我在加载一些对象时遇到了一个奇怪的问题。我正在使用 JPA 1、hibernate-core 版本 3.3.0.SP1 和 hibernate-entitymanager 版本 3.4.0.GA

假设我有这些 JPA 实体:

@Entity
@Table(name = "SLC_ELE")
@Inheritance(strategy = InheritanceType.JOINED)
@DiscriminatorColumn(discriminatorType = DiscriminatorType.INTEGER, name = ElementoPrograma.C_ID_CTG_ELE)
public class Element {
...
} 

@Entity
@Table(name = "SLC_ELE_ONE")
@Inheritance(strategy = InheritanceType.JOINED)
@DiscriminatorValue(Categories.ID_CTG_ONE)
public class ElementTypeOne extends Element {
    ...
}

@Entity
@Table(name = "SLC_ELE_TWO")
@Inheritance(strategy = InheritanceType.JOINED)
@DiscriminatorValue(Categories.ID_CTG_TWO)
public class ElementTypeTwo extends Element {
    ...
} 

@Entity
@Table(name = ThreeElementExample.TABLENAME)
@AssociationOverrides({
    @AssociationOverride(name = JpaMany3ManyEntity.ASOCIATION_OVERRIDE_ONE,
    joinColumns =
    @JoinColumn(name = Element.C_ID_ELE)),
    @AssociationOverride(name = JpaMany3ManyEntity.ASOCIATION_OVERRIDE_TWO,
    joinColumns =
    @JoinColumn(name = OneEntity.C_ID_TWO)),
    @AssociationOverride(name = JpaMany3ManyEntity.ASOCIATION_OVERRIDE_THREE,
    joinColumns =
    @JoinColumn(name = AnotherEntity.C_ID_THREE))}) 
public class ThreeElementExample extends JpaMany3ManyEntity<Element, OneEntity, AnotherEntity> {
 ...
}

问题是，当我加载这些实体的集合时，我总是希望获得子类(意思是 ElementTypeOne、ElementTypeTwo 而不是元素)。问题是多对多关系总是得到元素(父亲而不是 child )

假设我有一个包含元素集合的实体 A:

@Fetch(FetchMode.JOIN)
@OneToMany(cascade = CascadeType.ALL, mappedBy = "idEle")
private Collection<Element> elementCollection;

如果我得到集合，一切正常(我得到了预期的子类)。

当我有另一个实体 B 和 JpaMany3ManyEntity 的集合时，问题就来了(注意涉及相同的实体元素)

@OneToMany(cascade = CascadeType.ALL, mappedBy = JpaMany3ManyEntity.ASOCIATION_OVERRIDE_ONE, fetch = FetchType.LAZY)
private Collection<ThreeElementExample> threeElementExampleCollection;

如果我在尝试从 A 类获取 elementCollection 之前从 B 类循环 threeElementExampleCollection，当我从 elementCollection 加载对象时 我只获取父类(super class) (Element) 对象而不是子对象。

我猜想，出于任何原因，多对多关系总是获取元素对象(父亲)并将它们保存在 hibernate 缓存中，但我需要避免这种行为。

有任何想法或解决方法吗？非常感谢任何形式的帮助。

提前致谢。

编辑:多对多类:

@SuppressWarnings("serial")
@MappedSuperclass
@AssociationOverrides({
@AssociationOverride(name = JpaMany3ManyEntity.ASOCIATION_OVERRIDE_ONE,
joinColumns =
@JoinColumn(name = "changeMeWhenExtends")),
@AssociationOverride(name = JpaMany3ManyEntity.ASOCIATION_OVERRIDE_TWO,
joinColumns =
@JoinColumn(name = "changeMeWhenExtends")),
@AssociationOverride(name = JpaMany3ManyEntity.ASOCIATION_OVERRIDE_THREE,
joinColumns =
@JoinColumn(name = "changeMeWhenExtends"))})
public abstract class JpaMany3ManyEntity<A extends JpaBaseEntity, B extends JpaBaseEntity, C extends JpaBaseEntity> extends JpaBaseEntity {

public static final String ID_ATTNAME = "id";

public static final String ASOCIATION_OVERRIDE_ONE = JpaMany3ManyEntity.ID_ATTNAME + "." + JpaMany3ManyId.ID_ONE_ATTNAME;

public static final String ASOCIATION_OVERRIDE_TWO = JpaMany3ManyEntity.ID_ATTNAME + "." + JpaMany3ManyId.ID_TWO_ATTNAME;

public static final String ASOCIATION_OVERRIDE_THREE = JpaMany3ManyEntity.ID_ATTNAME + "." + JpaMany3ManyId.ID_THREE_ATTNAME; 

 ...
 }

最佳答案

这里有一个对我有用的解决方法:代理实体。

即使拥有实体的父代理 (jpa.inheritance.issue.Element_$$_javassist_1)，如果您取消代理，您将获得真实的实体(子实体)。

假设您想要从实体 A 循环您的(子)元素集合并用它们做一些事情。

类似于:

public void  loopDeproxyElements(List<Element> yourElementsCollection){
  for(Element p : yourElementsCollection){
      if(p instanceof HibernateProxy){
        Element child =   (Element) ((HibernateProxy) p).getHibernateLazyInitializer()
                    .getImplementation();

        if (child instanceof ElementTypeOne){

         //You can cast your object or do whatever you want, knowing for sure that's a child element)

          ElementTypeOne myRealElement =  (ElementTypeOne) child;
          ...
          } else {
           //It should be ElementTypeTwo (if u never create parent entities)
           ...
      }
    }        
  }
)

它总是会像我期望的那样获取子元素。

关于java - 继承 JPA 和 Hibernate 问题，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/24488455/