Interceptor

Interceptor helps us to intercept request sending from dispatcher to controller and from controller to dispatcher.

As shown in the inline image below, when a request is made from the browser, it first reaches the DispatcherServlet, which acts as the central entry point for handling requests in a Spring Boot application. By default, the request is then forwarded directly to the appropriate controller for processing.

However, interceptors allow us to intercept and modify both the incoming request before it reaches the controller and the outgoing response before it is sent back to the client. This enables us to implement pre-processing and post-processing logic, such as authentication, logging, or modifying request parameters, in between the DispatcherServlet and the controllers.

Implementation

We can create interceptor post implementing HandlerInterceptor interfacse and overriding preHandle(), postHandle() and afterCompletion() methods as shown below:

Implement WebMvcConfigurer and override addInterceptor() method to register our CustomInterceptor.

Now, create login controller having /login as request mapping URL and return success as response text.

Now hit request URL as http://localhost:8080/login in browser will give below output:

Now, verify the below logs in the console/terminal where pre handler got called before controller and post handler got called after controller. After completion will get call at the end.