Netty 进阶之路笔记 (一)

Netty服务端意外退出

案例一

服务端程序在bind()方法阻塞执行完后直接退出

public static void main(string[] args){
  ServerBootstrap serverBootstrap = new ServerBootstrap();
  EventLoopGroup bossGroup = new NioEventLoopGroup(1);
  EventLoopGroup workerGroup = new NioEventLoopGroup(8);
  try{
      serverBootstrap
        .channel(NioServerSocketChannel.class)
        .group(bossGroup, workerGroup)
        .childHandler(new RpcServerInitializer(60L, this.beanRegistry))
        .option(ChannelOption.SO_BACKLOG, 128);
  serverBootstrap.bind(serverAddress, serverPort).sync();
  }finally{
    bossGroup.shutDownGracefully();
    workerGroup.shutDownGracefully();
  }
}

原因：在服务端进行绑定后，无论如何都将执行finally代码块中的方法，将EventLoopGroup关闭掉，此时主线程也执行完退出，EventLoopGroup作为用户进程也被关闭，导致服务端bind到指定端口后直接退出

案例二

当添加服务端channel的closeFuture之后仍然出现服务端退出现象

public static void main(string[] args){
  ServerBootstrap serverBootstrap = new ServerBootstrap();
  EventLoopGroup bossGroup = new NioEventLoopGroup(1);
  EventLoopGroup workerGroup = new NioEventLoopGroup(8);
  try{
      serverBootstrap
        .channel(NioServerSocketChannel.class)
        .group(bossGroup, workerGroup)
        .childHandler(new RpcServerInitializer(60L, this.beanRegistry))
        .option(ChannelOption.SO_BACKLOG, 128);
      Channel channel = serverBootstrap.bind(serverAddress, serverPort).sync().channel();
      channel.closeFuture().addListener(ChannelFutureListener.CLOSE);
  }finally{
    bossGroup.shutDownGracefully();
    workerGroup.shutDownGracefully();
  }
}

原因： channel.closeFuture().addListener(ChannelFutureListener.CLOSE)执行过程完全是异步的，只是在channel关闭时回掉ChannelFutureListener进行相关操作，并不会因此阻塞在这行代码，所以相当于直接进入finally代码块，关闭了NioEventLoopGroup，没有了用户进程，服务端就直接退出了。

避免方法

方案一

让主线程在服务端channel的closeFuture上进行阻塞直到服务端channel关闭

public static void main(string[] args){
  ServerBootstrap serverBootstrap = new ServerBootstrap();
  EventLoopGroup bossGroup = new NioEventLoopGroup(1);
  EventLoopGroup workerGroup = new NioEventLoopGroup(8);
  try{
      serverBootstrap
        .channel(NioServerSocketChannel.class)
        .group(bossGroup, workerGroup)
        .childHandler(new RpcServerInitializer(60L, this.beanRegistry))
        .option(ChannelOption.SO_BACKLOG, 128);
      Channel channel = serverBootstrap.bind(serverAddress, serverPort).sync().channel();
      channel.closeFuture().sync().addListener(ChannelFutureListener.CLOSE);
  }finally{
    bossGroup.shutDownGracefully();
    workerGroup.shutDownGracefully();
  }
}

方案二（推荐）

使用JDK的ShutdownHook进行实现，删除掉finally块的NioEventLoopGroup的关闭代码，将其移动到ShutdownHook的关闭线程中。

public static void main(string[] args){
   ServerBootstrap serverBootstrap = new ServerBootstrap();
   EventLoopGroup bossGroup = new NioEventLoopGroup(1);
   EventLoopGroup workerGroup = new NioEventLoopGroup(8);
   serverBootstrap
      .channel(NioServerSocketChannel.class)
      .group(bossGroup, workerGroup)
      .childHandler(new RpcServerInitializer(60L, this.beanRegistry))
      .option(ChannelOption.SO_BACKLOG, 128);
   serverBootstrap.bind(serverAddress, serverPort).sync();
      Runtime.getRuntime().addShutdownHook(new Thread(()->{
          bossGroup.shutDownGracefully();
                  workerGroup.shutDownGracefully();
      }, "server-killer"));
}

方案三（推荐）

除了ShutdownHook还有通过监听信号量注册signalHanlder的方式进行优雅退出，但是需要判断不同操作系统的中断信号类型，如果是windows系统，则是SIGINT，否则选择SIGTERM信号，然后进行回掉注册：

Signal.handle(sig,shutdownHandler)