JPA - Single Table Strategy

Overview

Single-Table strategy takes all classes fields (both super and sub classes) and map them down into a single table known as SINGLE_TABLE strategy. Here discriminator value plays key role in differentiating the values of three entities in one table.

Let us consider the above example, TeachingStaff and NonTeachingStaff are the sub classes of class Staff. Remind the concept of inheritance (is a mechanism of inheriting the properties of super class by sub class) and therefore sid, sname are the fields which belongs to both TeachingStaff and NonTeachingStaff. Create a JPA project. All the modules of this project as follows:

Creating Entities

Create a package named com.tutorialspoint.eclipselink.entity under src package if not present. Create a new java class named Staff.java under given package. The Staff entity class is shown as follows:

Staff.java

package com.tutorialspoint.eclipselink.entity;

import java.io.Serializable;

import jakarta.persistence.DiscriminatorColumn;
import jakarta.persistence.Entity;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;
import jakarta.persistence.Id;
import jakarta.persistence.Inheritance;
import jakarta.persistence.InheritanceType;
import jakarta.persistence.Table;

@Entity
@Table
@Inheritance( strategy = InheritanceType.SINGLE_TABLE )
@DiscriminatorColumn( name = "type" )

public class Staff implements Serializable {
   private static final long serialVersionUID = 1L;
   @Id
   @GeneratedValue( strategy = GenerationType.AUTO )
   
   private int sid;
   private String sname;
   
   public Staff( int sid, String sname ) {
      super( );
      this.sid = sid;
      this.sname = sname;
   }
   
   public Staff( ) {
      super( );
   }
   
   public int getSid( ) {
      return sid;
   }
   
   public void setSid( int sid ) {
      this.sid = sid;
   }
   
   public String getSname( ) {
      return sname;
   }
   
   public void setSname( String sname ) {
      this.sname = sname;
   }
}

In the above code @DescriminatorColumn specifies the field name (type) and the values of it shows the remaining (Teaching and NonTeachingStaff) fields.

Create a subclass (class) to Staff class named TeachingStaff.java under the com.tutorialspoint.eclipselink.entity package. The TeachingStaff Entity class is shown as follows:

TeachingStaff.java

package com.tutorialspoint.eclipselink.entity;

import jakarta.persistence.DiscriminatorValue;
import jakarta.persistence.Entity;

@Entity
@DiscriminatorValue( value="TS" )
public class TeachingStaff extends Staff {
   private static final long serialVersionUID = 1L;
   private String qualification;
   private String subjectexpertise;

   public TeachingStaff( int sid, String sname, 
   
   String qualification,String subjectexpertise ) {
      super( sid, sname );
      this.qualification = qualification;
      this.subjectexpertise = subjectexpertise;
   }

   public TeachingStaff( ) {
      super( );
   }

   public String getQualification( ){
      return qualification;
   }

   public void setQualification( String qualification ){
      this.qualification = qualification;
   }

   public String getSubjectexpertise( ) {
      return subjectexpertise;
   }

   public void setSubjectexpertise( String subjectexpertise ){
      this.subjectexpertise = subjectexpertise;
   }
}

Create a subclass (class) to Staff class named NonTeachingStaff.java under the com.tutorialspoint.eclipselink.entity package. The NonTeachingStaff Entity class is shown as follows:

NonTeachingStaff.java

package com.tutorialspoint.eclipselink.entity;

import jakarta.persistence.DiscriminatorValue;
import jakarta.persistence.Entity;

@Entity
@DiscriminatorValue( value = "NS" )

public class NonTeachingStaff extends Staff {
   private static final long serialVersionUID = 1L;
   private String areaexpertise;

   public NonTeachingStaff( int sid, String sname, String areaexpertise ) {
      super( sid, sname );
      this.areaexpertise = areaexpertise;
   }

   public NonTeachingStaff( ) {
      super( );
   }

   public String getAreaexpertise( ) {
      return areaexpertise;
   }

   public void setAreaexpertise( String areaexpertise ){
      this.areaexpertise = areaexpertise;
   }
}

Persistence.xml

Persistence.xml file contains the configuration information of database and registration information of entity classes. The xml file is shown as follows:

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="3.0" xmlns="https://jakarta.ee/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="https://jakarta.ee/xml/ns/persistence https://jakarta.ee/xml/ns/persistence/persistence_3_0.xsd">
	<persistence-unit name="Eclipselink_JPA" transaction-type="RESOURCE_LOCAL">
	<provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
	<class>com.tutorialspoint.eclipselink.entity.Staff</class>
	<class>com.tutorialspoint.eclipselink.entity.NonTeachingStaff</class>
	<class>com.tutorialspoint.eclipselink.entity.TeachingStaff</class>
      <properties>
         <property name="jakarta.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/jpadb"/>
         <property name="jakarta.persistence.jdbc.user" value="guest"/>
         <property name="jakarta.persistence.jdbc.password" value="guest123"/>
         <property name="jakarta.persistence.jdbc.driver" value="com.mysql.cj.jdbc.Driver"/>
         <property name="eclipselink.logging.level" value="FINE"/>
         <property name="eclipselink.ddl-generation" value="create-tables"/>
      </properties>
	</persistence-unit>
</persistence>

Service class

Service classes are the implementation part of business component. Create a package under src package named com.tutorialspoint.eclipselink.service if not present.

Create a class named SaveClient.java under the given package to store Staff, TeachingStaff, and NonTeachingStaff class fields. The SaveClient class is shown as follows:

SaveClient.java

package com.tutorialspoint.eclipselink.service;

import jakarta.persistence.EntityManager;
import jakarta.persistence.EntityManagerFactory;
import jakarta.persistence.Persistence;
import com.tutorialspoint.eclipselink.entity.NonTeachingStaff;
import com.tutorialspoint.eclipselink.entity.TeachingStaff;

public class SaveClient {

   public static void main( String[ ] args ) {
   
      EntityManagerFactory emfactory = Persistence.createEntityManagerFactory( "Eclipselink_JPA" );
      EntityManager entitymanager = emfactory.createEntityManager( );
      entitymanager.getTransaction( ).begin( );

      //Teaching staff entity 
      TeachingStaff ts1=new TeachingStaff(1,"Gopal","MSc MEd","Maths");
      TeachingStaff ts2=new TeachingStaff(2, "Manisha", "BSc BEd", "English");
      
      //Non-Teaching Staff entity
      NonTeachingStaff nts1=new NonTeachingStaff(3, "Satish", "Accounts");
      NonTeachingStaff nts2=new NonTeachingStaff(4, "Krishna", "Office Admin");

      //storing all entities
      entitymanager.persist(ts1);
      entitymanager.persist(ts2);
      entitymanager.persist(nts1);
      entitymanager.persist(nts2);
      
      entitymanager.getTransaction().commit();
      
      entitymanager.close();
      emfactory.close();
   }
}

Output

After compilation and execution of the above program you will get notifications in the console panel of Eclipse IDE. Check MySQL workbench for output. The output in a tabular format is shown as follows:

Finally you will get single table which contains all three classs fields and differs with discriminator column named Type (field).