JPA with Java Generics

Question

I have several entities with a basic Long ID and other properties. These entities have a one-to-many relationship with another entity that holds different custom (user-entered) translations for the first entities. There's basically entity pairs with one holding all the non-translation stuff and the other holding multiple translations for text properties.

In order to reduce duplication of code and annotations, I want to create an abstract class for each entity in one of these pairs. For the multiple translations I created a class like this:

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

For the primary entity I have this:

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

So say one of the entity pairs is for Foo. I would have this:

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

and this:

@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();
    }
}

This all compiles just fine, but on server startup I get the following error:

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

So, it sounds like it can find the property locale on T for the Map. It seems like it should know that T is a CustomTranslations and therefore know that there is a locale field on CustomTranslations.

Is this just an issue with EclipseLink? Or is there just no way I can do it like this? I'd be interested to know how Hibernate handles this exact same code. Any information or suggestions on this would be greatly appreciated.

Answer 1

JPA specification says:

Persistent relationships defined by a mapped superclass must be unidirectional.

That's why you cannot define CustomTranslationsHolder as Mapped Superclass, because it has relationship ManyToOne with another Mapped Superclass. This class might be an entity.

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

But in this case JPA will be unable to extract T type at runtime unfortunately.

I suggest reviewing your design.

Answer 2

The accepted answer is wrong. Generics just don't seem to be well supported in JPA, or in EclipseLink (perhaps because they use byte code weaving instead of reflection). The information is definitely available at runtime using reflection, regardless of type erasure.

The actual runtime type of T is erased, but the fact that T extends CustomTranslations is not. This is specified at compile time and is stored in the class metadata accessible from the Class instance.

You can call CustomTranslationsHolder.class.getTypeParameters() which will return an array with one TypeVariable element with name "T", on which you can call getBounds() which will return "CustomTranslations". So the values in the map can be proven to contain the locale property.

I would file a bug and/or test with other JPA providers.

Answer 3

It seems like it should know that T is a CustomTranslations

Due to Type Erasure it does not know this.

http://docs.oracle.com/javase/tutorial/java/generics/erasure.html

Java generics - type erasure - when and what happens