HelloWorld with EclipseLink and MySQL

This is a simple example of standalone java application using Java Persistence API (JPA), EclipseLink, and MySql. It consists of an entity class, a main class, and a persistence.xml file. No need to create tables, as they are created and dropped automatically.

META-INF/persistence.xml must be at the root of the persistence unit. In my example project, it's located at $HOME/NetBeansProjects/greeting/build/classes/META-INF/persistence.xml.

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="1.0" 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_1_0.xsd">
<persistence-unit name="greetingPU" transaction-type="RESOURCE_LOCAL">
<provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
<class>javahowto.Greeting</class>
<properties>
<property name="eclipselink.jdbc.password" value=""/>
<property name="eclipselink.jdbc.user" value="root"/>
<property name="eclipselink.jdbc.driver" value="com.mysql.jdbc.Driver"/>
<property name="eclipselink.jdbc.url" value="jdbc:mysql://localhost:3306/test"/>
<property name="eclipselink.ddl-generation" value="drop-and-create-tables"/>
<property name="eclipselink.logging.level" value="INFO"/>
</properties>
</persistence-unit>
</persistence>

Entity class (generated by NetBeans 6.1):

package javahowto;
import java.io.Serializable;
import javax.persistence.Basic;
import javax.persistence.Entity;
import javax.persistence.Id;

@Entity
public class Greeting implements Serializable {
private static final long serialVersionUID = 1L;

@Id
private Long id;

@Basic
private String message;

public Long getId() {
return id;
}

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

public String getMessage() {
return message;
}

public void setMessage(String message) {
this.message = message;
}

@Override
public int hashCode() {
int hash = 0;
hash += (id != null ? id.hashCode() : 0);
return hash;
}

@Override
public boolean equals(Object object) {
// TODO: Warning - this method won't work in the case the id fields are not set
if (!(object instanceof Greeting)) {
 return false;
}
Greeting other = (Greeting) object;
if ((this.id == null && other.id != null) ||
 (this.id != null && !this.id.equals(other.id))) {
 return false;
}
return true;
}

@Override
public String toString() {
return "javahowto.Greeting[id=" + id + ", message='" + getMessage() +
"']";
}
}

Main class:

package javahowto;
import java.util.logging.Logger;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

public class Main {
private static final Logger logger = Logger.getLogger("javahowto.greeting");
private static final String PERSISTENCE_UNIT_NAME = "greetingPU";
private EntityManagerFactory emf;
private EntityManager em;

public static void main(String[] args) {
Main main = new Main();
main.initEntityManager();
main.createAndRead();
main.createAndRollback();
}

private void createAndRead() {
Greeting g = new Greeting();
g.setId(1L);
g.setMessage("hello, createAndRead");
em.getTransaction().begin();
em.persist(g);
em.getTransaction().commit();

//g should be written to database now.
//Read it from db (no transaction context needed for em.find method)
Greeting g2 = em.find(Greeting.class, g.getId());
logger.info("Greeting " + g.getId() + " from db: " + g2);
}

private void createAndRollback() {
Greeting g = new Greeting();
g.setId(2L);
g.setMessage("hello, createAndRollback");
em.getTransaction().begin();
em.persist(g);
em.getTransaction().rollback();

logger.info("Persisted " + g + ", but the transaction was rolled back.");
Greeting g2 = em.find(Greeting.class, g.getId());
logger.info("Greeting " + g.getId() + " from db: " + g2);  //should be null
}

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

To compile the project,

/NetBeansProjects/greeting/src >
javac -d ../build/classes -cp $GLASSFISH_HOME/modules/javax.persistence.jar javahowto/*.java

Start MySql on localhost, and run the following command. No need to create the table since our persistence.xml tells EclipseLink to automatically drop and create it.

/home/javahowto/mysql > bin/mysqld_safe &

/NetBeansProjects/greeting/build/classes >
java -cp $HOME/tmp/mysql-connector-java-5.1.5-bin.jar:$GLASSFISH_HOME/modules/javax.persistence.jar:$GLASSFISH_HOME/modules/org.eclipse.persistence.jpa.jar:$GLASSFISH_HOME/modules/org.eclipse.persistence.core.jar:. javahowto.Main

[EL Info]: 2008.10.20 11:56:24.712--ServerSession(4744654)--EclipseLink, version: Eclipse Persistence Services - 1.0.1 (Build 20080905)
[EL Info]: 2008.10.20 11:56:25.163--ServerSession(4744654)--file:/home/javahowto/NetBeansProjects/greeting/build/classes/-greetingPU login successful
Oct 20, 2008 11:56:25 AM javahowto.Main createAndRead
INFO: Greeting 1 from db: javahowto.Greeting[id=1, message='hello, createAndRead']
Oct 20, 2008 11:56:25 AM javahowto.Main createAndRollback
INFO: Persisted javahowto.Greeting[id=2, message='hello, createAndRollback'], but the transaction was rolled back.
Oct 20, 2008 11:56:25 AM javahowto.Main createAndRollback
INFO: Greeting 2 from db: null

To stop MySQL server:

/home/javahowto/mysql > bin/mysqladmin -u root shutdown