EntityManager injection - NullPointerException exception

Question

In my Spring + JPA / Hibernate + Wicket application, I have a QueryBuilder bean that I want to use in one of my DAOs that generates a typed query using the criteria API:

@Service(value="inboxQueryBuilder")
public class InboxQueryBuilder {

    @PersistenceContext
    EntityManager em;

    CriteriaBuilder cb;

    public InboxQueryBuilder() {
        cb = em.getCriteriaBuilder();
    }

    public TypedQuery<App> getQueryForApps(AppSearchObject aso) {
           ...
    }

    ...
}

However, when I launch the application, I get a null pointer exception for the string:

cb = em.getCriteriaBuilder();

i.e. EntityManager is not entered. Do you know, why?

Also, is this used correctly and thread safe, or should I create an InboxQueryBuilder instance for each query? In this case, should I also enter an EntityManager or just pass it as a constructor parameter (will InboxQueryBuilder get an instance for every query in the DAO that has an injected EntityManager instance)?

Answer 1

EntityManager . @PostConstruct-Annotation

@Service(value="inboxQueryBuilder")
public class InboxQueryBuilder {

@PersistenceContext
EntityManager em;

CriteriaBuilder cb;

public InboxQueryBuilder() {
    // em= null
}

@PostConstruct
public void toSomething(){
    // em set by Container  
    cb = em.getCriteriaBuilder();
}


public TypedQuery<App> getQueryForApps(AppSearchObject aso) {
       ...
}

...
}

EDIT: , , . , Java EE-Dependency-Injection JBoss , , spring -IOC.

Answer 2

bean - ?

<bean id="entityManagerFactory" class="org.springframework.orm.jpa.LocalEntityManagerFactoryBean">
  <property name="persistenceUnitName" value="myPersistenceUnit"/>
</bean>

Answer 3

Spring Java Beans, , :

@PersistenceContext
EntityManager em;

:

private EntityManager entityManager;

@PersistenceContext
public void setEntityManager(final EntityManager entityManager){
    this.entityManager = entityManager;
}