Question

I have a singleton Spring bean (and it has to stay a singleton) which needs a fresh instance of another bean (Lets call it X) every time a certain method executes.

So far I looked at the following approaches:

• just create X using new. This worked for a while but now we need spring AOP features for X, so this doesn’t work anymore, since the resulting instances are not Spring managed.

• I considered a FactoryBean as a dependency, but I would only get a single X instance from the FactoryBean, which doesn’t meet my first instance.

• the current plan is to manually lookup X in the Spring context, and declare it there with a prototype dependency. This should work, but I think it is really ugly.

=> How can I inject a factory in my bean so that I can call its factory method any time I see fit and getting a spring managed instance out of it.

Answer 1

The means of choice for a scenario like this is called lookup method injection. In short, this uses the approach of a call to a bean method resulting in a new bean instance created. You’d start by creating a class with an abstract method that will eventually provide the dependency instance:

abstract class MyClient implements Client {

  void businessMethod(…) {

    Dependency dependency = getDependencyInstance();
    …
  }

  abstract Dependency getDependencyInstance();
}

You now go ahead and configure a prototype bean definition for the dependency:

<bean id="dependency" class="….DependencyImpl" scope="prototype" />

As well as the client using the lookup-method element to always get a fresh instance of the dependency for each method call:

<bean class="….MyClient">
  <lookup-method name="getDependencyInstance" bean="dependency" />
</bean>

This will cause a CGLib proxy being created for MyClient and the method declaration of getDependencyInstance(…) being backed by a TargetSource with a reference to the BeanFactory and the name of the bean to be looked up. On each method invocation the bean lookup will be triggered and a fresh instance of the prototype configured bean is returned.