Spring Bean: Is autowired attribute initialised before constructor?

Question

@Component
public class BeanA {
...
}


@Component
public class BeanB {

    @Autowired
    BeanA beanA;

    public BeanB() {
        // Use beanA
        beanA.method();
    }
}

Can we assume BeanA is created and initialized before BeanB constructor is called? (I know we can pass BeanA as constructor arg to BeanB, this is more of curiosity question to understand spring/java initialisation sequence)

Answer 1

Take a look at http://docs.spring.io/spring/docs/current/spring-framework-reference/htmlsingle/#beans-autowired-annotation-qualifiers

Setting properties on bean would happen after it is constructed by means of a constructor or a factory method. By default, beans are autowired by name and values are set using setter methods. So in your case the field will be set after constructor.

This is because

@Autowired
BeanA beanA;

really means that you want to autowire the field of that class instance. beanA in your case is not really a constructor arg. (Well, here is a quick question, are the constructor argument names retained after compilation? Is there any debug flag related to this?)

As this example from spring documentation says, you can apply @Autowired to constructors and fields:

public class MovieRecommender {

    @Autowired
    private MovieCatalog movieCatalog;

    private CustomerPreferenceDao customerPreferenceDao;

    @Autowired
    public MovieRecommender(CustomerPreferenceDao customerPreferenceDao) {
        this.customerPreferenceDao = customerPreferenceDao;
    }

    // ...

}

Let me know if you need any more help with this.

Oh, and just a minor point. You seem to be calling method() on beanA in your constructor. It is not a good idea, if the method can be overridden. I know it is just an example you jotted down here, but just a word of caution.

Answer 2

No, the autowiring is handled by a BeanPostProcessor that will run after the constructor of the newly created bean. If, for some reason, you need to autowire some fields and then run some initialisation you can use a @PostConstruct annotated method. It will be called after all dependencies are injected. In most cases @Autowiring a constructor (and, perhaps, making the object immutable) is still the best option.

Answer 3

No, spring is very clever, but not that magical ... Internally, spring :

• creates an instance
• set instances properties
• stores eventually the bean in relevant scope (exept for prototype beans) and/or gives it to caller

But the creation uses a constructor and at the time it is called properties have not been set.