There are three types of inheritance mapping strategies −
Table per class hierarchy
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In this, only a single table is created for inheritance mapping. Disadvantages of this approach is that a lot of null values gets stored in the table.
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@Inheritance(strategy=InheritanceType.SINGLE_TABLE), @DiscriminatorColumn and @DiscriminatorValue are the annotations used in this strategy.
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@DiscriminatorColumn is used to create an additional column which is used to identify the hierarchy classes.
Consider the following example to understand this −
Steps to implement −
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Create entity classes and use proper annotations for them.
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Write the hibernate configuration file and add the mapping classes.
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Write the code to create and store the data in the table.
1.CREATE ENTITY CLASSES
Car.j**a
package com.tutorialspoint; @Entity @Table(name = "car") @Inheritance(strategy=InheritanceType.SINGLE_TABLE) @DiscriminatorColumn(name="category",discriminatorType=DiscriminatorType.STRING) @DiscriminatorValue(value="car") public class Car { @Id @GeneratedValue(strategy=GenerationType.AUTO) private int id; @Column(name = "name") private String name; @Column(name = "color") private String color; }
Sports_Car.j**a
package com.tutorialspoint; import j**ax.persistence.*; @Entity @DiscriminatorValue("sportscar") public class Sports_Car extends Car{ @Column(name="mileage") private int mileage; @Column(name="cost") private int cost; }
Taxi_Car.j**a
package com.tutorialspoint; import j**ax.persistence.*; @Entity @DiscriminatorValue("taxicar") public class Taxi_Car extends Car{ @Column(name="farePerKm") private int farePerKm; @Column(name="**ailable") private boolean **ailable; }
2. Hibernate Configuration file (hibernate.cfg.xml)
<!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd"> <hibernate-configuration> <session-factory> <property name="connection.driver_class">com.mysql.cj.jdbc.Driver</property> <property name="connection.url">jdbc:mysql://localhost:3306/demo?useSSL=false</property> <property name="connection.username">root</property> <property name="connection.password">root</property> <property name="connection.pool_size">4</property> <property name="show_sql">true</property> <property name="dialect">org.hibernate.dialect.MySQL5Dialect</property> <property name="hbm2ddl.auto">create-drop</property> <mapping class="com.tutorialspoint.Car"/> <mapping class="com.tutorialspoint.Sports_Car"/> <mapping class="com.tutorialspoint.Taxi_Car"/> </session-factory> </hibernate-configuration>
3. Code to create a table and store the data
package com.tutorialspoint; import org.hibernate.Session; import org.hibernate.SessionFactory; import org.hibernate.Transaction; import org.hibernate.cfg.Configuration; public class StoreTest { public static void main(String args[]){ SessionFactory sessionFactory = new Configuration() .configure("com/tutorialspoint/hibernate.cfg.xml") .buildSessionFactory(); Session session=sessionFactory.openSession(); Transaction t=session.beginTransaction(); Car c1=new Car(); c1.setName("Mercedes"); c1.setColor("Black"); Sport_Car c2=new Sport_Car(); c2.setName("Porsche"); c2.setColor("Red"); c2.setMileage(20); c2.setCost(5000000); Taxi_Car c3=new Taxi_Car(); c3.setName("Innova"); c3.setColor("White"); c3.setFarePerKm(7); c3.setAvailable(true); session.persist(c1); session.persist(c2); session.persist(c3); t.commit(); session.close(); } }
MySQL Table
