EclipseLink @MappedSuperclass和泛型

Question

我的Web应用程序中有一些域模型类,它们与自己有层次关系.一个示例是用于对用户发布进行分类的分层类别结构.

有一些逻辑与这些类的层次性有关,这是常见的.所以我试图将逻辑移到一个通用的@MappedSuperclass注释超类中.

就像是 :

@MappedSuperclass
public abstract class HierarchicalBaseEntity<N extends HierarchicalBaseEntity<N>>
        extends BaseEntity {

    @ManyToOne(optional=true)
    @JoinColumn(name="parent")
    private N parent;

    private int depth;

    public N getParent() { ...
    public void setParent(N newParent) { ...

    public boolean isRoot() { ...
    public int getDepth() { ...

    public boolean isDescendantOf(N ancestor) { ...
    public static <N extends HierarchicalBaseEntity<N>> N getCommonAncestor(N a, N b) { ...
    public static <N extends HierarchicalBaseEntity<N>> Collection<N> reduceToCommonAncestors(Collection<N> entities) { ...
}

然后子类扩展HierarchicalBaseEntity,将自己作为泛型类型N:

@Entity
public class CategoryBean extends HierarchicalBaseEntity<CategoryBean> {

在Java中,这一切都非常干净.但不幸的是,EclipseLink似乎不喜欢通用的"父"字段:

private N parent;

它给出了以下例外:

Caused by: Exception [EclipseLink-7250] (Eclipse Persistence Services - 2.1.0.v20100614-r7608): org.eclipse.persistence.exceptions.ValidationException
Exception Description: [class net.timp.yaase.core.model.HierarchicalBaseEntity] uses a non-entity [class java.lang.String] as target entity in the relationship attribute [field parent].
at org.eclipse.persistence.exceptions.ValidationException.nonEntityTargetInRelationship(ValidationException.java:1341)
at org.eclipse.persistence.internal.jpa.metadata.accessors.mappings.RelationshipAccessor.getReferenceDescriptor(RelationshipAccessor.java:416)
at org.eclipse.persistence.internal.jpa.metadata.accessors.mappings.ObjectAccessor.processOneToOneForeignKeyRelationship(ObjectAccessor.java:609)
at org.eclipse.persistence.internal.jpa.metadata.accessors.mappings.ObjectAccessor.processOwningMappingKeys(ObjectAccessor.java:678)
at org.eclipse.persistence.internal.jpa.metadata.accessors.mappings.ManyToOneAccessor.process(ManyToOneAccessor.java:107)

为什么抱怨非实体字符串？

作为测试,我尝试删除泛型,并将父字段定义为:

private HierarchicalBaseEntity parent;

没有泛型,EclipseLink给出了这个例外:

Caused by: Exception [EclipseLink-7250] (Eclipse Persistence Services - 2.1.0.v20100614-r7608): org.eclipse.persistence.exceptions.ValidationException
Exception Description: [class net.timp.yaase.core.model.OnymBean] uses a non-entity [class net.timp.yaase.core.model.HierarchicalBaseEntity] as target entity in the relationship attribute [field parent].
at org.eclipse.persistence.exceptions.ValidationException.nonEntityTargetInRelationship(ValidationException.java:1341)
at org.eclipse.persistence.internal.jpa.metadata.accessors.mappings.RelationshipAccessor.getReferenceDescriptor(RelationshipAccessor.java:416)
at org.eclipse.persistence.internal.jpa.metadata.accessors.mappings.ObjectAccessor.processOneToOneForeignKeyRelationship(ObjectAccessor.java:609)
at org.eclipse.persistence.internal.jpa.metadata.accessors.mappings.ObjectAccessor.processOwningMappingKeys(ObjectAccessor.java:678)
at org.eclipse.persistence.internal.jpa.metadata.accessors.mappings.ManyToOneAccessor.process(ManyToOneAccessor.java:107)

True HierarchicalBaseEntity在任何一种情况下都不是实体,是@MappedSuperclass ..但有没有办法用泛型或其他方式做到这一点？看来你的@MappedSuperclass中没有一个字段可以引用它的一个子类.

Answer 1

问题是，当使用泛型作为关系的字段类型时，EclipseLink 无法知道目标类型是什么，直到运行时检查实际实例。因此映射必须在运行时动态创建，而这是不受支持的。

您可以继续使用泛型超类，但这需要将字段移动到实体，在那里它们将定义类型，然后为那些返回对象的字段提供抽象的内部 getter/setter，泛型方法将调用转换为泛型类型。令人费解，但它允许通用 MappedSuperclass。