JPA - Entity Managers

This chapter takes you through simple example with JPA. Let us consider employee management as example. It means the employee management is creating, updating, finding, and deleting an employee. As mentioned above we are using MySQL database for database operations.

The main modules for this example are as follows:

• Model or POJO

  Employee.java

• Persistence

  Persistence.xml

• Service

  CreatingEmployee.java

  UpdatingEmployee.java

  FindingEmployee.java

  DeletingEmployee.java

Let us take the package hierarchy which we have used in the JPA installation with Eclipselink. Follow the hierarchy for this example as below:

Creating Entities

Entities are nothing but beans or Models, in this example we will use Employee as entity. eid, ename, salary, and deg are the attributes of this entity. It contains default constructor, setter and getter methods of those attributes.

In the above shown hierarchy, create a package named com.tutorialspoint.eclipselink.entity, under src (Source) package. Create a class named Employee.java under given package as follows:

Employee.java

package com.tutorialspoint.eclipselink.entity;

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

@Entity
@Table
public class Employee {

   @Id
   @GeneratedValue(strategy = GenerationType.AUTO) 	
   
   private int eid;
   private String ename;
   private double salary;
   private String deg;
   
   public Employee(int eid, String ename, double salary, String deg) {
      super( );
      this.eid = eid;
      this.ename = ename;
      this.salary = salary;
      this.deg = deg;
   }

   public Employee( ) {
      super();
   }

   public int getEid( ) {
      return eid;
   }
   
   public void setEid(int eid) {
      this.eid = eid;
   }
   
   public String getEname( ) {
      return ename;
   }
   
   public void setEname(String ename) {
      this.ename = ename;
   }

   public double getSalary( ) {
      return salary;
   }
   
   public void setSalary(double salary) {
      this.salary = salary;
   }

   public String getDeg( ) {
      return deg;
   }
   
   public void setDeg(String deg) {
      this.deg = deg;
   }
   
   @Override
   public String toString() {
      return "Employee [eid=" + eid + ", ename=" + ename + ", salary=" + salary + ", deg=" + deg + "]";
   }
}

In the above code, we have used @Entity annotation to make this POJO class as entity.

Before going to next module we need to create database for relational entity, which will register the database in persistence.xml file. Open MySQL workbench and type query as follows:

create database jpadb
use jpadb

Persistence.xml

This module plays a crucial role in the concept of JPA. In this xml file we will register the database and specify the entity class.

In the above shown package hierarchy, persistence.xml under JPA Content package is as follows:

persistence.xml

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="3.0" xmlns="https://jakarta.ee/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="https://jakarta.ee/xml/ns/persistence https://jakarta.ee/xml/ns/persistence/persistence_3_0.xsd">
	<persistence-unit name="Eclipselink_JPA" transaction-type="RESOURCE_LOCAL">
    <provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
	<class>com.tutorialspoint.eclipselink.entity.Employee</class>

      <properties>
         <property name="jakarta.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/jpadb"/>
         <property name="jakarta.persistence.jdbc.user" value="guest"/>
         <property name="jakarta.persistence.jdbc.password" value="guest123"/>
         <property name="jakarta.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
         <property name="eclipselink.logging.level" value="FINE"/>
         <property name="eclipselink.ddl-generation" value="create-tables"/>
      </properties>
	</persistence-unit>
</persistence>

In the above xml, <persistence-unit> tag is defined with specific name for JPA persistence. The <class> tag defines entity class with package name. The <properties> tag defines all the properties, and <property> tag defines each property such as database registration, URL specification, username, and password. These are the Eclipselink properties. This file will configure the database.

Persistence Operations

Persistence operations are used against database and they are load and store operations. In a business component all the persistence operations fall under service classes.

In the above shown package hierarchy, create a package named com.tutorialspoint.eclipselink.service, under src (source) package. All the service classes named as CreateEmloyee.java, UpdateEmployee.java, FindEmployee.java, and DeleteEmployee.java. comes under the given package.

After completion of all the modules in this example, the package and file hierarchy is shown as follows: