Plain old singleton or spring singleton bean ?

Question

I have a service in my application which is a singleton. My application is being bloated with the use of spring framework.

I am confused over to use the singleton service as

1: Plain Old Singleton [Access them statically when required]

OR as a

2: Spring singleton bean. [Use DI to inject when required]

Which approach is correct ?

Answer 1

The Spring singleton scope is not the same as the Singleton design pattern, which is not the same as a class with static methods.

From the documentation

"Please be aware that Spring's concept of a singleton bean is quite different from the Singleton pattern as defined in the seminal Gang of Four (GoF) patterns book. The GoF Singleton hardcodes the scope of an object such that one and only one instance of a particular class will ever be created per ClassLoader. The scope of the Spring singleton is best described as per container and per bean. This means that if you define one bean for a particular class in a single Spring container, then the Spring container will create one and only one instance of the class defined by that bean definition."

Also, note you need to be very careful using a Spring singleton as a service that web requests will utilize. Since each request is on its own thread, you can have threading issues if your singleton maintains any state.

To answer your question: Create a class that implements an interface, and use Spring to DI it appropriately. If your service does not maintain state, you can scope it to singleton, otherwise you can scope it to prototype.

Answer 2

I use Spring's beans whenever possible. The framework was designed to manage these things, and it is probably better than me at it. Another reason to use Spring's dependency injection is the possibility to unit-test with mocks instead of the real utility code, thus focusing the unit-test to the exact scope.

EDIT: To answer the question in the comment, the only case I can think of for a non-bean singleton would be a utility code class, which would contain short pieces of generally reusable code in public static methods. Anything else requires instantiation, and therefore - a bean.