Inheritance in Hibernate Annotations?

Question

How can I configure Hibernate inheritance mappings using Java annotations? What are the advantages of using inheritance in Annotations?

Answer 1

3 possible types :

1. Single table per class hierarchy strategy:

 @Entity  @Inheritance(strategy=InheritanceType.SINGLE_TABLE)  @DiscriminatorColumn(      name="planetype",   discriminatorType=DiscriminatorType.STRING      )  @DiscriminatorValue("Plane")  public class Plane { ... }   @Entity  @DiscriminatorValue("A320")  public class A320 extends Plane { ... }         <hibernate-mapping>  <subclass name="DomesticCat" extends="Cat" discriminator-value="D">       <property name="name" type="string"/>  </subclass> 

• Pros : Simplest. No JOINs required
• Cons : Cannot use nulls. # of columns increase with the Object graph depth.

2. Joined subclass strategy:

Database Tables

CREATE TABLE SUPER_TABLE( id_col number primary key, sup_Name varchar2(20));  CREATE TABLE  SUB_TABLE( SUP_ID primary key, sub_name varchar2(20), constraint SUB_TABLE_fk foreign key (sup_Id)  references    super_table(id_col));   @Entity @Table(name= "SUPER_TABLE") @Inheritance(strategy= InheritanceType.JOINED)     public class TestSuperClass { @Id @GeneratedValue(         strategy=GenerationType.SEQUENCE,          generator="SEQ_GEN")     @SequenceGenerator(         name="SEQ_GEN",         sequenceName="hibernate_sequence"     )     @Column(name ="id_col") private long idcol; @Column(name ="sup_name") private String supName;   @Entity  @Table(name="SUB_TABLE") @PrimaryKeyJoinColumn(name="SUP_ID")  <class name="Payment" table="PAYMENT"> <id name="id" type="long" column="PAYMENT_ID">     <generator class="native"/> </id> <property name="amount" column="AMOUNT"/> ... <joined-subclass name="CreditCardPayment" table="CREDIT_PAYMENT">     <key column="PAYMENT_ID"/>     <property name="creditCardType" column="CCTYPE"/>     ... </joined-subclass> <joined-subclass name="CashPayment" table="CASH_PAYMENT">     <key column="PAYMENT_ID"/>     ... </joined-subclass>  public class TestSubClass extends TestSuperClass{      private String sub_name;  } 

Test Module

TestSubClass sub = new TestSubClass("sub1"); sub.setSupName("supersuper"); session1.save(sub); 

SQL Generated

 Hibernate: insert into SUPER_TABLE (sup_name, id_col) values (?, ?)      Hibernate: insert into SUB_TABLE (sub_name, SUP_ID) values (?, ?) 

• Pros : Normalized data structures.
• Cons : JOINS are always reqd.

3. Table per concrete class strategy:

create table CREDIT_CARD( payment_id number primary key, amount number, creditCardType varchar2(2) );     @Entity   @Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)   public abstract class Payment { @Id @GeneratedValue(         strategy=GenerationType.SEQUENCE,          generator="SEQ_GEN")     @SequenceGenerator(         name="SEQ_GEN",         sequenceName="hibernate_sequence"     ) @Column(name = "payment_id") private long id;  private double amount;     @Entity  @Table(name="CREDIT_CARD")  public class CreditCardPayment extends Payment { private String creditCardType;    <class name="Payment"> <id name="id" type="long" column="PAYMENT_ID">     <generator class="sequence"/> </id> <property name="amount" column="AMOUNT"/> ... <union-subclass name="CreditCardPayment" table="CREDIT_PAYMENT">     <property name="creditCardType" column="CCTYPE"/>     ... </union-subclass> 

Test module

CreditCardPayment credit = new CreditCardPayment("C",1.0); session1.save(credit); 

SQL Generated

  Hibernate: insert into CREDIT_CARD (amount, creditCardType, payment_id) values (?, ?, ?) 

There's also a @MappedSuperClass which we have used in our application.

Answer 2

This is a very general question, but I'd advise taking a look at the following resources:

• The documentation on how inheritance is declared via Hibernate annotations.
• This PDF file (chapter 2 of a book on Hibernate). Page 38 forwards deals with Hibernate annotations.

But the very basic answer to your question is that you should use the @Inheritance annotation, like so:

@Entity @Inheritance(strategy = InheritanceType.TABLE_PER_CLASS) public class Flight implements Serializable {    ... }