Apparantly the library integrates natively only with spring-boot applications like you mentioned in your comment. If you want to use spring, it's possible but the integration details aren't documented, because it really depends on the version/module and the nature of you spring application.

You can check the FAQ to see if it answers your questions.

There are some more answers here on SO.

Your controller should like below:

@RestController
public class DefaultController implements ErrorController {

    @Override
    public String getErrorPath() {
        return "/error";
    }

    @RequestMapping("/error")
    public void handleErrorWithRedirect(HttpServletResponse response) throws IOException {
        response.sendRedirect("/swagger-ui.html");
    }

    @RequestMapping(value = "/")
    public void redirect(HttpServletResponse response) throws IOException {
        response.sendRedirect("/swagger-ui.html");
    }

}

I also have put together a working model for you in my github spring-boot project.
For default/index page or error page, it will always redirect to swagger-ui.html.
Let me know if you still have questions.

Statement : Generate Swagger UI for the listing of all the REST APIs through Spring Boot Application.

Follow the below steps to generate the Swagger UI through Spring Boot application:

1. Add following dependency in pom.xml –

    <dependency>
        <groupId>io.springfox</groupId>
        <artifactId>springfox-swagger2</artifactId>
        <version>2.6.1</version>
    </dependency>

    <dependency>
        <groupId>io.springfox</groupId>
        <artifactId>springfox-swagger-ui</artifactId>
        <version>2.6.1</version>
    </dependency>

2. Add the following piece of code in your main application class having the @EnableSwagger2 annotation.

    @EnableSwagger2
    @SpringBootApplication
    public class MyApp {
    public static void main(String[] args) {
        SpringApplication.run(MyApp.class, args);
    }

    @Bean
    public Docket api() {
        return new Docket(DocumentationType.SWAGGER_2).select()  
           .apis(RequestHandlerSelectors.withClassAnnotation(Api.class))
           .paths(PathSelectors.any()).build().pathMapping("/")
           .apiInfo(apiInfo()).useDefaultResponseMessages(false);
    }

    @Bean
    public ApiInfo apiInfo() {
        final ApiInfoBuilder builder = new ApiInfoBuilder();
        builder.title("My Application API through Swagger UI").version("1.0").license("(C) Copyright Test")
        .description("List of all the APIs of My Application App through Swagger UI");
        return builder.build();
        }
    }

3. Add the below RootController class in your code to redirect to the Swagger UI page. In this way, you don’t need to put the dist folder of Swagger-UI in your resources directory.

    import org.springframework.stereotype.Controller;
    import org.springframework.web.bind.annotation.RequestMapping;
    import org.springframework.web.bind.annotation.RequestMethod;
    @Controller
    @RequestMapping("/")
    public class RootController {
        @RequestMapping(method = RequestMethod.GET)
        public String swaggerUi() {
            return "redirect:/swagger-ui.html";
        }
    }

4. Being the final steps, add the @Api and @ApiOperation notation in all your RESTControllers like below –

    import static org.springframework.web.bind.annotation.RequestMethod.GET;
    import org.springframework.http.HttpStatus;
    import org.springframework.web.bind.annotation.RequestMapping;
    import org.springframework.web.bind.annotation.RequestMethod;
    import org.springframework.web.bind.annotation.ResponseStatus;
    import org.springframework.web.bind.annotation.RestController;
    import io.swagger.annotations.Api;
    import io.swagger.annotations.ApiOperation;

    @RestController
    @RequestMapping("/hello")
    @Api(value = "hello", description = "Sample hello world application")
    public class TestController {

        @ApiOperation(value = "Just to test the sample test api of My App Service")
        @RequestMapping(method = RequestMethod.GET, value = "/test")
        // @Produces(MediaType.APPLICATION_JSON)
        public String test() {
            return "Hello to check Swagger UI";
        }

        @ResponseStatus(HttpStatus.OK)
        @RequestMapping(value = "/test1", method = GET)
        @ApiOperation(value = "My App Service get test1 API", position = 1)
        public String test1() {
            System.out.println("Testing");
            if (true) {
                return "Tanuj";
            }
            return "Gupta";
        }
    }

Now your are done. Now to run your Spring Boot Application, go to browser and type localhost:8080. You will see Swagger UI having all the details of your REST APIs.

Happy Coding.
The source code of the above implementation is also on my blog if you feel like checking it out.

if for some reason you don't want redirect to /swagger-ui.html you can set itself contents as home view, setting an index.html at resources/static/index.html:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Welcome to another awesome Microservice</title>
</head>
<body>
    <script>
        document.body.innerHTML = '<object type="text/html" data="/swagger-ui.html" style="overflow:hidden;overflow-x:hidden;overflow-y:hidden;height:100%;width:100%;position:absolute;top:0px;left:0px;right:0px;bottom:0px"></object>';
    </script>
</body>
</html>

then accessing to your http://localhost:8080/ you will see your swagger docs.

finally you can customize path and .html file using:

registry.addViewController("/swagger").setViewName("forward:/index.html");

like suggests this answer

The solution above does not work for me, running spring boot 3.3.0 and springdoc-openapi-starter-webmvc-ui in a kubernetes environment.

What works both locally and from a kubernetes container for me is:

springdoc:
  api-docs:
    path: /admin/swagger-ui/v3/api-docs
  swagger-ui:
    path: /admin/
    url: /admin/swagger-ui/v3/api-docs

then swagger is accessed at http://localhost:8080/admin/swagger-ui/index.html