Question

I was wondering how the lifecycle of a singleton bean (annotated with @Scope(value="singleton")) in Spring is exactly defined.

Specifically, is it guaranteed that within the same ApplicationContext consecutive lookups of that bean return the same bean object instance?

Answer 1

Yes, that is the definition of singleton scope: only one instance returned by every call to getBean(). Note that non-lazy singleton is the default scope in Spring.

Also if singleton is defined as lazy and no other non-lazy bean references it, it will be created during first explicit lookup.

EDIT: To answer your comment: occasionally you might discover that your singleton is created twice. Take this bean as an example:

@Service
@Transactional
public class Singleton {
    public Singleton() {
        System.out.println("Created: " + this.getClass());
    }
}

Even though this is a singleton, your program might produce the following output:

Created: class com.example.Singleton
Created: class com.example.Singleton$$EnhancerByCGLIB$$f8ccc422

Constructor has been called twice – once for the original class and second time for the class created by CGLIB to implement proxying on a class without any interfaces. To preserve Singleton public interface, the class generated by CGLIB must subclass from the original class. This way CGLIB class might be used where Singleton was expected (polymorphism). But subclass must call the base class constructor, hence two constructor calls.

But don’t worry, the CGLIB-generated class is just a stub that redirects all the calls to a “normal” bean, applying AOP stuff in the meantime.