In Java Persistence API (JPA), the EntityManager is the primary interface for interacting with the persistence context. JPQL (Java Persistence Query Language) is used to query entities using object-oriented syntax rather than SQL. This article explains these concepts with examples.
The EntityManager interface provides methods to manage entity lifecycle and perform database operations. Some key methods include:
persist(Object entity) - Saves an entity to the database.merge(Object entity) - Updates an existing entity or saves a new one.remove(Object entity) - Deletes an entity.find(Class entityClass, Object primaryKey) - Fetches an entity by its primary key.createQuery(String jpql) - Executes JPQL queries.
First, set up the persistence.xml file:
<persistence xmlns="http://xmlns.jcp.org/xml/ns/persistence" version="2.1">
<persistence-unit name="examplePU">
<properties>
<property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/jpa_example"/>
<property name="javax.persistence.jdbc.user" value="root"/>
<property name="javax.persistence.jdbc.password" value="password"/>
<property name="javax.persistence.jdbc.driver" value="com.mysql.cj.jdbc.Driver"/>
<property name="hibernate.hbm2ddl.auto" value="update"/>
<property name="hibernate.show_sql" value="true"/>
</properties>
</persistence-unit>
</persistence>
Define an entity class:
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
@Entity
public class Product {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private int id;
private String name;
private double price;
// Getters and setters
public int getId() {
return id;
}
public void setId(int 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;
}
}
Using EntityManager to persist data:
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
public class MainApp {
public static void main(String[] args) {
EntityManagerFactory emf = Persistence.createEntityManagerFactory("examplePU");
EntityManager em = emf.createEntityManager();
Product product = new Product();
product.setName("Smartphone");
product.setPrice(699.99);
em.getTransaction().begin();
em.persist(product);
em.getTransaction().commit();
em.close();
emf.close();
System.out.println("Product saved successfully!");
}
}
JPQL (Java Persistence Query Language) is a query language for entities. Unlike SQL, it uses entity names and field names instead of table and column names. JPQL can be used for retrieval, updates, and deletions.
JPQL syntax:
SELECT e FROM EntityName e - Fetch all records of an entity.WHERE - Filter results based on conditions.ORDER BY - Sort results.
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.Query;
import java.util.List;
public class JPQLExample {
public static void main(String[] args) {
EntityManagerFactory emf = Persistence.createEntityManagerFactory("examplePU");
EntityManager em = emf.createEntityManager();
em.getTransaction().begin();
// JPQL query
Query query = em.createQuery("SELECT p FROM Product p WHERE p.price > :minPrice");
query.setParameter("minPrice", 500.00);
List<Product> products = query.getResultList();
for (Product product : products) {
System.out.println(product.getName() + " - " + product.getPrice());
}
em.getTransaction().commit();
em.close();
emf.close();
}
}
em.getTransaction().begin();
Query updateQuery = em.createQuery("UPDATE Product p SET p.price = :newPrice WHERE p.name = :name");
updateQuery.setParameter("newPrice", 799.99);
updateQuery.setParameter("name", "Smartphone");
int rowsUpdated = updateQuery.executeUpdate();
em.getTransaction().commit();
System.out.println("Rows updated: " + rowsUpdated);
em.getTransaction().begin();
Query deleteQuery = em.createQuery("DELETE FROM Product p WHERE p.name = :name");
deleteQuery.setParameter("name", "Smartphone");
int rowsDeleted = deleteQuery.executeUpdate();
em.getTransaction().commit();
System.out.println("Rows deleted: " + rowsDeleted);
Named queries are predefined JPQL queries with fixed syntax. They are defined using the @NamedQuery annotation.
@Entity
@NamedQuery(name = "Product.findAll", query = "SELECT p FROM Product p")
public class Product {
// Fields, getters, and setters
}
// Using NamedQuery
Query namedQuery = em.createNamedQuery("Product.findAll");
List<Product> products = namedQuery.getResultList();
The EntityManager interface and JPQL are core components of JPA, enabling developers to interact with databases using object-oriented concepts. Understanding their usage helps create efficient and maintainable data-driven applications.