Spring lookup-method and scoped proxy usage

Question

I'm a bit confused about using method injection (lookup-method) and aop scoped-proxy (Since both used for different scoped beans injection) so

1) When to use method injection and when to use aop-scoped proxy ? 2) What is the reason why a aop-scoped proxy will not be used for a prototype bean ?

Answer 1

Both lookup method injection and scoped proxy are means to inject shorter lived beans into longer lived beans. However, they serve different use cases.

Method injection is useful in cases where a singleton-scoped bean has a dependency on a prototype-scoped bean.

A proxy gets injected in place of the desired bean and provides that bean depending on the context. For example, if a singleton bean (such as a Spring MVC controller) auto-wires a session scoped bean, then the proxy delivers that bean belonging to the current HTTP session.

Such a proxy doesn't apply well to a situation where a prototype bean shall be obtained at runtime. Lookup method injection is one way to obtain prototype instances at runtime.

However, method injection has limitations because it builds upon abstract methods. Hence, certain things like writing unit tests are more cumbersome, as you need to provide a stub implementation of the abstract method. Component scanning doesn't work with abstract classes either.

One alternative to method injection is Spring's ObjectFactory, or its JSR equivalent Provider.

Another, straightforward way of creating prototype bean instances at runtime (which even makes it possible to provide constructor arguments) is to implement a bean factory like the following:

@Configuration
public class MyProvider {

    @Bean
    @Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
    public MyThing create(String name) {
        return new MyThing(name);
    }

}

Usage:

@Component
public class MySingleton {

    @Autowired
    private MyProvider myProvider;

    public void doStuffThatNeedsAPrototypeBeanInstance() {
        MyThing thing = myProvider.create("some name");
        ...
    }
}