Java Hibernate/JPA-访问属性参数时出现NullPointerException

我试图在Hibernate 5.0.7.Final中使用JPA2类型安全标准查询

...
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

。

Person

的元模型类

Person

由

org.hibernate.jpamodelgen.jpametamodelenticyprocessor

生成

在中提出了类似的问题，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)

我的代码：

接口，我使用它来确保它们将具有

getId（）

因此，问题似乎是将
SingularAttribute
参数传递给方法
readBy（SingularAttribute列，字符串值）

我在main中测试了这段代码，结果显示
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

这就是它应该做的吗？如何将
SingularAttribute
对象作为参数传递给方法？
在方法调用以某种方式工作之前，在main中调用
HibernateUtil.getEntityManager（）
当我调用方法的一个空块时，它甚至可以工作
init（）
它可能与类的初始化有关。下面是代码片段

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();
    }
}

谢谢你，在我的HibernateSessionFactory类中添加
public void init（）{}//并从外部调用它，也解决了我的问题。真奇怪。有人知道为什么、如何和为什么吗？你可以回答你自己的问题，而不是用答案编辑它-这是更推荐的。谢谢@Samoth。我接受了这个解决方案，尽管它背后的原因仍然不清楚。
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;

}

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;
    }
}

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

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;
}

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

//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
}

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();
    }
}