What is the number of prototype beans injected into a singleton bean in Spring?

In Spring, if a singleton bean directly injects a prototype bean through regular autowiring, then the prototype bean is created once when the singleton bean is created and reused — in other words, only one instance of the prototype bean is effectively injected.

To obtain a new instance of the prototype bean on each call, you need to use special mechanisms, such as:

• ObjectFactory or Provider to request a new instance
• a method annotated with @Lookup

Example with ObjectFactory:

@Component
public class SingletonBean {
    @Autowired
    private ObjectFactory<PrototypeBean> prototypeBeanFactory;

    public void doSomething() {
        PrototypeBean prototypeBean = prototypeBeanFactory.getObject();
        prototypeBean.action();
    }
}

Thus, the number of prototype beans injected into the singleton bean depends on the injection method: with direct injection — one instance, using a factory — as many new instances as requested.