            super.userEventTriggered(ctx, evt);        }    }}

编解码工具类

我们使用Jboss Marshalling进行编解码。以下是自定义编解码工具类：

public final class MarshallingCodeFactory {    /** 创建Jboss marshalling解码器 */    public static MarshallingDecoder buildMarshallingDecoder() {        MarshallerFactory factory = Marshalling.getProvidedMarshallingFactory("serial");        MarshallingConfiguration configuration = new MarshallingConfiguration();        configuration.setVersion(5);        DefaultUnmarshallerProvider provider = new DefaultUnmarshallerProvider(factory, configuration);        return new MarshallingDecoder(provider, 1024);    }    /** 创建Jboss marshalling编码器 */    public static MarshallingEncoder buildMarshallingEncoder() {        MarshallerFactory factory = Marshalling.getProvidedMarshallingFactory("serial");        MarshallingConfiguration configuration = new MarshallingConfiguration();        configuration.setVersion(5);        DefaultMarshallerProvider provider = new DefaultMarshallerProvider(factory, configuration);        return new MarshallingEncoder(provider);    }}

公共实体类

以下是用于数据传输的实体类：

public class UserInfo implements Serializable {    private static final long serialVersionUID = 6271330872494117382L;    private String username;    private int age;    public UserInfo() {    }    public UserInfo(String username, int age) {        this.username = username;        this.age = age;    }    //省略getter、setter、toString等方法}

客户端实现

客户端实现需要处理连接和断线重连。以下是客户端的核心代码：

public class SimpleClientHandler extends ChannelInboundHandlerAdapter {    private static final InternalLogger log = InternalLoggerFactory.getInstance(SimpleClientHandler.class);    private NettyClient client;    public SimpleClientHandler(NettyClient client) {        this.client = client;    }    @Override    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {        log.info("client receive: {}", msg);    }}

断线重连实现

在实际应用中，客户端断线重连涉及以下关键问题：

总结

通过上述实现，我们可以看到Netty在断线重连中的关键作用。服务端需要处理连接状态和心跳机制，而客户端则需要实现自动重连功能。线程配置也是一个重要考虑因素，需要根据实际需求进行优化。

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