java - How to understand Spring @ComponentScan

Question

I'm following a tutorial about Spring MVC and I cannot understand something about the @ComponentScan annotation even after reading the spring API doc, so here is the sample code:

Configuring View Controllers

package com.apress.prospringmvc.bookstore.web.config;
import org.springframework.web.servlet.config.annotation.ViewControllerRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurerAdapter;
// Other imports ommitted
@Configuration
@EnableWebMvc
@ComponentScan(basePackages = { "com.apress.prospringmvc.bookstore.web" })
public class WebMvcContextConfiguration extends WebMvcConfigurerAdapter {
    // Other methods ommitted
    @Override
    public void addViewControllers(final ViewControllerRegistry registry) {
        registry.addViewController("/index.htm").setViewName("index");
    }
}

Annotation-based Controllers

package com.apress.prospringmvc.bookstore.web;    
import org.springframework.stereotype.Controller;    
import org.springframework.web.bind.annotation.RequestMapping;    
import org.springframework.web.servlet.ModelAndView;    
@Controller    
public class IndexController {    
@RequestMapping(value = "/index.htm")    
    public ModelAndView indexPage() {     
        return new ModelAndView("index");    
    }    
}     

My question is, for View Controllers, by adding @Configuration and @ComponentScan(basePackages = { "com.apress.prospringmvc.bookstore.web" }), what will be done in the background? Will the package com.apress.prospringmvc.bookstore.web offer something for these view controllers?

Answer 1

Simply put - @ComponentScan tells Spring in which packages you have annotated classes which should be managed by Spring. So, for example, if you have a class annotated with @Controller which is in a package which is not scanned by Spring, you will not be able to use it as Spring controller.

Classes annotated with @Configuration is a new way of configuring Spring using annotations instead of XML files (it's called Java configuration). Spring needs to know which packages contain spring beans, otherwise you would have to register each bean individually. That's what @ComponentScan is used for.

In your example, you tell Spring that package com.apress.prospringmvc.bookstore.web contains classes which should be handled by Spring. Spring then finds a class annotated with @Controller, and processes it, which results in all requests coming to /index.htm being intercepted by the controller.

When the request is intercepted, Spring needs to know what response to send to the caller. Since you return an instance of ModelAndView, it will try to find a view (JSP page) called index in your project (details of this depend on configured view resolvers), and renders it to the user.

If @Controller annotation wasn't present, or that package wasn't scanned by Spring, all of this would not be possible.