Hibernate HQL Query Example with MySQL Database

This tutorial will guide you through setting up and using Hibernate Query Language (HQL) with a MySQL database using the Product entity. We will demonstrate various HQL queries to interact with the database.

Introduction

Hibernate Query Language (HQL) is a powerful query language similar to SQL but designed for Hibernate. It allows you to perform database operations using the object-oriented paradigm. This tutorial will show you how to set up HQL queries using a MySQL database and the Product entity.

In this tutorial, we will:

  1. Set up a Maven project with necessary dependencies.
  2. Configure Hibernate and MySQL.
  3. Create an entity class (Product).
  4. Demonstrate various HQL queries.

Step 1: Set Up Your Project

1.1 Create a Maven Project

Open your IDE and create a new Maven project.

1.2 Add Dependencies

Update your pom.xml file to include dependencies for Hibernate and MySQL.

<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>

    <groupId>com.example</groupId>
    <artifactId>hibernate-hql-example</artifactId>
    <version>1.0-SNAPSHOT</version>

    <dependencies>
        <!-- Hibernate ORM -->
        <dependency>
            <groupId>org.hibernate.orm</groupId>
            <artifactId>hibernate-core</artifactId>
            <version>6.4.0.Final</version>
        </dependency>

        <!-- MySQL Connector -->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.29</version>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.10.1</version>
                <configuration>
                    <source>21</source>
                    <target>21</target>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

Step 2: Configure Hibernate

2.1 Create hibernate.cfg.xml

Create a hibernate.cfg.xml file in the src/main/resources directory to configure database connection settings and Hibernate properties.

<!DOCTYPE hibernate-configuration PUBLIC
    "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>
    <session-factory>
        <!-- JDBC Database connection settings -->
        <property name="hibernate.connection.driver_class">com.mysql.cj.jdbc.Driver</property>
        <property name="hibernate.connection.url">jdbc:mysql://localhost:3306/hibernate_db</property>
        <property name="hibernate.connection.username">root</property>
        <property name="hibernate.connection.password">password</property>

        <!-- JDBC connection pool settings -->
        <property name="hibernate.c3p0.min_size">5</property>
        <property name="hibernate.c3p0.max_size">20</property>
        <property name="hibernate.c3p0.timeout">300</property>
        <property name="hibernate.c3p0.max_statements">50</property>
        <property name="hibernate.c3p0.idle_test_period">3000</property>

        <!-- SQL dialect -->
        <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>

        <!-- Echo all executed SQL to stdout -->
        <property name="hibernate.show_sql">true</property>
        <property name="hibernate.format_sql">true</property>

        <!-- Drop and re-create the database schema on startup -->
        <property name="hibernate.hbm2ddl.auto">update</property>

        <!-- Entities -->
        <mapping class="com.example.entity.Product"/>
    </session-factory>
</hibernate-configuration>

Replace hibernate_db, root, and password with your MySQL database name and credentials.

Explanation:

  • hibernate.connection.driver_class specifies the JDBC driver class for MySQL.
  • hibernate.connection.url specifies the JDBC URL for the database connection.
  • hibernate.connection.username and hibernate.connection.password specify the database credentials.
  • hibernate.c3p0 properties configure the connection pool settings using C3P0.
  • hibernate.dialect specifies the SQL dialect to be used.
  • hibernate.show_sql and hibernate.format_sql properties are used to display and format the generated SQL statements.
  • hibernate.hbm2ddl.auto specifies the schema generation strategy.
  • The <mapping class="com.example.entity.Product"/> line maps the Product entity to the database.

Step 3: Create the Entity Class

Create an entity class Product that will be mapped to a table in the database. This class uses annotations to define the entity and its fields.

package com.example.entity;

import jakarta.persistence.Entity;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;
import jakarta.persistence.Id;

@Entity
public class Product {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    private double price;

    // Getters and setters
    public Long getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) {
        this.price = price;
    }
}

Explanation:

  • The @Entity annotation specifies that the class is an entity and is mapped to a database table.
  • The @Id annotation specifies the primary key of the entity.
  • The @GeneratedValue(strategy = GenerationType.IDENTITY) annotation specifies that the primary key is auto-incremented.

Step 4: Demonstrate Various HQL Queries

Create a main class to demonstrate various HQL queries using the Product entity.

