使用JPA（+ Hibernate）嵌入抽象类

Question

I am trying to implement this JPA model:

• Class Owner is an Entity . It has @Embedded instance of class AbstractParent .
• Class AbstractParent is @Embeddable . It is an abstract class, and this class has two children.The children are concrete classes.

The exception is: Cannot instantiate abstract class or interface: AbstractParent

It seems that (1) I need component inheritance, but (2) component inheritance is not allowed in JPA and (3) it was not implemented in Hibernate. Could you confirm that (1), (2) and (3) are correct? If they are correct could you provide any advice or workaround? I use hibernate-jpa 2.0, hibernate-core 3.5.1, hibernate-core-annotations 3.2.0

Answer 1

To answer questions why one may need this implementation: here is an example. Database table has information about computers, computers have monitors, if monitor is LCD then it's parameter is "pixels". If monitor is a TV then it's parameter is "lines". All monitors also have parameter "weight" Data structure of this table includes: id, RAM, monitor_indicator, weight, lines, pixels.

This data structure can be implemented in classes:

• Class Computer {id, RAM, display}
  • It has information about computers.
• Abstract class AbstractDisplay {weight}
  • It has generic information about displays.
• Class LCDDisplay {pixels} inherits class AbstractDisplay
  • it has LCD specific information (pixels).
• Class TVDisplay {lines} inherits class AbstractDisplay
  • it has TV specific information (lines).

The basic idea is that main class (Owner/Computer) embeds another class (Parent/AbstractDisplay) that can have different set of parameters depending on type of the embedded class.

Regarding how embedded abstract class can be instantiated: same way as an abstract class is instantiated now in JPA: in the above example "monitor_indicator" indicates children class that should be used, this field must exist during the time when class Computer is instantiated.

Answer 2

Embedding an Abstract class doesn't make any logical sense, it can't be instantiated if it is Abstract . You need to re-work your logic on why you think you need to embed an Abstract class and not a specific implementation.