Hibernate Inheritance Mapping (Strategy 4) - Table per subclass

In strategy-4, each persistent class in the inheritance hierarchy, whether concrete or abstract, is mapped to a table each in the Database schema. The inheritance relationship between the domain classes is reflected in the mapping files through special XML elements and in the Database schema as relationship foreign-key associations between the tables.

1) Create the persistent classes inheritance hierarchy with S4 as the superclass and A4 and B4 as the subclasses that extend from the superclass.

S4.java

package info.icontraining.hibernate;

public abstract class S4 {

   private Long id;
   private String p1;
   private String p2;
 
   public Long getId() {
      return id;
   }
   public void setId(Long id) {
      this.id = id;
   } 
   public String getP1() {
      return p1;
   }
   public void setP1(String p1) {
      this.p1 = p1;
   }
   public String getP2() {
      return p2;
   }
   public void setP2(String p2) {
      this.p2 = p2;
   }
}

A4.java

package info.icontraining.hibernate;

public class A4 extends S4 {

   private String p3;
   private String p4;
 
   public A4() { }
 
   public A4(String p1, String p2, String p3, String p4) {
      this.setP1(p1);
      this.setP2(p2);
      this.setP3(p3);
      this.setP4(p4);
   }
 
   public String getP3() {
      return p3;
   }
   public void setP3(String p3) {
      this.p3 = p3;
   }
   public String getP4() {
      return p4;
   }
   public void setP4(String p4) {
      this.p4 = p4;
   }
}

B4.java

package info.icontraining.hibernate;

public class B4 extends S4 {

   private String p5;
   private String p6;
 
   public B4() { }
 
   public B4(String p1, String p2, String p5, String p6) {
      this.setP1(p1);
      this.setP2(p2);
      this.setP5(p5);
      this.setP6(p6);
   }
 
   public String getP5() {
      return p5;
   }
   public void setP5(String p5) {
      this.p5 = p5;
   }
   public String getP6() {
      return p6;
   }
   public void setP6(String p6) {
      this.p6 = p6;
   }
}

2) Create the mapping file, AB4.hbm.xml,

AB4.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> 
   <class name="info.icontraining.hibernate.S4" table="S4_TABLE" > 
      <id name="id" column="ID" type="long" >
         <generator class="native" /> 
      </id> 
      <property name="p1" column="P1" type="string" />
      <property name="p2" column="P2" type="string" />
      <joined-subclass name="info.icontraining.hibernate.A4" table="A4_TABLE">
         <key column="A4_ID"/>
         <property name="p3" column="P3" type="string" />
         <property name="p4" column="P4" type="string" />
      </joined-subclass>
      <joined-subclass name="info.icontraining.hibernate.B4" table="B4_TABLE">
         <key column="B4_ID" />
         <property name="p5" column="P5" type="string" />
         <property name="p6" column="P6" type="string" />
      </joined-subclass>
   </class> 
</hibernate-mapping>

3) In the hibernate.cfg.xml file, add the following configuration,

<mapping resource="info/icontraining/hibernate/AB4.hbm.xml" />

4) Create the client code in a servlet class, as follows,

package info.icontraining.servlets;

import java.io.*; 
import java.util.*; 
import javax.servlet.*; 
import javax.servlet.http.* ; 
import org.hibernate.*; 
import info.icontraining.hibernate.*;

public class InheritanceDemo4 extends HttpServlet { 
  
   public void doGet(HttpServletRequest req, HttpServletResponse res) throws IOException, ServletException{ 
  
      PrintWriter out = res.getWriter(); 

      Session session = HibernateUtil.getSessionFactory().openSession(); 
      Transaction tx = session.beginTransaction(); 
      A4 a1 = new A4("str1", "str2", "str3", "str4");
      B4 b1 = new B4("str1", "str2", "str5", "str6");
      session.save(a1); 
      session.save(b1);
      tx.commit(); 
      session.close(); 

      Session newSession = HibernateUtil.getSessionFactory().openSession(); 
      Transaction newTransaction = newSession.beginTransaction(); 
      List<S4> a = newSession.createQuery("from S4").list(); 
      out.println( a.size() + " items found:" ); 
      for ( Iterator<S4> iter = a.iterator(); iter.hasNext(); ) { 
         S4 a2 = iter.next(); 
         out.println( a2.getP1() ); 
      } 
   
      newTransaction.commit(); 
      newSession.close(); 
   } 
}

5) Configure the servlet in the web.xml file,

<servlet>
   <servlet-name>InheritanceDemo4</servlet-name>
   <servlet-class>info.icontraining.servlets.InheritanceDemo4</servlet-class>
</servlet>
<servlet-mapping>
   <servlet-name>InheritanceDemo4</servlet-name>
   <url-pattern>/inheritanceDemo4</url-pattern>
</servlet-mapping>

6) Enter the following URL in the browser to test,

http://localhost:8080/WebAppName/inheritanceDemo4