Tweaking Hibernate inheritance

In previous post, I had explained single table strategy of hibernate inheritance. In this post, I am going to explain how to tweak the table that is generated by hibernate.

@DiscriminatorColumn(name="product_type", discriminatorType=DiscriminatorType.STRING)

public class Product {

        ....

}

“DiscriminatorColumn” annotation is used to change the name of column DTYPE to “product_type”

@DiscriminatorValue("book_type")

public class Book extends Product{

        ....

}

“DiscriminatorValue” tells hibernate use the name “book_type” for decriminator column of the table. By default it takes as class name. You can see previous post, how it generates.

package myFirstHibernate;

import javax.persistence.DiscriminatorColumn;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.DiscriminatorType;

@Entity
@DiscriminatorColumn(name="product_type", discriminatorType=DiscriminatorType.STRING)
public class Product {
  @Id @GeneratedValue
  int id;
  private int noOfProducts;

  public int getNoOfProducts() {
    return noOfProducts;
  }

  public void setNoOfProducts(int noOfProducts) {
    this.noOfProducts = noOfProducts;
  }
  
}

package myFirstHibernate;

import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;

@Entity
@DiscriminatorValue("book_type")
public class Book extends Product{
  private String title;
  private String author;
  
  public String getTitle() {
    return title;
  }
  
  public void setTitle(String title) {
    this.title = title;
  }
  
  public String getAuthor() {
    return author;
  }
  
  public void setAuthor(String author) {
    this.author = author;
  }
  
}

package myFirstHibernate;

import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;

@Entity
@DiscriminatorValue("laptop_type")
public class Laptop extends Product {
  private String brand;
  private String model;
  
  public String getBrand() {
    return brand;
  }
  
  public void setBrand(String brand) {
    this.brand = brand;
  }
  
  public String getModel() {
    return model;
  }
  
  public void setModel(String model) {
    this.model = model;
  }
  
}

hibernate.cfg.xml

<?xml version='1.0' encoding='utf-8'?>
<!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>
  
    <!--  Database Connection settings -->
    <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
    <property name="connection.url">jdbc:mysql://localhost/sample</property>
    <property name="connection.username">root</property>
    <property name="connection.password">tiger</property>
    
    <!-- Enable the logging of all the generated SQL statements to the console -->
    <property name="show_sql">true</property>
    
    <!-- Format the generated SQL statement to make it more readable, -->
    <property name="format_sql">false</property>
    
    <!-- Hibernate will put comments inside all generated SQL statements to hint what’s the generated SQL trying to do -->
    <property name="use_sql_comments">false</property>
    
    <!-- This property makes Hibernate generate the appropriate SQL for the chosen database. -->
    <property name="dialect">org.hibernate.dialect.MySQLDialect</property>
    
    <!-- Drop and re-create the database schema on startup -->
    <property name="hbm2ddl.auto">create</property>
    
    <!-- mappings for annotated classes -->
    <mapping class="myFirstHibernate.Product" />
    <mapping class="myFirstHibernate.Book" />
     <mapping class="myFirstHibernate.Laptop" />
    
  </session-factory>
  
</hibernate-configuration>

package myFirstHibernate;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
import org.hibernate.cfg.Configuration;

public class TestInheritance {
  
  /* Step 1: Create session factory */
  private static SessionFactory getSessionFactory() {
    Configuration configuration = new Configuration().configure();
    StandardServiceRegistryBuilder builder = new StandardServiceRegistryBuilder().
    applySettings(configuration.getProperties());
    SessionFactory factory = configuration.buildSessionFactory(builder.build());
        return factory;
    }
  
  public static void main(String args[]){
    Product prod = new Product();
    Book book = new Book();
    Laptop laptop = new Laptop();
    
    prod.setNoOfProducts(2234);
    
    book.setAuthor("eckhart tolle");
    book.setNoOfProducts(1234);
    book.setTitle("The Power of Now");
    
    laptop.setBrand("Dell");
    laptop.setModel("Alienware 17 Laptop");
    laptop.setNoOfProducts(1000);
    
    /* To persist data */
    SessionFactory sessionFactory = getSessionFactory();
    Session session = sessionFactory.openSession();
    session.beginTransaction();
    session.save(prod);
    session.save(book);
    session.save(laptop);
    session.getTransaction().commit();
    session.close(); 
  }
}

Run TestInheritance class, you will get output like below.

Hibernate: drop table if exists Product
Hibernate: create table Product (product_type varchar(31) not null, id integer not null auto_increment, noOfProducts integer not null, author varchar(255), title varchar(255), brand varchar(255), model varchar(255), primary key (id))
Dec 22, 2014 10:37:14 AM org.hibernate.tool.hbm2ddl.SchemaExport execute
INFO: HHH000230: Schema export complete
Hibernate: insert into Product (noOfProducts, product_type) values (?, 'Product')
Hibernate: insert into Product (noOfProducts, author, title, product_type) values (?, ?, ?, 'book_type')
Hibernate: insert into Product (noOfProducts, brand, model, product_type) values (?, ?, ?, 'laptop_type')

MySQL structure looks like below.

mysql> describe product;
+--------------+--------------+------+-----+---------+----------------+
| Field        | Type         | Null | Key | Default | Extra          |
+--------------+--------------+------+-----+---------+----------------+
| product_type | varchar(31)  | NO   |     | NULL    |                |
| id           | int(11)      | NO   | PRI | NULL    | auto_increment |
| noOfProducts | int(11)      | NO   |     | NULL    |                |
| author       | varchar(255) | YES  |     | NULL    |                |
| title        | varchar(255) | YES  |     | NULL    |                |
| brand        | varchar(255) | YES  |     | NULL    |                |
| model        | varchar(255) | YES  |     | NULL    |                |
+--------------+--------------+------+-----+---------+----------------+
7 rows in set (0.01 sec)

mysql> select * from product;
+--------------+----+--------------+---------------+------------------+-------+---------------------+
| product_type | id | noOfProducts | author        | title            | brand | model               |
+--------------+----+--------------+---------------+------------------+-------+---------------------+
| Product      |  1 |         2234 | NULL          | NULL             | NULL  | NULL                |
| book_type    |  2 |         1234 | eckhart tolle | The Power of Now | NULL  | NULL                |
| laptop_type  |  3 |         1000 | NULL          | NULL             | Dell  | Alienware 17 Laptop |
+--------------+----+--------------+---------------+------------------+-------+---------------------+
3 rows in set (0.00 sec)

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