问题
I'm trying to use JPA2 type-safe criteria queries with Hibernate 5.0.7.Final.
...
criteria.where( builder.equal( root.get(SingularAttribute.attr), value ));
//where parameters are
//criteria.where( builder.equal( root.get(Person_.name), "Can" ));
...
The root.get always throw NullPointerException.
The metamodel class Person_ for Person is generated by org.hibernate.jpamodelgen.JPAMetaModelEntityProcessor.
A similar problem was asked in JPA/Hibernate Static Metamodel Attributes not Populated -- NullPointerException but this time both classes are in the same package.
Stack trace:
java.lang.NullPointerException
at org.hibernate.jpa.criteria.path.AbstractPathImpl.get(AbstractPathImpl.java:123)
My code:
Interface that i use to make sure they will have getId();.
package it.unibz.db.hibernate.model;
public interface ModelInterface<PK extends Serializable> extends Serializable {
PK getId();
}
Model class
package it.unibz.db.hibernate.model;
@Entity
@Table(name ="person")
public class Person implements ModelInterface<Integer> {
@Id
private Integer id;
private String name;
public Integer getId() {
return id;
}
//other getter and setters
}
Generated metamodel Person_ class
package it.unibz.db.hibernate.model;
@Generated(value = "org.hibernate.jpamodelgen.JPAMetaModelEntityProcessor")
@StaticMetamodel(Person.class)
public abstract class Person_ {
public static volatile SingularAttribute<Person, String> name;
public static volatile SingularAttribute<Person, Integer> id;
}
Generic DAO class that i inherit with PersonDao
public class GenericDao<E extends ModelInterface<PK>, PK extends Serializable> implements DaoInterface<E, PK> {
private Class<E> type;
public GenericDao(Class<E> type) {
this.type = type;
//called as super(ClassName.class); eg. super(Person.class);
}
public List<E> readBy(SingularAttribute column, String value) throws Exception {
EntityManager em = HibernateUtil.getEntityManager();
CriteriaBuilder builder = em.getCriteriaBuilder();
CriteriaQuery<E> criteria = builder.createQuery(type);
Root<E> root = criteria.from(type);
criteria.select(root);
criteria.where( builder.equal( root.get(column), value ));
List<E> entityList = em.createQuery(criteria).getResultList();
em.close();
return entityList;
}
}
Some of my dependencies
hibernate-c3p0 5.0.7.Final
hibernate-entitymanager 5.0.7.Final
postgresql 9.4.1207.jre7
hibernate-jpamodelgen 5.0.7.Final
EDIT:
Running this code in the main method works
EntityManager em = HibernateUtil.getEntityManager();
CriteriaBuilder builder = em.getCriteriaBuilder();
CriteriaQuery<Person> criteria = builder.createQuery(Person.class);
Root<Person> root = criteria.from(Person.class);
criteria.select(root);
criteria.where( builder.equal( root.get(Person_.name), "Can" ));
List<Person> entityList = em.createQuery(criteria).getResultList();
//do stuff with entityList
but the same code covered in a method throws NullPointerException.
public List<Person> readBy(SingularAttribute column, String value) throws Exception {
log.debug("Reading entity by");
EntityManager em = HibernateUtil.getEntityManager();
CriteriaBuilder builder = em.getCriteriaBuilder();
CriteriaQuery<Person> criteria = builder.createQuery(Person.class);
Root<Person> root = criteria.from(Person.class);
criteria.select(root);
criteria.where( builder.equal( root.get(column), value ));
List<Person> entityList = em.createQuery(criteria).getResultList();
em.close();
return entityList;
}
So it seems that the problem is passing SingularAttribute parameter to the method readBy(SingularAttribute column, String value).
I tested this code in main and this prints false
EntityManager em = HibernateUtil.getEntityManager();
CriteriaBuilder builder = em.getCriteriaBuilder();
CriteriaQuery<Person> criteria = builder.createQuery(Person.class);
Root<Person> root = criteria.from(Person.class);
System.out.println(root.get(Person_.name) == null); //false
meanwhile this throws InvocationTargetException caused by NullPointerException at root.get(column).
//invoked as personDao.test(Person_.name) from main
public void test(SingularAttribute column) {
EntityManager em = HibernateUtil.getEntityManager();
CriteriaBuilder builder = em.getCriteriaBuilder();
CriteriaQuery<Person> criteria = builder.createQuery(Person.class);
Root<Person> root = criteria.from(Person.class);
System.out.println(root.get(column) == null); //InvocationTargetException
}
Is this what it's supposed to do? How can i pass SingularAttribute object to a method as a parameter?
回答1:
Calling HibernateUtil.getEntityManager() in main before method call somehow works. It even works when i call literally a empty block of a method init().
It could be related with initialization of classes. Here's the code snippet.
public class HibernateUtil {
private static EntityManagerFactory emFactory;
private static EntityManager em;
private static final Logger log = LoggerFactory.getLogger(HibernateUtil.class);
private static final String PERSISTENCE_UNIT = "pt";
static{
log.info("Creating entity manager factory");
emFactory = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT);
}
//calling this from main before PersonDao's method calls somehow works...
public void init(){} //does nothing
public static EntityManager getEntityManager(){
if(em != null && em.isOpen()){
closeEntityManager();
}
log.debug("Creating new entity manager");
em = emFactory.createEntityManager();
return em;
}
public static void close() throws Exception {
if(em != null && em.isOpen()){
closeEntityManager();
}
log.info("Closing entity manager factory");
emFactory.close();
}
private static void closeEntityManager(){
log.info("Closing last entity manager");
em.close();
}
}
来源：https://stackoverflow.com/questions/34823602/hibernate-jpa-nullpointerexception-when-accessing-singularattribute-parameter