[JAVA] How to dynamically switch between FIN and RST in Netty

When building a server using Netty, you may want to use FIN and RST properly depending on the situation when closing Channel (socket). For example, there is a requirement that FIN (RST when delayed timeout) is set normally, but RST is set immediately when an abnormality is detected.

Specifying the default behavior when closing

Whether to use FIN / RST when Channel is closed can be specified when building ServerBootstrap.

EventLoopGroup bossGroup = new NioEventLoopGroup(1);
EventLoopGroup workerGroup = new NioEventLoopGroup(0);
ServerBootstrap bootstrap = new ServerBootstrap()
    .option(ChannelOption.SO_BACKLOG, 1024)
    .group(bossGroup, workerGroup)
    .childOption(ChannelOption.SO_LINGER, 10) //FIN → Delayed timeout(10 seconds)→ Specify to close with RST
    .childHandler(new MyChannelInitializer());
// ...


Release by RST that specifies> -1 is disabled.

Select FIN / RST when closing

If you want to close the ServerBootstrap in a way different from the default behavior specified when building it, you can do so by getting the ChannelConfig from the Channel and changing the SO_LINGER setting.

class MyHandler extends SimpleChannelInboundHandler<byte[]> {
  protected void channelRead0(ChannelHandlerContext ctx, byte[] msg) {
    // ...
  public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
    ctx.channel().config().setOption(ChannelOption.SO_LINGER, 0); //Specify to close with RST immediately


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