Hibernate Inheritance: Table Per Concrete Class (Annotation & XML mapping)

Welcome to Hibernate Tutorial Series. In previous tutorials we saw how to implement Inheritance in Hibernate: One Table per Subclass.

Today we will see how to implement Hibernate Inheritance: One Table per Concrete Class scheme.

You may want to look at previous tutorials. Here is the complete list for you.

Introduction to Inheritance in Hibernate

Java is an object oriented language. It is possible to implement Inheritance in Java. Inheritance is one of the most visible facets of Object-relational mismatch. Object oriented systems can model both “is a” and “has a” relationship. Relational model supports only “has a” relationship between two entities. Hibernate can help you map such Objects with relational tables. But you need to choose certain mapping strategy based on your needs.

One Table per Concrete Class example

Suppose we have a class Person with subclasses Employee and Owner. Following the class diagram and relationship of these classes.

In One Table per Concrete class scheme, each concrete class is mapped as normal persistent class. Thus we have 3 tables; PERSON, EMPLOYEE and OWNER to persist the class data. In this scheme, the mapping of the subclass repeats the properties of the parent class.

Following are the advantages and disadvantages of One Table per Subclass scheme.

Advantages

• This is the easiest method of Inheritance mapping to implement.

Disadvantages

• Data thats belongs to a parent class is scattered across a number of subclass tables, which represents concrete classes.
• This hierarchy is not recommended for most cases.
• Changes to a parent class is reflected to large number of tables
• A query couched in terms of parent class is likely to cause a large number of select operations

This strategy has many drawbacks (esp. with polymorphic queries and associations) explained in the JPA spec, the Hibernate reference documentation, Hibernate in Action, and many other places. Hibernate work around most of them implementing this strategy using SQL UNION queries. It is commonly used for the top level of an inheritance hierarchy:

Create Database Table to persist Concrete classes

CREATE TABLE `person` (
	`person_id` BIGINT(20) NOT NULL AUTO_INCREMENT,
	`firstname` VARCHAR(50) NOT NULL DEFAULT '0',
	`lastname` VARCHAR(50) NOT NULL DEFAULT '0',
	PRIMARY KEY (`person_id`)
)

CREATE TABLE `employee` (
	`person_id` BIGINT(10) NOT NULL AUTO_INCREMENT,
	`firstname` VARCHAR(50) NOT NULL,
	`lastname` VARCHAR(50) NOT NULL,
	`joining_date` DATE NULL DEFAULT NULL,
	`department_name` VARCHAR(50) NULL DEFAULT NULL,
	PRIMARY KEY (`person_id`)
)

CREATE TABLE `owner` (
	`person_id` BIGINT(20) NOT NULL AUTO_INCREMENT,
	`firstname` VARCHAR(50) NOT NULL DEFAULT '0',
	`lastname` VARCHAR(50) NOT NULL DEFAULT '0',
	`stocks` BIGINT(11) NULL DEFAULT NULL,
	`partnership_stake` BIGINT(11) NULL DEFAULT NULL,
	PRIMARY KEY (`person_id`)
)

Project Structure in Eclipse

For XML mapping

For Annotation mapping

Hibernate Inheritance: XML Mapping

Following is the example where we map Person, Employee and Owner entity classes using XML mapping.

File: Person.java

package net.viralpatel.hibernate;

public class Person {

	private Long personId;
	private String firstname;
	private String lastname;

	// Constructors and Getter/Setter methods, 
}

File: Employee.java

package net.viralpatel.hibernate;

import java.util.Date;

public class Employee extends Person {

	private Date joiningDate;
	private String departmentName;

	// Constructors and Getter/Setter methods, 
}

File: Owner.java

package net.viralpatel.hibernate;

public class Owner extends Person {

		private Long stocks;
		private Long partnershipStake;

	// Constructors and Getter/Setter methods, 
}

File: Person.hbm.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="net.viralpatel.hibernate">

	<class name="Person" table="PERSON">
		<id name="personId" column="PERSON_ID">
			<generator class="native" />
		</id>
		<property name="firstname" />
		<property name="lastname" column="lastname" />

	</class>

	<class name="Employee">
		<id name="personId" column="PERSON_ID">
			<generator class="native" />
		</id>
		<property name="firstname" />
		<property name="lastname" column="lastname" />
		<property name="departmentName" column="department_name" />
		<property name="joiningDate" type="date" column="joining_date" />
	</class>

	<class name="Owner">
		<id name="personId" column="PERSON_ID">
			<generator class="native" />
		</id>
		<property name="firstname" />
		<property name="lastname" column="lastname" />
		<property name="stocks" column="stocks" />
		<property name="partnershipStake" column="partnership_stake" />
	</class>
</hibernate-mapping>

Note that we have defined only one hibernate mapping (hbm) file Person.hbm.xml. Both Person and Employee model class are defined within one hbm file.

We mapped all the classes using simple <class> tag in hbm. This is the usual way of mapping classes in XML.

Hibernate Inheritance: Annotation Mapping

Following is the example where we map Person, Employee and Owner entity classes using JPA Annotations.