4.1 Create MainApp

package com.example.main;

import com.example.entity.Product;
import com.example.util.HibernateUtil;
import org.hibernate.Session;
import org.hibernate.query.Query;

import java.util.List;

public class MainApp {
    public static void main(String[] args) {
        // Load the configuration and build the SessionFactory
        Session session = HibernateUtil.getSessionFactory().openSession();

        // Insert sample data
        insertSampleData(session);

        // Perform HQL queries
        performHQLQueries(session);

        // Close the session
        session.close();

        // Shut down the SessionFactory
        HibernateUtil.shutdown();
    }

    private static void insertSampleData(Session session) {
        session.beginTransaction();
        session.persist(new Product("Laptop", 1000.00));
        session.persist(new Product("Phone", 500.00));
        session.persist(new Product("Tablet", 300.00));
        session.getTransaction().commit();
    }

    private static void performHQLQueries(Session session) {
        // Select all products
        Query<Product> query1 = session.createQuery("from Product", Product.class);
        List<Product> products = query1.list();
        System.out.println("All Products:");
        products.forEach(product -> System.out.println(product.getName()));

        // Select products with price greater than 400
        Query<Product> query2 = session.createQuery("from Product where price > 400", Product.class);
        List<Product> expensiveProducts = query2.list();
        System.out.println("Products with price greater than 400:");
        expensiveProducts.forEach(product -> System.out.println(product.getName()));

        // Update product price
        session.beginTransaction();
        Query updateQuery = session.createQuery("update Product set price = :price where name = :name");
        updateQuery.setParameter("price", 1200.00);
        updateQuery.setParameter("name", "Laptop");
        int result = updateQuery.executeUpdate();
        System.out.println("Number of products updated: " + result);
        session.getTransaction().commit();

        // Delete a product
        session.beginTransaction();
        Query deleteQuery = session.createQuery("delete Product where name = :name");
        deleteQuery.setParameter("name", "Tablet");
        result = deleteQuery.executeUpdate();
        System.out.println("Number of products deleted: " + result);
        session.getTransaction().commit();
    }
}

Explanation:

  • The MainApp class demonstrates how to perform various HQL queries using Hibernate.
  • The insertSampleData method inserts sample data into the Product table.
  • The performHQLQueries method demonstrates different HQL queries:
    • Selecting all products.
    • Selecting products with a price greater than 400.
    • Updating the price of a product.
    • Deleting a product.

4.2 Create HibernateUtil Class

Create a utility class HibernateUtil to manage the Hibernate SessionFactory.

package com.example.util;

import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

public class HibernateUtil {
    private static final SessionFactory sessionFactory = buildSessionFactory();

    private static SessionFactory buildSessionFactory() {
        try {
            // Load the configuration and build the SessionFactory
            return new Configuration().configure().buildSessionFactory();
        } catch (Throwable ex) {
            System.err.println("Initial SessionFactory creation failed." + ex);
            throw new ExceptionInInitializerError(ex);
        }
    }

    public static SessionFactory getSessionFactory() {
        return sessionFactory;
    }

    public static void shutdown() {
        getSessionFactory().close();
    }
}

Explanation:

  • The HibernateUtil class provides a singleton SessionFactory and a method to shut it down.
  • The buildSessionFactory method loads the Hibernate configuration from hibernate.cfg.xml and builds the SessionFactory.

Step 5: Run the Application

  1. Ensure your MySQL database is running and the connection details in hibernate.cfg.xml are correct.
  2. Run the MainApp class to load the Hibernate configuration, build the SessionFactory, perform HQL queries, and print the results.

Sample Output

If everything is set up correctly, running the MainApp class should produce output similar to the following:

All Products:
Laptop
Phone
Tablet
Products with price greater than 400:
Laptop
Phone
Number of products updated: 1
Number of products deleted: 1

Conclusion

In this tutorial, we have successfully demonstrated how to create and configure a Hibernate configuration file (hibernate.cfg.xml) to connect to a MySQL database using the Product entity and perform various HQL queries. We configured the project dependencies, created an entity class, set up the Hibernate configuration file, and demonstrated different HQL queries. This guide provides a solid foundation for using HQL with Hibernate and a MySQL database in your applications.


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