I have this entity. It should work on both, internal and external, users.

@Entity(name = "TokenAuthentication")
class TokenAuthenticationEntity<T>(

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
var id: Long? = null,

...

@NotNull
@ManyToOne(fetch = FetchType.EAGER)
val user: T,

) : BaseEntity()

When I run this, I get

Property TokenAuthenticationEntity.user has an unbound type and no explicit target entity. Resolve this Generic usage issue or set an explicit target attribute

So Hibernate tells me to f*** off with my generics, it needs explicit definitions. Any of you have an idea how to get this running with generics?

CodePudding user response：

This is not possible. The reason being T type erasure at compile-time. This T type is used at compile-time to check types and guarantee type safety, but then it is removed. Having said that, each instance of TokenAuthenticationEntity can have a different value for T, and as explained before this information is lost.

What you can actually do is the following:

@MappedSuperclass
public abstract class TokenAuthenticationEntity<T> {
  private T user;
}

Now you can create your entities based on this generic superclass:

@Entity
public class InternalUser extends TokenAuthenticationEntity<IUser> { }

@Entity
public class ExternalUser extends TokenAuthenticationEntity<EUser> { }

Why? Because each concrete subclass of TokenAuthenticationEntity has a type T which is defined and is retained and unique inside the subclasses. In this case, JPA will store your subclasses in one or more tables, depending on the chosen @InheritanceStrategy (if the Inheritance annotation is not specified or if no inheritance type is specified for an entity class hierarchy, the SINGLE_TABLE mapping strategy is used). Additional details at https://www.baeldung.com/hibernate-inheritance#single-table.