解决方法：启动类加上：

@Bean
public WebServerFactoryCustomizer<ConfigurableWebServerFactory> webServerFactoryCustomizer() {
    //第二种写法：java8 lambda写法
    return (factory -> {
        ErrorPage errorPage404 = new ErrorPage(HttpStatus.NOT_FOUND, "/index.html");
        factory.addErrorPages(errorPage404);
    });
}

package com.example.demo222;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.web.server.ConfigurableWebServerFactory;
import org.springframework.boot.web.server.ErrorPage;
import org.springframework.boot.web.server.WebServerFactoryCustomizer;
import org.springframework.context.annotation.Bean;
import org.springframework.http.HttpStatus;

@SpringBootApplication
public class Demo222Application {

    public static void main(String[] args) {
        SpringApplication.run(Demo222Application.class, args);
    }


    @Bean
    public WebServerFactoryCustomizer<ConfigurableWebServerFactory> webServerFactoryCustomizer() {
        //第二种写法：java8 lambda写法
        return (factory -> {
            ErrorPage errorPage404 = new ErrorPage(HttpStatus.NOT_FOUND, "/index.html");
            factory.addErrorPages(errorPage404);
        });
    }

}