Java Spring Boot: How to map my app root (“/”) to index.html?

It would have worked out of the box if you hadn't used @EnableWebMvc annotation. When you do that you switch off all the things that Spring Boot does for you in WebMvcAutoConfiguration. You could remove that annotation, or you could add back the view controller that you switched off:

@Override
public void addViewControllers(ViewControllerRegistry registry) {
    registry.addViewController("/").setViewName("forward:/index.html");
}

An example of Dave Syer's answer:

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.servlet.config.annotation.ViewControllerRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurerAdapter;

@Configuration
public class MyWebMvcConfig {

    @Bean
    public WebMvcConfigurerAdapter forwardToIndex() {
        return new WebMvcConfigurerAdapter() {
            @Override
            public void addViewControllers(ViewControllerRegistry registry) {
                // forward requests to /admin and /user to their index.html
                registry.addViewController("/admin").setViewName(
                        "forward:/admin/index.html");
                registry.addViewController("/user").setViewName(
                        "forward:/user/index.html");
            }
        };
    }

}

if it is a Spring boot App.

Spring Boot automatically detects index.html in public/static/webapp folder. If you have written any controller @Requestmapping("/") it will override the default feature and it will not show the index.html unless you type localhost:8080/index.html

@Configuration  
@EnableWebMvc  
public class WebAppConfig extends WebMvcConfigurerAdapter {  

    @Override
    public void addViewControllers(ViewControllerRegistry registry) {
        registry.addRedirectViewController("/", "index.html");
    }

}

Update: Jan-2019

First create public folder under resources and create index.html file. Use WebMvcConfigurer instead of WebMvcConfigurerAdapter.

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

@Configuration
public class WebAppConfig implements WebMvcConfigurer {

    @Override
    public void addViewControllers(ViewControllerRegistry registry) {
        registry.addViewController("/").setViewName("forward:/index.html");
    }

}