Tefa Tefa - 20 days ago 8
Java Question

JPA EntityManager not being injected in Java EE app

I am receiving a

NullPointerException
with the following code and configuration, and I am not sure why. I would appreciate some help in debugging this issue.

File persistence.xml:

<persistence-unit name="adismPersistenceUnit" transaction-type="RESOURCE_LOCAL" >
<provider>org.hibernate.ejb.HibernatePersistence</provider>
<class>com.adism.domain.Ad</class>
<properties>
<property name="hibernate.connection.url" value="jdbc:mysql://localhost:3306/adism" />
<property name="hibernate.connection.driver_class" value="com.mysql.jdbc.Driver" />
<property name="hibernate.dialect" value="org.hibernate.dialect.MySQL5Dialect" />
<property name="hibernate.connection.username" value="root" />
<property name="hibernate.connection.password" value="" />
</properties>

</persistence-unit>


Entity Class:

@Entity @Table(name = "ads") public class Ad {
private Integer adId;
private String adTitle;

public Ad(){}

@Id
@Column(name="adid")
@GeneratedValue
public Integer getAdId(){
return adId;
}


public void setAdId(Integer adId){
this.adId = adId;
}

@Column(name="adtitle")
public String getAdTitle(){
return this.adTitle;
}

public void setAdTitle(String title){
this.adTitle = title;
}

}


DAO Implementation:

public class AdDaoImpl implements AdDao{

@PersistenceContext
public EntityManager entityManager;

@Override
public void save(Ad ad){
entityManager.persist(ad);
}


}

When I run following code in JSP, I get NullPointerException

Ad ad = new Ad();
ad.setAdId(1000);
ad.setAdTitle("JPA pure");
AdDao newDao = new AdDaoImpl();
newDao.save(ad);

Answer

If you just do AdDao newDao = new AdDaoImpl(); your container will not known where to inject the EntityManager.

If you are using JBoss or Glassfish (or someother kind of EJB Containner) you need to declare AdDao as EJB:

@Stateless
public class AdDao () {}

And you will use it in your servlet like:

@EJB
public AdDao ejb;

PS.: I would not inject a DAO in a controller. The best is to use other classes between both, but if you are new to this kind of technology start with it.


If you are using a solution without JPA you can do something like: private static EntityManagerFactory emf = Persistence.createEntityManagerFactory("YOUR_PERSISTENCE_UNIT"); // store it in your class

public void yourMethod(){
    final EntityManager entityManager = entityManagerFactory.createEntityManager();
    entityManager.getTransaction().begin();
    // do your stuff here
    entityManager.getTransaction().commit();
    entityManager.close();
}