Sunday, March 27, 2011

Simple example on JPA with eclipse link

Create a Java project "com.vaani.jpa.simple". Create a folder "lib" and place the required JPA jars and derby.jar into this folder. Add the libs to the project classpath.
Afterwards create the package "com.vaani.jpa.simple.model" and create the following classes.
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

public class Todo {
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String summary;
private String description;

public String getSummary() {
return summary;

public void setSummary(String summary) {
this.summary = summary;

public String getDescription() {
return description;

public void setDescription(String description) {
this.description = description;

public String toString() {
return "Todo [summary=" + summary + ", description=" + description
+ "]";


Persistence Unit

Create a directory "META-INF" in your "src" folder and create the file "persistence.xml". This examples uses EclipseLink specific flags for example via the parameter "eclipselink.ddl-generation" you specify that the database scheme will be automatically dropped and created.

<?xml version="1.0" encoding="UTF-8" ?>
<persistence xmlns:xsi=""
version="2.0" xmlns="">
<persistence-unit name="todos" transaction-type="RESOURCE_LOCAL">
<property name="javax.persistence.jdbc.driver" value="org.apache.derby.jdbc.EmbeddedDriver" />
<property name="javax.persistence.jdbc.url"
value="jdbc:derby:/home/vogella/databases/simpleDb;create=true" />
<property name="javax.persistence.jdbc.user" value="test" />
<property name="javax.persistence.jdbc.password" value="test" />

<!-- EclipseLink should create the database schema automatically -->
<property name="eclipselink.ddl-generation" value="create-tables" />
<property name="eclipselink.ddl-generation.output-mode"
value="database" />


The database specified via "javax.persistence.jdbc.url" will be automatically created by the Derby Driver. You may want to adjust the path, it currently is based on Linux notations and points to my home directory on my Linux system.

To see the SQL generated for the the databases set eclipselink.ddl-generation.output-mode value from "database" to "sql-script" or "both". Two files will get generated 'createDDL.jdbc' and 'dropDDL.jdbc'

Test your installation

Create the following Main class which will create a new entry every time it it called. After the first call you need to remove the property "eclipselink.ddl-generation" from persistence.xml otherwise you will receive an error as EclipseLink tries to create the database scheme again. Alternative you could set the property to "drop-and-create-tables" but this would drop your database schema at every run.

import java.util.List;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.Query;

import com.vaani.jpa.simple.model.Todo;

public class Main {
private static final String PERSISTENCE_UNIT_NAME = "todos";
private static EntityManagerFactory factory;

public static void main(String[] args) {
factory = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
EntityManager em = factory.createEntityManager();
// Read the existing entries and write to console
Query q = em.createQuery("select t from Todo t");
List<Todo> todoList = q.getResultList();
for (Todo todo : todoList) {
System.out.println("Size: " + todoList.size());

// Create new todo
Todo todo = new Todo();
todo.setSummary("This is a test");
todo.setDescription("This is a test");


Run you program several times to see that the database is filled.

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