JPA EntityManager Example

Chandan SinghJanuary 14th, 2015Last Updated: February 27th, 2019

1 662 2 minutes read

In this example, we shall try to demonstrate how to useJPA EntityManager. As the name suggests, anEntityManager is a class that manages the state of theEntity(Persist/Update/Delete etc).

EveryEntityManager object has an instance ofEntityTransaction associated with it.EntityTransaction is used to manage the transactions.

We shall be usingHibernate as the JPA Vendor. The underlying database shall be MySQL.

The benefit of using theJPA over any specific ORM related libraries like Hibernate, iBatis is that we need not change the code when we change the vendor. The code is decoupled (or loosely coupled) with the underlying ORM framework.

Let’s build a sample application to see how we can avoid usingHibernate specific interfaces and useJPA interfaces instead:

Employee.java:

package com.jcg.pojo;import javax.persistence.Column;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.GenerationType;import javax.persistence.Id;import javax.persistence.Table;@Entity@Table(name="employee")public class Employee{protected Long employeeId;protected String name;@Id@Column(name="employee_id")@GeneratedValue(strategy=GenerationType.IDENTITY)public  Long getEmployeeId(){return employeeId;}public  void setEmployeeId(Long employeeId){this.employeeId = employeeId;}@Column(name="employee_name")public  String getName(){return name;}public  void setName(String name){this.name = name;}@Override    public String toString()    {    return "Employee [employeeId=" + employeeId + ", name=" + name + "]";    }}

JPADemo.java:

package com.jcg;import javax.persistence.EntityManager;import javax.persistence.EntityManagerFactory;import javax.persistence.Persistence;import com.isg.maps.Employee;/** * @author Chandan Singh * */public class JPADemo{public static void main(String[] args)    {EntityManagerFactory emf = Persistence.createEntityManagerFactory("jcg-JPA");EntityManager em = emf.createEntityManager();em.getTransaction().begin();Employee employee = new Employee();employee.setName("Chandan");System.out.println("COMIITING");em.persist(employee);em.getTransaction().commit();    }}

persistence.java:

<?xml version="1.0" encoding="UTF-8"?><persistence xmlns="http://java.sun.com/xml/ns/persistence"        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"        xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"        version="2.0">                <persistence-unit name="jcg-JPA">        <provider>org.hibernate.ejb.HibernatePersistence</provider><properties><property name="hibernate.dialect" value="org.hibernate.dialect.MySQL5InnoDBDialect" /><property name="hibernate.hbm2ddl.auto" value="update" /><property name="hibernate.connection.driver_class" value="com.mysql.jdbc.Driver" /><property name="hibernate.connection.username" value="myusername" /><property name="hibernate.connection.password" value="mypwd" /><property name="hibernate.connection.url" value="jdbc:mysql://localhost/research_development" /></properties>        </persistence-unit></persistence>

We define the connection properties in thepersistence.xml.
Then we look up the persistence unit from thecreateEntityManagerFactory method ofPersistence class of JPA. This returns an object ofEntityManagerFactory.
Finally, we can get theEntityManager object from theEntityManagerFactory class.
We ,now, use theEntityManager object to performCRUD operation on theEntities under the scope of anEntityTransaction object.
The last step is tocommit thetransaction back tot the database.

TIP:
It is mandatory to placepersistence.xml in theMETA-INF folder.

Conclusion:

Thus we studied about theJPA EntityManager and how we can use it to avoid dependency on any particular ORM Framework.

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