Spring Boot Application Starts But API Not Responding

If you’ve ever started your Spring Boot application, seen the familiar “Started Application in X seconds” log line with no errors at all, and then opened Postman only to watch your request spin forever or return absolutely nothing — you’re not alone. This is one of the most common (and most confusing) issues Spring Boot developers run into, precisely because the application looks healthy from the outside.

Typical symptoms:– The app starts cleanly, logs show no exceptions- Tomcat/Netty reports it’s running on a port (e.g., 8080)- Hitting the endpoint from Postman or curl either hangs, times out, or returns a blank response / 404- No stack trace anywhere to point you in the right direction

The good news: there are only a handful of root causes, and once you know what to check, this usually takes
5–10 minutes to diagnose

Common Causes

1. Controller Not Being Scanned

If your @RestController class lives outside the package (or sub-package) of your @SpringBootApplication
class, Spring’s component scanning won’t pick it up. The app starts fine because nothing is “wrong” — your
controller bean simply never gets registered.
// Main application class
com.example.demo.DemoApplication
// Controller sitting OUTSIDE the scanned package — won’t be detected
com.otherpackage.controllers.UserController
“