<< Back to Hibernate Tutorial

In the previous article, we discussed the mapped superclass inheritance strategy. In this article, we will discuss The single table strategy which maps all entities of the inheritance structure to the same database table. This approach makes polymorphic queries very efficient and provides the best performance.

Note that when no explicit inheritance strategy is registered, Hibernate/JPA will choose the SINGLE_TABLE inheritance strategy by default.

SINGLE_TABLE inheritance performs the best in terms of executed SQL statements. However, you cannot use NOT NULL constraints on the column level. You can still use triggers and rules to enforce such constraints, but it’s not as straightforward.

Creating Models + Hibernate/JPA Mappings

Each subclass in a hierarchy must define a unique discriminator value, which is used to differentiate between rows belonging to separate subclass types. If this is not specified, the DTYPE column is used as a discriminator, storing the associated subclass name.

The inheritance strategy is defined on the abstract superclass, using the @Inheritance annotation.

In this example, we used InheritanceType.SINGLE_TABLE. This means all concrete subclasses will be stored in one table. You can optionally specify a discriminator column name. This column is registered by the @DiscriminatorColumn if omitted the default DTYPE name is used.

Hibernate/JPA Single Table Inheritance Step-By-Step Example

Let's demonstrate the SINGLE_TABLE inheritance strategy with a complete hibernate example.

Technologies and tools used

Hibernate 6.1.7.Final
IDE - Eclipse
Maven 3.5.3
JavaSE 17
MySQL - 8.0.32

Development Steps

Create a Simple Maven Project
Project Directory Structure
Add jar Dependencies to pom.xml
Creating the JPA Entities
Create a Hibernate configuration file - hibernate.cfg.xml
Create a Hibernate utility class
Create the Main class and Run an Application

1. Create a Simple Maven Project

Use the How to Create a Simple Maven Project in Eclipse article to create a simple Maven project in Eclipse IDE.

2. Project Directory Structure

The project directory structure for your reference -

3. Add jar Dependencies to pom.xml

<project
    xmlns="http://maven.apache.org/POM/4.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>net.javaguides.hibernate</groupId>
        <artifactId>hibernate-tutorial</artifactId>
        <version>0.0.1-SNAPSHOT</version>
    </parent>
    <artifactId>hibernate-singletable-example</artifactId>
    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>
    <dependencies>
        <!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.32</version>
        </dependency>
        <!-- https://mvnrepository.com/artifact/org.hibernate/hibernate-core -->
        <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate-core</artifactId>
            <version>6.1.7.Final</version>
        </dependency>
    </dependencies>
    <build>
        <sourceDirectory>src/main/java</sourceDirectory>
        <plugins>
            <plugin>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.5.1</version>
                <configuration>
                    <source>17</source>
                    <target>17</target>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

4. Creating the JPA Entities

Let's define the following Account base class:

Account.java

package net.javaguides.hibernate.entity;

import jakarta.persistence.*;

@Entity(name = "Account")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
public abstract class Account {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String owner;

    private double balance;

    private double interestRate;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getOwner() {
        return owner;
    }

    public void setOwner(String owner) {
        this.owner = owner;
    }

    public double getBalance() {
        return balance;
    }

    public void setBalance(double balance) {
        this.balance = balance;
    }

    public double getInterestRate() {
        return interestRate;
    }

    public void setInterestRate(double interestRate) {
        this.interestRate = interestRate;
    }
}

CreditAccount.java

package net.javaguides.hibernate.entity;

import jakarta.persistence.Entity;

@Entity(name = "CreditAccount")
public class CreditAccount extends Account {

    private double creditLimit;

    public double getCreditLimit() {
        return creditLimit;
    }

    public void setCreditLimit(double creditLimit) {
        this.creditLimit = creditLimit;
    }
}

DebitAccount.java

package net.javaguides.hibernate.entity;

import jakarta.persistence.Entity;

@Entity(name = "DebitAccount")
public class DebitAccount extends Account {

    private double overdraftFee;

    public double getOverdraftFee() {
        return overdraftFee;
    }

    public void setOverdraftFee(double overdraftFee) {
        this.overdraftFee = overdraftFee;
    }
}

5. Create a Hibernate configuration file - hibernate.cfg.xml

The configuration file contains information about the database and mapping file. Conventionally, its name should be hibernate.cfg.xml.

Let's create an XML file named hibernate.cfg.xml under the /resources folder and write the following code in it.

