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The combination of Apache OpenJPA and Derby provides a fantastic and simple way to prototype database apps in Java. The only downside is that there are a couple of gotchas I keep on finding myself hitting each time I start a new project. Here’s the simplest app I can think of and all the associated files for my later use. If it’s useful to you as well, have fun!

File: src/main/java/test/Account.java

package test;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

@Entity
public class Account {

private String name;

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private long id;

public Account(String name) {
this.name = name;
}

public String getName() {
return name;
}

public long getId() {
return id;
}
}

File: src/main/java/META-INF/persistence.xml

<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="account">
		<class>test.Account</class>
		<properties>
			<property name="openjpa.jdbc.SynchronizeMappings" value="buildSchema" />
			<property name="openjpa.ConnectionURL" value="jdbc:derby:testdb;create=true" />
			<property name="openjpa.ConnectionDriverName" value="org.apache.derby.jdbc.EmbeddedDriver" />
		</properties>
	</persistence-unit>
</persistence>

File: src/test/java/test/AccountTest.java

package test;

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

import junit.framework.TestCase;

public class AccountTest extends TestCase {

	public void testAccount() {
		EntityManagerFactory emf = Persistence.createEntityManagerFactory("account");
		EntityManager em = emf.createEntityManager();
		EntityTransaction et = em.getTransaction();
		et.begin();
		Account a = new Account("Bob");
		em.persist(a);
		et.commit();
		System.out.println("a"+a.getId());
	}
}

File: pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
	<modelVersion>4.0.0</modelVersion>
	<groupId>test</groupId>
	<artifactId>test</artifactId>
	<packaging>jar</packaging>
	<version>1.0-SNAPSHOT</version>
	<name>test</name>
	<dependencies>
		<dependency>
			<groupId>org.apache.openjpa</groupId>
			<artifactId>openjpa-all</artifactId>
			<version>2.0.0</version>
		</dependency>
		<dependency>
			<groupId>org.apache.derby</groupId>
			<artifactId>derby</artifactId>
			<version>10.5.3.0_1</version>
		</dependency>
		<dependency>
			<groupId>junit</groupId>
			<artifactId>junit</artifactId>
			<version>4.8.1</version>
			<type>jar</type>
			<scope>test</scope>
		</dependency>
	</dependencies>
	<build>
		<plugins>
			<plugin>
				<groupId>org.apache.maven.plugins</groupId>
				<artifactId>maven-compiler-plugin</artifactId>
				<version>2.0.2</version>
				<configuration>
					<source>1.5</source>
					<target>1.5</target>
				</configuration>
			</plugin>
			<plugin>
				<groupId>org.codehaus.mojo</groupId>
				<artifactId>openjpa-maven-plugin</artifactId>
				<version>1.0</version>
				<configuration>
					<includes>**/entities/*.class</includes>
					<excludes>**/entities/XML*.class</excludes>
					<addDefaultConstructor>true</addDefaultConstructor>
					<enforcePropertyRestrictions>true</enforcePropertyRestrictions>
				</configuration>
				<executions>
					<execution>
						<id>enhancer</id>
						<phase>process-classes</phase>
						<goals>
							<goal>enhance</goal>
						</goals>
					</execution>
				</executions>
				<dependencies>
					<dependency>
						<groupId>org.apache.openjpa</groupId>
						<artifactId>openjpa</artifactId>
						<version>2.0.0</version>
					</dependency>
				</dependencies>
			</plugin>
		</plugins>
	</build>
</project>