JPA with Java Generics

Question

我有几个具有基本LongID和其他属性的实体.这些实体与另一个实体具有一对多的关系,该实体为第一个实体保存不同的自定义(用户输入的)翻译.基本上是实体对,其中一个包含所有非翻译内容,另一个包含文本属性的多个翻译.

为了减少代码和注释的重复,我想为其中一对中的每个实体创建一个抽象类.对于多个翻译,我创建了一个这样的类:

@MappedSuperclass
public abstract class CustomTranslations
{
    @Id
    protected Long id;
    @Id
    protected String locale;
}

对于主要实体,我有这个:

@MappedSuperclass
public abstract class CustomTranslationsHolder<T extends CustomTranslations>
{
    @OneToMany(fetch=FetchType.EAGER)
    @JoinColumn(name="ID")
    @MapKey(name="locale")
    protected Map<String, T> translationsByLocale;
}

所以说其中一个实体对是Foo.我会这样的:

@Entity
@Table(name="FOO_TRANSLATIONS")
public class FooTranslations extends CustomTranslations
{
    private String title;
    private String description;
    ....
}

还有这个:

@Entity
public class Foo extends CustomTranslationsHolder<FooTranslations>
{
    @Id
    private Long id;
    private String whatever;
    private Integer blah;
    ...
    public String getTitle(String locale)
    {
        return translationsByLocale.get(locale).getTitle();
    }
}

这一切都编译得很好,但在服务器启动时我收到以下错误:

Exception Description: Neither the instance method or field named [locale] exists for the item class [class java.lang.Void], and therefore cannot be used to create a key for the Map.
at org.eclipse.persistence.exceptions.ValidationException.mapKeyNotDeclaredInItemClass(ValidationException.java:1332)
at org.eclipse.persistence.internal.queries.MapContainerPolicy.initializeKey(MapContainerPolicy.java:517)
at org.eclipse.persistence.internal.queries.MapContainerPolicy.getKeyType(MapContainerPolicy.java:438)
at org.eclipse.persistence.internal.jpa.metamodel.MapAttributeImpl.<init>(MapAttributeImpl.java:167)
at org.eclipse.persistence.internal.jpa.metamodel.ManagedTypeImpl.initialize(ManagedTypeImpl.java:1158)
at org.eclipse.persistence.internal.jpa.metamodel.MetamodelImpl.initialize(MetamodelImpl.java:459)
at org.eclipse.persistence.internal.jpa.metamodel.MetamodelImpl.<init>(MetamodelImpl.java:111)
at org.eclipse.persistence.internal.jpa.metamodel.MetamodelImpl.<init>(MetamodelImpl.java:130)
at org.eclipse.persistence.internal.jpa.EntityManagerSetupImpl.getMetamodel(EntityManagerSetupImpl.java:2566)
at org.eclipse.persistence.internal.jpa.EntityManagerFactoryDelegate.getMetamodel(EntityManagerFactoryDelegate.java:592)
at org.eclipse.persistence.internal.jpa.EntityManagerFactoryImpl.getMetamodel(EntityManagerFactoryImpl.java:506)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.springframework.orm.jpa.AbstractEntityManagerFactoryBean.invokeProxyMethod(AbstractEntityManagerFactoryBean.java:376)
at org.springframework.orm.jpa.AbstractEntityManagerFactoryBean$ManagedEntityManagerFactoryInvocationHandler.invoke(AbstractEntityManagerFactoryBean.java:517)
at $Proxy8.getMetamodel(Unknown Source)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.springframework.orm.jpa.SharedEntityManagerCreator$SharedEntityManagerInvocationHandler.invoke(SharedEntityManagerCreator.java:176)
at $Proxy12.getMetamodel(Unknown Source)
at org.springframework.data.jpa.repository.support.JpaEntityInformationSupport.getMetadata(JpaEntityInformationSupport.java:60)
at org.springframework.data.jpa.repository.support.JpaRepositoryFactory.getEntityInformation(JpaRepositoryFactory.java:149)
at org.springframework.data.jpa.repository.support.JpaRepositoryFactory.getTargetRepository(JpaRepositoryFactory.java:87)
at org.springframework.data.jpa.repository.support.JpaRepositoryFactory.getTargetRepository(JpaRepositoryFactory.java:70)
at org.springframework.data.repository.core.support.RepositoryFactorySupport.getRepository(RepositoryFactorySupport.java:137)
at org.springframework.data.repository.core.support.RepositoryFactoryBeanSupport.getObject(RepositoryFactoryBeanSupport.java:125)
at org.springframework.data.repository.core.support.RepositoryFactoryBeanSupport.getObject(RepositoryFactoryBeanSupport.java:41)
at org.springframework.beans.factory.support.FactoryBeanRegistrySupport.doGetObjectFromFactoryBean(FactoryBeanRegistrySupport.java:142)
... 44 more

所以,它听起来就像是可以找到的财产locale上T的地图.它似乎应该知道这T是一个CustomTranslations,因此知道有一个locale字段CustomTranslations.

这只是EclipseLink的一个问题吗？或者我不能这样做吗？我有兴趣知道Hibernate如何处理这个完全相同的代码.任何有关此的信息或建议将不胜感激.

Answer 1

接受的答案是错误的.Generics似乎在JPA或EclipseLink中得不到很好的支持(可能是因为它们使用字节代码编织而不是反射).无论类型擦除如何,使用反射在运行时都可以获得该信息.

实际的运行时类型T被删除,但T扩展的事实CustomTranslations并非如此.这在编译时指定,并存储在可从Class实例访问的类元数据中.

您可以调用CustomTranslationsHolder.class.getTypeParameters()哪个将返回一个TypeVariable名为"T"的元素的数组,您可以调用哪个元素getBounds()将返回"CustomTranslations".因此,可以证明地图中的值包含locale属性.

我会向其他JPA提供商提交错误和/或测试.