File: Person.java

package net.viralpatel.hibernate;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.Inheritance;
import javax.persistence.InheritanceType;
import javax.persistence.Table;


@Entity
@Table(name = "PERSON")
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public class Person {

	@Id
	@Column(name = "PERSON_ID")
	private Long personId;
	
	@Column(name = "FIRSTNAME")
	private String firstname;
	
	@Column(name = "LASTNAME")
	private String lastname;
	
	public Person() {
	
	}
	public Person(String firstname, String lastname) {
		this.firstname = firstname;
		this.lastname = lastname;
	}
	// Getter and Setter methods, 
}

@Inheritance – Defines the inheritance strategy to be used for an entity class hierarchy. It is specified on the entity class that is the root of the entity class hierarchy.

@InheritanceType – Defines inheritance strategy options. TABLE_PER_CLASS is a strategy to map table per concrete class.

File: Employee.java

package net.viralpatel.hibernate;

import java.util.Date;

import javax.persistence.AttributeOverride;
import javax.persistence.AttributeOverrides;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Table;

@Entity
@Table(name="EMPLOYEE")
@AttributeOverrides({
    @AttributeOverride(name="firstname", column=@Column(name="FIRSTNAME")),
    @AttributeOverride(name="lastname", column=@Column(name="LASTNAME"))
})
public class Employee extends Person {

	@Column(name="joining_date")
	private Date joiningDate;
	
	@Column(name="department_name")
	private String departmentName;

	public Employee() {
	}
	
	public Employee(String firstname, String lastname, String departmentName, Date joiningDate) {
		
		super(firstname, lastname);
		
		this.departmentName = departmentName;
		this.joiningDate = joiningDate;
	}
	
	// Getter and Setter methods, 
}

File: Owner.java

package net.viralpatel.hibernate;

import javax.persistence.AttributeOverride;
import javax.persistence.AttributeOverrides;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Table;

@Entity
@Table(name="OWNER")
@AttributeOverrides({
    @AttributeOverride(name="firstname", column=@Column(name="FIRSTNAME")),
    @AttributeOverride(name="lastname", column=@Column(name="LASTNAME"))
})
public class Owner extends Person {

	@Column(name="stocks")
	private Long stocks;
	
	@Column(name="partnership_stake")
	private Long partnershipStake;

	public Owner() {
	}
	
	public Owner(String firstname, String lastname, Long stocks, Long partnershipStake) {
		
		super(firstname, lastname);
		
		this.stocks = stocks;
		this.partnershipStake = partnershipStake;
	}

	// Getter and Setter methods, 
}

Both Employee and Owner classes are child of Person class. Thus while specifying the mappings, we used @AttributeOverrides to map them.

@AttributeOverrides – This annotation is used to override mappings of multiple properties or fields.

@AttributeOverride – The AttributeOverride annotation is used to override the mapping of a Basic (whether explicit or default) property or field or Id property or field.

The AttributeOverride annotation may be applied to an entity that extends a mapped superclass or to an embedded field or property to override a basic mapping defined by the mapped superclass or embeddable class. If the AttributeOverride annotation is not specified, the column is mapped the same as in the original mapping.

Thus in over case, firstname and lastname are defined in parent class Person and mapped in child class Employee and Owner using @AttributeOverrides annotation.

This strategy supports one-to-many associations provided that they are bidirectional. This strategy does not support the IDENTITY generator strategy: the id has to be shared across several tables. Consequently, when using this strategy, you should not use AUTO nor IDENTITY. Note that in below Main class we specified the primary key explicitly.

Main class

package net.viralpatel.hibernate;

import java.util.Date;

import org.hibernate.Session;
import org.hibernate.SessionFactory;

public class Main {

	public static void main(String[] args) {

		SessionFactory sf = HibernateUtil.getSessionFactory();
		Session session = sf.openSession();
		session.beginTransaction();

		
		Person person = new Person("Steve", "Balmer");
		person.setPersonId(1L);
		session.save(person);

		Employee employee = new Employee("James", "Gosling", "Marketing", new Date());
		employee.setPersonId(2L);
		session.save(employee);

		Owner owner = new Owner("Bill", "Gates", 300L, 20L);
		owner.setPersonId(3L);
		session.save(owner);

		
		session.getTransaction().commit();
		session.close();

	}
}

The Main class is used to persist Person, Employee and Owner object instances. Note that these classes are persisted in different tables and parent attributes (firstname, lastname) are repeated across all tables.

Output

Hibernate: insert into PERSON (FIRSTNAME, LASTNAME, PERSON_ID) values (?, ?, ?)
Hibernate: insert into EMPLOYEE (FIRSTNAME, LASTNAME, department_name, joining_date, PERSON_ID) values (?, ?, ?, ?, ?)
Hibernate: insert into OWNER (FIRSTNAME, LASTNAME, partnership_stake, stocks, PERSON_ID) values (?, ?, ?, ?, ?)

Download Source Code

Hibernate-inheritance-table-per-concrete-class_xml.zip (9 KB)
Hibernate-inheritance-table-per-concrete-class_annotation.zip (9 KB)