Hibernate/JPA - 访问 SingularAttribute 参数时出现 NullPointerException

我正在尝试将 JPA2 类型安全标准查询与 Hibernate 5.0.7.Final 一起使用。

...
criteria.where( builder.equal( root.get(SingularAttribute.attr), value ));
//where parameters are
//criteria.where( builder.equal( root.get(Person_.name), "Can" ));
...

root.get 总是抛出NullPointerException。 元模型类Person_ for Person是由生成的org.hibernate.jpamodelgen.JPAMetaModelEntityProcessor.

类似的问题被问到JPA/Hibernate 静态元模型属性未填充 -- NullPointerException但这次两个类都在同一个包中。

堆栈跟踪：

java.lang.NullPointerException
at org.hibernate.jpa.criteria.path.AbstractPathImpl.get(AbstractPathImpl.java:123)

My code:

我用来确保他们拥有的界面getId();.

package it.unibz.db.hibernate.model;

public interface ModelInterface<PK extends Serializable> extends Serializable {
    PK getId();
}

模型类

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
}

生成元模型Person_类

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;

}

我用 PersonDao 继承的通用 DAO 类

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;
    }
}

我的一些依赖项

• hibernate-c3p0 5.0.7.Final

• hibernate-entitymanager 5.0.7.Final

• postgresql 9.4.1207.jre7

• hibernate-jpamodelgen 5.0.7.Final

EDIT:

在 main 方法中运行此代码有效

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

但方法中包含的相同代码会抛出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;
}

所以看来问题已经过去了SingularAttribute方法的参数readBy(SingularAttribute column, String value).

我在 main 中测试了这段代码并打印了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

同时这会抛出InvocationTargetException引起的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
}

这是它应该做的吗？我怎样才能通过SingularAttribute对象作为参数的方法？

Calling HibernateUtil.getEntityManager()在方法调用之前的 main 中以某种方式起作用。当我从字面上调用方法的空块时它甚至可以工作init().这可能与类的初始化有关。这是代码片段。

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();
    }
}