Spring Security integrates as a chain of servlet Filters (registered as a single DelegatingFilterProxy that delegates to an internal FilterChainProxy) sitting in front of the DispatcherServlet — each request passes through an ordered sequence of security filters (e.g., authentication filters, CSRF protection, authorization checks) before ever reaching application code, and any filter in the chain can short-circuit the request (e.g., reject it with 401/403) before it gets further.

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#Security Filter Chain

Filter Order Is Deliberate and Meaningful

The filters aren't arbitrarily ordered — authentication filters must run before authorization filters (you can't check permissions for an unidentified user), and CSRF checks typically run early to reject forged requests before any expensive processing happens. Customizing the SecurityFilterChain means inserting your own filter at a specific, deliberate position relative to these existing filters, not just appending it anywhere.

Request → DelegatingFilterProxy → ordered SecurityFilterChain (auth, CSRF, authorization...) → DispatcherServlet, only if all pass