What is the prototype Spring Bean used for?

Question

By default, the Bean created by Spring is singleton. They are thread-safe because they are stateless. When we want Spring to create a stateful Bean, we need to use prototype scope for the Bean definition. We need to take care of the thread-safe issues for them. All stateless Bean will be polluted when they are injected by the prototype bean. So, I just can not image where we can use the prototype scope. Can you give some typical scenario that we can / need to use prototype Spring Bean? Also how can we void the stateful pollution on other singleton beans?

Answer 1

There are many reasons to use prototype scope, e.g., any time you'd use "new" instead of using a singleton. A per-user bean, a per-request bean, a collection of unique beans, etc. After all, in any non-trivial application, don't you use non-singletons far more than singletons?

Singleton-scoped beans aren't thread-safe simply because they're singletons–they must be written to be thread-safe. They don't become thread-safe magically. A bean's scope is just that, its scope: it doesn't make the bean appropriate for the particular scope–that's up to the developer.

Answer 2

I perceive prototype scoped beans as an alternative to factory classes used to create objects. The difference is in case of prototype beans spring will save you some code for dependency injection and will also automatically proxy your objects for transactions etc. when appropriate.

I myself prefer the factory approach. One reasonable scenario for prototype scope I encountered was a stateful object, needed by different known beans and each required its own copy. A dedicated factory class would be redundant in this scenario since I did not need to create objects on the fly but only during other beans' instantiation.