Java – EJB: using entitymanager in postconstruct method

After constructing the bean, I want to use the entitymanager to retrieve data from the database It is not possible in the constructor because entitymanager. Is injected after calling the constructor So I try to do this in the @ postconstruct annotation method According to API, the postconstruct method is called after all the injection is completed. Executing a query is valid, but it always returns an empty list If I use the same query in other methods, the correct result is returned Who knows why it doesn't work in the postconstruct method?

@Stateful(mappedName = "price")
@Singleton
@Startup
public class PriceManagementBean implements PriceManagement {

    @PersistenceContext
    private EntityManager em;

    private List<PriceStep> priceSteps =  Collections.synchronizedList(new ArrayList<PriceStep>());


    public PriceManagementBean(){


    }


    @postconstruct
    public void init(){
        javax.persistence.Query query = em.createQuery("SELECT ps FROM PriceStep ps");
        List<PriceStep> res = query.getResultList();
            .....
       }
}

Solution

Reason 1 you can't create @ stateful and @ singleton beans at the same time (well, you can, but singletons are also stateful, so it doesn't make any sense). This is one of the reasons why you are in trouble There are no exceptions, but there are conflicts, and you need to solve this problem first

Just remember:

>A singleton bean is a bean that maintains its state There is only one singleton instance in the application, which is shared among all users of the application In addition, since it is a shared (possibly better concurrent) bean, you need to use the @ lock annotation to implement some kind of locking mechanism. > Stateful beans are beans that retain each state after a transaction Stateful beans at work, each user gets a copy of the bean that will last as long as the session - or until the @ remove annotated method is called

Reason 2 even if it is valid, you cannot access the results because you store them in an object named res, which can only be accessed from within the method init() I think you want to assign the returned value to the variable pricestaps

Anyway, there are a lot of errors in your code because you don't say everything I don't know what your system requirements are, but here I will give you a simple solution to allow you to access the database:

I think you are trying to return the data in the bean life cycle in some way, because if the bean is @ stateful, you want to avoid sending queries again and again The problem is that you don't have to, you can still make your bean @ stateless and avoid using many queries to compress your database All you need to do is create a @ namedquery

Therefore, annotate your entity pricestap with @ namedquery, and then enter the query string you wrote In this link, you will find information on how to use @ namedqueries: http://docs.oracle.com/cd/B31017_01/web.1013/b28221/ent30qry001.htm

I suggest you pay attention to your class pricemanagementbean as * @ stateless * Don't worry, if a new entity manager is created in each request, it will not put pressure on the database at all because it interacts with the domain model You don't need @ postconstruct, just call @ namedquery when you need it Instead of interacting with the database all the time, the application server will cache it and return it to every user who needs it

Here is a code:

@Entity
@NamedQuery(
    name="allPriceSteps",queryString="SELECT ps FROM PriceStep ps"
)
public class PriceStep implements Serializable {
...
}

Now beans:

@Stateless
public class PriceManagementBean implements PriceManagement {

    @PersistenceContext
    private EntityManager em;

    public List<PriceStep> getAllPriceSteps() {
         Query query =  em.createNamedQuery("allPriceSteps");
         return query.getResultList();
     }
}

I hope it's useful If you provide more information about system requirements, we can provide you with best practice suggestions