<!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>
        <!-- JDBC Database connection settings -->
        <property name="connection.driver_class">com.mysql.cj.jdbc.Driver</property>
        <property name="connection.url">jdbc:mysql://localhost:3306/hibernate_db?useSSL=false</property>
        <property name="connection.username">root</property>
        <property name="connection.password">root</property>
        <!-- JDBC connection pool settings ... using built-in test pool -->
        <property name="connection.pool_size">1</property>
        <!-- Echo the SQL to stdout -->
        <property name="show_sql">true</property>
        <!-- Set the current session context -->
        <property name="current_session_context_class">thread</property>
        <!-- Drop and re-create the database schema on startup -->
        <property name="hbm2ddl.auto">create-drop</property>
        <!-- dbcp connection pool configuration -->
        <property name="hibernate.dbcp.initialSize">5</property>
        <property name="hibernate.dbcp.maxTotal">20</property>
        <property name="hibernate.dbcp.maxIdle">10</property>
        <property name="hibernate.dbcp.minIdle">5</property>
        <property name="hibernate.dbcp.maxWaitMillis">-1</property>
        <mapping class="net.javaguides.hibernate.entity.CreditAccount" />
 <mapping class="net.javaguides.hibernate.entity.DebitAccount" />
    </session-factory>
</hibernate-configuration>

6. Create a Hibernate Utility Class

Create a helper class to bootstrap hibernate SessionFactory. In most Hibernate applications, the SessionFactory should be instantiated once during application initialization. The single instance should then be used by all code in a particular process, and any Session should be created using this single SessionFactory.

The SessionFactory is thread-safe and can be shared; a Session is a single-threaded object.

The bootstrapping API is quite flexible, but in most cases, it makes the most sense to think of it as a 3 step process:

Build the StandardServiceRegistry
Build the Metadata
Use those 2 to build the SessionFactory

package net.javaguides.hibernate.util;

import org.hibernate.SessionFactory;
import org.hibernate.boot.Metadata;
import org.hibernate.boot.MetadataSources;
import org.hibernate.boot.registry.StandardServiceRegistry;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;

public class HibernateUtil {
    private static StandardServiceRegistry registry;
    private static SessionFactory sessionFactory;

    public static SessionFactory getSessionFactory() {
        if (sessionFactory == null) {
            try {
                // Create registry
                registry = new StandardServiceRegistryBuilder().configure().build();

                // Create MetadataSources
                MetadataSources sources = new MetadataSources(registry);

                // Create Metadata
                Metadata metadata = sources.getMetadataBuilder().build();

                // Create SessionFactory
                sessionFactory = metadata.getSessionFactoryBuilder().build();

            } catch (Exception e) {
                e.printStackTrace();
                if (registry != null) {
                    StandardServiceRegistryBuilder.destroy(registry);
                }
            }
        }
        return sessionFactory;
    }

    public static void shutdown() {
        if (registry != null) {
            StandardServiceRegistryBuilder.destroy(registry);
        }
    }
}

7. Create the main App class and Run an Application

package net.javaguides.hibernate;

import org.hibernate.Session;
import org.hibernate.Transaction;

import net.javaguides.hibernate.entity.CreditAccount;
import net.javaguides.hibernate.entity.DebitAccount;
import net.javaguides.hibernate.util.HibernateUtil;

public class App {
    public static void main(String[] args) {


        Transaction transaction = null;
        try (Session session = HibernateUtil.getSessionFactory().openSession()) {
            // start a transaction
            transaction = session.beginTransaction();
            // save the account object
            CreditAccount account = new CreditAccount();
            account.setBalance(10000.0);
            account.setInterestRate(10.0);
            account.setOwner("Ramesh");
            account.setCreditLimit(10000.0);
            session.save(account);

            DebitAccount debitAccount = new DebitAccount();
            debitAccount.setBalance(10000.0);
            debitAccount.setInterestRate(10.0);
            debitAccount.setOwner("Ramesh");
            debitAccount.setOverdraftFee(100.0);
            session.persist(debitAccount);
            // commit transaction
            transaction.commit();
        }

        try (Session session = HibernateUtil.getSessionFactory().openSession()) {
            // start a transaction
            Transaction transaction1 = session.beginTransaction();
            // save the account object
            CreditAccount creditAccount = session.get(CreditAccount.class, 1 L);
            System.out.println(creditAccount.getBalance());
            System.out.println(creditAccount.getId());
            // commit transaction
            transaction1.commit();
        }
    }
}

Output

GitHub Repository

The complete source code of this article is available on my GitHub Repository - https://github.com/RameshMF/Hibernate-ORM-Tutorials

Conclusion

In this article, we have discussed the single table strategy which maps all entities of the inheritance structure to the same database table. This approach makes polymorphic queries very efficient and provides the best performance.

You can learn more about Hibernate ORM Framework at Hibernate Tutorial