Spring MVC Java WebMvcConfigurerAdapter.addResourceHandlers - spring-mvc

Question

I'm looking at a project with the following Java Configuration file:
#Configuration
#EnableWebMvc
#ComponentScan("spittr.web")
public class WebConfig extends WebMvcConfigurerAdapter {
#Bean
public ViewResolver viewResolver() {
InternalResourceViewResolver resolver = new InternalResourceViewResolver();
resolver.setPrefix("/WEB-INF/views/");
resolver.setSuffix(".jsp");
return resolver;
}
#Override
public void configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer) {
configurer.enable();
}
#Override
public void addResourceHandlers(ResourceHandlerRegistry registry) {
super.addResourceHandlers(registry);
}
}
The project has some static resources in the webapps/resources directory that are being served up. My question is I'm not sure why. It seems for this to work, the above call to addResourceHandlers(...) should be
#Override
public void addResourceHandlers(ResourceHandlerRegistry registry) {
registry.addResourceHandler("/resources/**").addResourceLocations("/resources");
}
I tried to look for something in the Spring documentation possibly indicating default values but could not, so I'm not sure why the project works.

Answer 1

The project can serve static resources thanks to DefaultServletHandlerConfigurer:
Configures a request handler for serving static resources by forwarding the request to the Servlet container's "default" Servlet.
I use Jetty in the following example, but other servlet containers (e.g. Tomcat) should behave similarly. Download the Jetty 9 distribution, then inside webapps, create these files:
example/
hello.txt
Start the Jetty server. We have deployed the simplest application imaginable. You should be able to access the file at:
http://localhost:8080/example/hello.txt
The servlet container can serve static resources without any extra configurations. This behavior is suppressed as soon as Spring MVC comes into play. Spring MVC will create a front controller, DispatcherServlet, and park it at "/". Every incoming request will go through this single servlet, and the servlet will find the appropriate component in the app to actually process the request (e.g. a #RequestMapping method in a #Controller class). If no handlers for the request can be found, we have 404s.
The handler DefaultServletHandlerConfigurer has the lowest precedence. If enabled, it allows the front controller to handle the request first. When that fails, it forwards the request to the servlet container, where the request is treated as a static resource read.
This is a quick and dirty way of serving static files. In practice, you don't want anything off the beaten path to be available via a GET. You want to allow only specific files and folders, and you would use WebMvcConfigurerAdapter.addResourceHandlers().