JPA/Hibernate Difference MappedSuperclass and Entity Abstract Class

Question

I would like to know what is the difference between MappedSuperclass and Entity Abstract Class when one wants to derive from a super class in Hibernate. I know that Hibernate does not create a Table in the Database for a MappedSuperclass. I read in the JavaEE doc "Abstract entities are like concrete entities but cannot be instantiated". Since they cannot be instantiated, I deduce that there is no matching Table in the Database. Am I right?

If so, what are virtually the difference between using a MappedSuperclass and an Entity Abstract Class and what is the impact on the Software and in the Database in each case?

Answer 1

A MappedSuperclass uses inheritance for field and code reuse. For example, if you want all your entities to have a Long id and a Long version field, you could make them all extend a BaseEntity class annotated with MappedSuperclass containing these two fields, along with their getters, setters, etc. But you would never have an entity having an association with a BaseEntity: the association would always be with a specific subclass of BaseEntity.

A parent entity is used for "entity polymorphism". For example, you could imagine having two kinds of Message: an EmailMessage and a SmsMessage. Both would contain a source, a target, and a body. But EmailMessage would have an email address and a subject, whereas SmsMessage would have a phone number.

And you could imagine having a Person entity containing a collection of sent messages, of type Message. The collection would in fact contain instances of EmailMessage and of SmsMessage. Hibernate would decide which one to instantiate depending on the inheritance strategy used for the inheritance mapping:

• all the messages could be stored in the same table, and Hibernate would use a discriminator column containing the type of the message
• the EmailMessage could be stored in one table, and the SmsMessage stored in another one
• or the fields common to both entities (source, target, body) could be stored in a comon table, the fields specific to EmailMessage in a second table, and the fields specific to SmsMessage in a third table.