Is it possible to implement a Spring component that behaves both as a singleton and as a prototype, and how to do it?

In Spring, by default, beans are either singletons or prototypes, but you cannot declare a bean as both a singleton and a prototype at the same time.

However, it is possible to implement a singleton bean that creates new instances of a prototype internally upon request.

How to do this:

1. Define a singleton bean (by default).
2. Define a prototype bean.
3. Inject ObjectProvider or ApplicationContext into the singleton bean to obtain new instances of the prototype on demand.

Example with ObjectProvider:

@Component
@Scope("singleton")
public class SingletonBean {

    private final ObjectProvider<PrototypeBean> prototypeBeanProvider;

    public SingletonBean(ObjectProvider<PrototypeBean> prototypeBeanProvider) {
        this.prototypeBeanProvider = prototypeBeanProvider;
    }

    public void usePrototype() {
        PrototypeBean prototype = prototypeBeanProvider.getObject();
        prototype.doSomething();
    }
}

@Component
@Scope("prototype")
public class PrototypeBean {
    public void doSomething() {
        System.out.println("Prototype instance: " + this);
    }
}

Thus, the singleton bean can create and use new instances of the prototype each time.