How can I serve static html from spring boot?

I ran the spring-boot-sample-web-static project from here, made this alteration to the pom

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-tomcat</artifactId>
</dependency>

And added this class to serve a duplicate page index2.html from the same static folder location:

import org.springframework.http.MediaType;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;

@Controller
public class Rester {

    @RequestMapping(value = "/rand", produces = MediaType.APPLICATION_JSON_VALUE)
    @ResponseBody
    private RandomObj jsonEndpoint() {
        return new RandomObj();
    }

    @RequestMapping(value = "/tw")
    public String somePg() {
        return "index2";
    }
}

The json url works fine, but when I try to access localhost:8080/tw I get a blank page, and this error in the console:

2017-02-22 15:37:22.076 ERROR 21494 --- [nio-8080-exec-9] o.s.boot.web.support.ErrorPageFilter     : Cannot forward to error page for request [/tw] as the response has already been committed. As a result, the response may have the wrong status code. If your application is running on WebSphere Application Server you may be able to resolve this problem by setting com.ibm.ws.webcontainer.invokeFlushAfterService to false

Am I doing something wrong?

Static files should be served from resources, not from controller.

Spring Boot will automatically add static web resources located within any of the following directories:

/META-INF/resources/  
/resources/  
/static/  
/public/

refs:
https://spring.io/blog/2013/12/19/serving-static-web-content-with-spring-boot
https://spring.io/guides/gs/serving-web-content/

In Spring boot, /META-INF/resources/, /resources/, static/ and public/ directories are available to serve static contents.

So you can create a static/ or public/ directory under resources/ directory and put your static contents there. And they will be accessible by: http://localhost:8080/your-file.ext. (assuming the server.port is 8080)

You can customize these directories using spring.resources.static-locations in the application.properties.

For example:

spring.resources.static-locations=classpath:/custom/

Now you can use custom/ folder under resources/ to serve static files.

This is also possible using Java config in Spring Boot 2:

@Configuration
public class StaticConfig implements WebMvcConfigurer {

    @Override
    public void addResourceHandlers(ResourceHandlerRegistry registry) {
        registry.addResourceHandler("/static/**").addResourceLocations("classpath:/custom/");
    }
}

This confugration maps contents of custom directory to the http://localhost:8080/static/** url.

I am using :: Spring Boot :: (v2.0.4.RELEASE) with Spring Framework 5

Spring Boot 2.0 requires Java 8 as a minimum version. Many existing APIs have been updated to take advantage of Java 8 features such as: default methods on interfaces, functional callbacks, and new APIs such as javax.time.

By default, Spring Boot serves static content from a directory called /static (or /public or /resources or /META-INF/resources) in the classpath or from the root of the ServletContext. It uses the ResourceHttpRequestHandler from Spring MVC so that you can modify that behavior by adding your own WebMvcConfigurer and overriding the addResourceHandlers method.

By default, resources are mapped on /** and located on /static directory. But you can customize the static loactions programmatically inside our web context configuration class.

@Configuration @EnableWebMvc
public class Static_ResourceHandler implements WebMvcConfigurer {

    @Override
    public void addResourceHandlers(ResourceHandlerRegistry registry) {
        // When overriding default behavior, you need to add default(/) as well as added static paths(/webapp).

        // src/main/resources/static/...
        registry
            //.addResourceHandler("/**") // « /css/myStatic.css
            .addResourceHandler("/static/**") // « /static/css/myStatic.css
            .addResourceLocations("classpath:/static/") // Default Static Loaction
            .setCachePeriod( 3600 )
            .resourceChain(true) // 4.1
            .addResolver(new GzipResourceResolver()) // 4.1
            .addResolver(new PathResourceResolver()); //4.1

        // src/main/resources/templates/static/...
        registry
            .addResourceHandler("/templates/**") // « /templates/style.css
            .addResourceLocations("classpath:/templates/static/");

        // Do not use the src/main/webapp/... directory if your application is packaged as a jar.
        registry
            .addResourceHandler("/webapp/**") // « /webapp/css/style.css
            .addResourceLocations("/");

        // File located on disk
        registry
            .addResourceHandler("/system/files/**")
            .addResourceLocations("file:///D:/");
    }
}

http://localhost:8080/handlerPath/resource-path+name
                    /static         /css/myStatic.css
                    /webapp         /css/style.css
                    /templates      /style.css

In Spring every request will go through the DispatcherServlet. To avoid Static file request through DispatcherServlet(Front contoller) we configure MVC Static content.

As @STEEL said static resources should not go through Controller. Thymleaf is a ViewResolver which takes the view name form controller and adds prefix and suffix to View Layer.