Hibernate / JPA Joined Table Inheritance Example

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In a previous couple of articles, we have discussed a below two inheritance strategies:
In this article, we will discuss The Joined table strategy or table-per-subclass mapping strategy.
Each subclass can also be mapped to its own table. This is also called the table-per-subclass mapping strategy. An inherited state is retrieved by joining with the table of the superclass.
A discriminator column is not required for this mapping strategy. Each subclass must, however, declare a table column holding the object identifier.
The JOINED table inheritance strategy addresses the data integrity concerns because every subclass is associated with a different table. Polymorphic queries or @OneToMany base class associations don’t perform very well with this strategy. However, polymorphic @ManyToOne associations are fine, and they can provide a lot of value.

Technologies and tools used

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

Development Steps

  1. Create a Simple Maven Project
  2. Project Directory Structure
  3. Add jar Dependencies to pom.xml
  4. Creating the JPA Entities
  5. Create a Hibernate configuration file - hibernate.cfg.xml
  6. Create a Hibernate utility class
  7. 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

The inheritance strategy is defined on the abstract superclass, using the @Inheritance annotation. In this example, we used InheritanceType.JOINED. This means all concrete subclasses and superclasses will be stored in their own table. You can optionally specify a discriminator column name. This column is registered by the @DiscriminatorColumn if omitted no default is used.
Let's define the following Account base class:

Account.java

package net.javaguides.hibernate.entity;

import jakarta.persistence.*;

@Entity(name = "Account")
@Inheritance(strategy = InheritanceType.JOINED)
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

The primary key of this table is also a foreign key to the superclass table and is described by the @PrimaryKeyJoinColumns.
The table name still defaults to the non-qualified class name. Also, if @PrimaryKeyJoinColumn is not set, the primary key / foreign key columns are assumed to have the same names as the primary key columns of the primary table of the superclass.
Let's join a table with @PrimaryKeyJoinColumn annotation:
package net.javaguides.hibernate.entity;

import jakarta.persistence.Entity;

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

    private double creditLimit;

    public double getCreditLimit() {
        return creditLimit;
    }

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

DebitAccount.java

Let's use the @PrimaryKeyJoinColumn annotation to join the table.
package net.javaguides.hibernate.entity;

import jakarta.persistence.Entity;

@Entity(name = "DebitAccount")
@PrimaryKeyJoinColumn(name = "account_id")
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

Let's 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:
  1. Build the StandardServiceRegistry
  2. Build the Metadata
  3. 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.save(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();
        }
    }
}
Since we used the JOINED inheritance strategy, all subclasses, and superclass have their own table.
CREATE TABLE Account (
    id BIGINT NOT NULL ,
    balance NUMERIC(19, 2) ,
    interestRate NUMERIC(19, 2) ,
    owner VARCHAR(255) ,
    PRIMARY KEY ( id )
)

CREATE TABLE CreditAccount (
    creditLimit NUMERIC(19, 2) ,
    id BIGINT NOT NULL ,
    PRIMARY KEY ( id )
)

CREATE TABLE DebitAccount (
    overdraftFee NUMERIC(19, 2) ,
    id BIGINT NOT NULL ,
    PRIMARY KEY ( id )
)

ALTER TABLE CreditAccount
ADD CONSTRAINT FKihw8h3j1k0w31cnyu7jcl7n7n
FOREIGN KEY (id) REFERENCES Account

ALTER TABLE DebitAccount
ADD CONSTRAINT FKia914478noepymc468kiaivqm
FOREIGN KEY (id) REFERENCES Account

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 Joined table strategy or table-per-subclass mapping strategy.

Check out the previous articles:
You can learn more about Hibernate ORM Framework at Hibernate Tutorial

Comments

  1. Unknown5 April 2019 at 14:37

    What if the subclass has its own primary key and the foreign key(which is the primary key of the super class)?

    ReplyDelete

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