Spring Boot Quiz - Is a @RestController singleton by default?

Question

A. Yes, one per request

B. Yes, it’s a singleton

C. No, it’s prototype scoped

D. Depends on the HTTP method

Answer

B. Yes, it's a singleton

Spring Controller Lifecycle

Spring controllers annotated with @RestController (or @Controller) are singleton beans by default. This means:

• One instance of the controller is created per application context

• All requests are handled by the same controller instance

• Multiple threads can access the same controller instance simultaneously

Thread Safety Implications

Since controllers are singletons, we must be careful about shared mutable state:

Best Practices for Thread-Safe Controllers

1. Avoid Instance Variables for Request Data
2. Use Dependency Injection Safely
3. If You Need Mutable State, Use Thread-Safe Options.

When Controllers Aren't Singletons

We can change the scope, but it's rarely needed:

Conclusion

• Controllers are singletons by default

• Multiple requests = multiple threads accessing the same controller instance

• Never store request-specific data in instance variables

• Stick to stateless controllers with injected dependencies

• Use method parameters and local variables for request data