default page +load +onstartup +springboot

@Configuration
public class DefaultView extends WebMvcConfigurerAdapter{

    @Override
    public void addViewControllers( ViewControllerRegistry registry ) {
        registry.addViewController( "/" ).setViewName( "forward:/yourpage.html" );
        registry.setOrder( Ordered.HIGHEST_PRECEDENCE );
        super.addViewControllers( registry );
    }
}

package org.gwtproject.tutorial.configuration;

import org.springframework.context.annotation.Configuration;
import org.springframework.core.Ordered;
import org.springframework.web.servlet.config.annotation.ViewControllerRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurerAdapter;

/**
 * Configure the welcome page 
 * 
 */
@Configuration
public class SpringBootWelcomePageConfiguration extends WebMvcConfigurerAdapter implements WebMvcConfigurer {

    /**
     * redirect a user to the welcome page when he visits tha app without a
     * destination url.
     */
    @Override
    public void addViewControllers(ViewControllerRegistry registry) {
        registry.addViewController("/").setViewName("forward:/ForExampleAGwtEntrypoint.html");
        registry.setOrder(Ordered.HIGHEST_PRECEDENCE);
        super.addViewControllers(registry);
    }
}