Spring Injection - access to the injected object within a constructor

Question

I have a resource (Spring bean) which has some of its fields injected by Spring, for example:

@Repository(value="appDao")
public class AppDaoImpl implements AppDao {
   @PersistenceContext
   EntityManager entityManager;

   public Resource() {
      ... use entityManager ... // doesn't work
   }
}

I know that I can't access the injected entityManager in the constructor and should use a @PostConstruct annotation on a different method. But what are the reasons for this?

Answer 1

Because Spring can't access any fields or methods before the object is created (which is done through the constructor). So Spring instantiates the object using the constructor and then injects the properties.

The only way around this is to use Constructor Injection (which can be cumbersome if you have multiple dependencies). I think what you should do is move your code out of the constructor and into an initialization method using the @PostConstruct annotation:

@PostConstruct
public void init(){
    // do stuff with entitymanager here
}

Answer 2

The reason is in the lifecycle of the bean:

• The container (spring application context) instantiates the object
• then it sets all the dependencies (incl. the entityManager in your example)
• after all dependencies have been set, it invokes the @PostConstruct method, if any.

Spring (and no one) can set fields to an object before actually constructing that object.

You can use constructor-injection - passing the dependencies to a non-default constructor, but it is not possible with @PersistenceContext