When using @Component + @WebFilter, you're actually registering the filter twice with Embedded Tomcat:

• @Component registers it through Spring's component scanning
• @WebFilter registers it through servlet container's scanning

This is why using both annotations works, but it's not the recommended approach

but in Traditional Spring MVC (External Tomcat):

• @WebFilter is a standard Jakarta EE annotation that's processed by the servlet container (Tomcat)
• @Component is a Spring framework annotation

When using external Tomcat, the servlet container handles @WebFilter registration directly This is why you get the "already registered" error when using both annotations

=> Working with Just @WebFilter require using @ServletComponentScan in the Spring Boot application, This annotation enables scanning for servlet components including @WebFilter