EntityManager injection - NullPointerException

Question

In my Spring+JPA/Hibernate+Wicket app, I have a QueryBuilder bean that I want to use in one of my DAOs which generates a typed query with the help of 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 run the app, I get a null pointer exception for line:

cb = em.getCriteriaBuilder();

i.e. the EntityManager doesn't get injected. Do you know why?

Also, is this use correct and thread-safe or should I instantiate my InboxQueryBuilder for each query? In that case, should I also inject the EntityManager or should I just pass it as a constructor parameter (the InboxQueryBuilder would get instantiated for each query in the DAO which has an injected instance of EntityManager)?

in the example given, it's a spring bean. but i don't know if making it a singleton is good. — John Manak, Nov 09 '10 at 10:14
Don't use singletons in a spring project. Use spring singleton beans instead. — Sean Patrick Floyd, Nov 09 '10 at 10:32

score 11 · Accepted Answer · edited Jul 24 '13 at 06:58

11

You can't access the EntityManager within the constructor. Take a look at the @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: After reading your post again, I start to became unsure, if I'm right. I know the Java EE-Dependency-Injection within a JBoss works as I described, but I'm not sure about spring-IOC.

edited Jul 24 '13 at 06:58

Arjan Tijms

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answered Nov 09 '10 at 11:23

Christian Kuetbach

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This is where spring is your friend! It is doing the autowire of the entity manager for you - without spring this would not work - the auto injection of the Entity Manager only works (via the Application Server) if you're running inside an EJB. – Michael Wiles Nov 09 '10 at 14:09

score 0 · Answer 2 · answered Nov 09 '10 at 10:27

0

Do you have this bean somewhere in your application context?

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

answered Nov 09 '10 at 10:27

nanda

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yes i do. entitymanager gets injected into the DAO that calls getQueryForApps method without a problem. – John Manak Nov 09 '10 at 10:29
then the comment from seanizer is the answer. You should make it spring singleton – nanda Nov 09 '10 at 10:35

score 0 · Answer 3 · answered Nov 09 '10 at 10:31

0

Spring uses the Java Beans mechanism, so I am pretty sure this is insufficient:

@PersistenceContext
EntityManager em;

Here's the standard way:

private EntityManager entityManager;

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

answered Nov 09 '10 at 10:31

Sean Patrick Floyd

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OK, do you have autowiring turned on? – Sean Patrick Floyd Nov 09 '10 at 10:34

EntityManager injection - NullPointerException

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