Spring Boot Hello World JSON No Annotations

MainController.java

package app;

import org.springframework.beans.MutablePropertyValues;
import org.springframework.beans.factory.support.BeanDefinitionRegistry;
import org.springframework.beans.factory.support.GenericBeanDefinition;
import org.springframework.boot.SpringApplication;
import org.springframework.http.MediaType;
import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
import org.springframework.http.server.ServletServerHttpResponse;
import org.springframework.web.servlet.ModelAndView;
import org.springframework.web.servlet.handler.SimpleUrlHandlerMapping;
import org.springframework.web.servlet.mvc.Controller;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.util.HashMap;
import java.util.Map;

public class MainController implements Controller {
  private static final Map<String, String> RESPONSE = new HashMap<String, String>() {{
    put("response", "Hello World!");
  }};

  static final Class[] BOOT_CLASSES = new Class[]{
      MainController.class,
      org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration.class,
      org.springframework.boot.autoconfigure.MessageSourceAutoConfiguration.class,
      org.springframework.boot.autoconfigure.PropertyPlaceholderAutoConfiguration.class,
      org.springframework.boot.autoconfigure.web.DispatcherServletAutoConfiguration.class,
      org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration.class,
      org.springframework.boot.autoconfigure.web.HttpMessageConvertersAutoConfiguration.class,
      org.springframework.boot.autoconfigure.web.MultipartAutoConfiguration.class,
      org.springframework.boot.autoconfigure.web.ServerPropertiesAutoConfiguration.class,
      org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration.class,
  };

  public static void main(String[] args) throws Exception {
    SpringApplication app = new SpringApplication(BOOT_CLASSES);
    app.addInitializers(applicationContext -> {
      GenericBeanDefinition definition = new GenericBeanDefinition();
      MutablePropertyValues values = new MutablePropertyValues();
      values.add("urlMap", new HashMap<String, String>() {{
        put("/main*", "mainController");
      }});
      values.add("order", 0);
      definition.setPropertyValues(values);
      definition.setBeanClass(SimpleUrlHandlerMapping.class);
      ((BeanDefinitionRegistry) applicationContext.getBeanFactory())
          .registerBeanDefinition("mainUrlMapping", definition);
    });
    app.run();
  }

  @Override
  public ModelAndView handleRequest(HttpServletRequest request, HttpServletResponse response) throws Exception {
    new MappingJackson2HttpMessageConverter().write(RESPONSE, MediaType.APPLICATION_JSON, new ServletServerHttpResponse(response));
    return null;
  }
}

curl localhost:8080/main