rest-internal-java.md

🔁 What Happens When You Hit a REST Endpoint in Spring Boot?

1. Client Sends a Request

   • The request first reaches the embedded server (like Tomcat, Jetty, etc.).

2. DispatcherServlet Takes Control

   • Spring’s DispatcherServlet is the front controller that receives all incoming HTTP requests.

3. Filters are Triggered

   • Any configured filters run first. These can handle:

     • Authentication & Authorization
     • CORS policies
     • Logging, etc.

4. HandlerMapping Matches the Request

   • Spring uses HandlerMapping to find the correct controller method based on URL and HTTP method.

5. Interceptors Run

   • Any registered interceptors execute:

     • preHandle() runs before the controller
     • postHandle() and afterCompletion() run after

6. Controller Logic Executes

   • The actual controller method runs and returns a response object.

7. Exception Handlers Kick In (If Needed)

   • If something goes wrong, global exception handlers (@ControllerAdvice, etc.) catch and handle the error.

8. Filters Execute Again (on the Way Out)

   • Response flows back through the filter chain again before leaving the server.

9. Response Sent to the Client

   • Finally, the response is serialized (e.g., to JSON) and sent back to the client.

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🛠 Bonus Tip from Real Experience

I once had a bug where an interceptor failed because it was trying to access authentication info before the filter had processed it.

🔍 Fixed it by adjusting the filter order and adding detailed logging to each phase of the request flow — which helped trace exactly where things broke.