How @RequestMapping internally works in Spring Boot?

Question

@RestController
@RequestMapping("/employee")
public class Employee {
  @RequestMapping("/save")
  public void saveEmployee() {
    // saving employee
  }
}

How does @RequestMapping will work internally to map the request to the saveEmployee method?

Answer 1

During application startup, Spring will identify all Beans by way of XML Config, Java Config, or Component Scanning and store them in the ApplicationContext.

Spring Boot autoconfigures many Beans for you, including RequestMappingHandlerMapping.

When this Bean is initialized it scans the ApplicationContext for any Beans annotated with @Controller.

Then it iterates over each Controller bean and looks for methods annotated with @RequestMapping. Finally it persists these mapping/handler pairs in the MappingRegistry

The DispatcherServlet is the central HTTP request handler for your application and it will search the ApplicationContext for any Beans that implement the HandlerMapping interface, which the RequestMappingHandlerMapping Bean does (by way of inheritance).

Then it iterates over these beans asking them to resolve the corresponding handler for this request. The RequestMappingHandlerMapping bean will resolve the handler by searching its MappingRegistry.

When a match is found, the handler method is eventually invoked.