JPA Hello World Example

Entity class

package com.prg;
import java.io.Serializable;
import javax.persistence.Basic;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;

@Entity
public class Greeting implements Serializable {
 @Id @GeneratedValue private int id;
 @Basic private String message;
 @Basic private String language;

 public Greeting() {}
 public Greeting(String message, String language) {
     this.message = message;
     this.language = language;
 }

 public String toString() {
     return "Greeting id=" + id + ", message=" + message + ", language=" + language;
 }
}


Main class

package com.prg;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

public class HelloWorld {
 private EntityManagerFactory emf;
 private EntityManager em;
 private String PERSISTENCE_UNIT_NAME = "hello-world";

 public static void main(String[] args) {
     HelloWorld hello = new HelloWorld();
     hello.initEntityManager();
     hello.create();
     hello.read();
     hello.closeEntityManager();
 }

 private void initEntityManager() {
     emf = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
     em = emf.createEntityManager();
 }

 private void closeEntityManager() {
     em.close();
     emf.close();
 }

 private void create() {
     em.getTransaction().begin();
     Greeting g_en = new Greeting("hello world", "en");
     Greeting g_es = new Greeting("hola, mundo", "es");
     Greeting[] greetings = new Greeting[]{g_en, g_es};
     for(Greeting g : greetings) {
         em.persist(g);
     }
     em.getTransaction().commit();
 }

 private void read() {
     Greeting g = (Greeting) em.createQuery(
             "select greetingg where g.language = :language")
             .setParameter("language", "en").getSingleResult();
     System.out.println("Query returned: " + g);
 }
}