HelloWorld with JPA, TopLink and MySql

This is a simple example of standalone java application using Java Persistence API (JPA), TopLink persistence engine, 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 C:\ws\nb\scrap\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">
<persistence-unit name="hello-world" transaction-type="RESOURCE_LOCAL">

<!-- comment out to use the default provider  
<provider>oracle.toplink.essentials.ejb.cmp3.EntityManagerFactoryProvider</provider>
-->

<class>com.foo.Greeting</class>
<properties>
<property name="toplink.jdbc.url" value="jdbc:mysql://localhost:3306/test"/>
<property name="toplink.jdbc.user" value="root"/>
<property name="toplink.jdbc.driver" value="com.mysql.jdbc.Driver"/>
<property name="toplink.jdbc.password" value=""/>
<property name="toplink.ddl-generation" value="drop-and-create-tables"/>
</properties>
</persistence-unit>
</persistence>

Entity class:

package com.foo;
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.foo;
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 g from Greeting g where g.language = :language")
            .setParameter("language", "en").getSingleResult();
    System.out.println("Query returned: " + g);
}
}

To compile the project,

C:\ws\nb\scrap\src\com\foo>
javac -d C:\ws\nb\scrap\build\classes
-classpath C:\Sun\AppServer\lib\toplink-essentials-agent.jar
Greeting.java  HelloWorld.java

To run it, start MySql on localhost, and run

C:\ws\nb\scrap\build\classes>
java -javaagent:C:\Sun\AppServer\lib\toplink-essentials-agent.jar
-cp .;C:\tools\mysql-java\mysql-connector-java-3.1.13-bin.jar
com.foo.HelloWorld

[TopLink Info]: 2006.07.27 01:34:26.484--ServerSession(20003078)--TopLink, version: Oracle TopLink Essentials - 2006.4 (Build 060412)
[TopLink Info]: 2006.07.27 01:34:27.686--ServerSession(20003078)--file:/C:/ws/nb/scrap/build/classes-hello-world login successful
[TopLink Warning]: 2006.07.27 01:34:27.926--ServerSession(20003078)--Exception [TOPLINK-4002] (Oracle TopLink Essentials - 2006.4 (Build 060412)): oracle.toplink.essentials.exceptions.DatabaseException
Internal Exception: java.sql.SQLException: Table 'sequence' already existsError Code: 1050
Call:CREATE TABLE SEQUENCE (SEQ_NAME VARCHAR(50) NOT NULL, SEQ_COUNT DECIMAL(38), PRIMARY KEY (SEQ_NAME))
Query:DataModifyQuery()
Query returned: Greeting id=1, message=hello world, language=en
[TopLink Info]: 2006.07.27 01:34:28.447--ServerSession(20003078)--file:/C:/ws/nb/scrap/build/classes-hello-world logout successful

Tags: EJB 3, EJB3, java, JPA, java persistence API, toplink, mysql, java persistence, hello world