GAE + JPA :java.lang.NoClassDefFoundError: Could not initialize class EMF - servlets

Question

When i tried to connect my Cloud SQL via JPA the following error is generated :
2012-10-25 10:07:38.439
Error for /jpatest
java.lang.NoClassDefFoundError: Could not initialize class com.my.jpa.EMF
at com.my.jpa.ContactService.createContact(ContactService.java:20)
at com.my.jpa.JPATestServlet.doGet(JPATestServlet.java:14)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
2012-10-25 10:07:38.440
Uncaught exception from servlet
java.lang.NoClassDefFoundError: Could not initialize class com.my.jpa.EMF
at com.my.jpa.ContactService.createContact(ContactService.java:20)
at com.my.jpa.JPATestServlet.doGet(JPATestServlet.java:14)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
My EMF Class is
public final class EMF {
private static final EntityManagerFactory emfInstance = Persistence
.createEntityManagerFactory("JPATest");
private EMF() {
}
public static EntityManagerFactory get() {
return emfInstance;
}
}
EMF initialising portion is
public class ContactService {
private static Logger logger = Logger.getLogger(ContactService.class
.getName());
public void createContact(Contact c) {
logger.info("Entering createContact: [" + c.getFirstName() + ","
+ c.getLastName() + "]");
EntityManager mgr = EMF.get().createEntityManager();
try {
mgr.getTransaction().begin();
mgr.persist(c);
mgr.getTransaction().commit();
} finally {
mgr.close();
}
logger.info("Exiting createContact");
}
}
My Servlet is :
public class JPATestServlet extends HttpServlet {
public void doGet(HttpServletRequest req, HttpServletResponse resp)
throws IOException {
ContactService service = new ContactService();
service.createContact(new Contact("Manu", "Mohan", "686019", "TVM"));
resp.setContentType("text/plain");
resp.getWriter().println("Hello, world");
}
}
web.xml is
<?xml version="1.0" encoding="utf-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" version="2.5">
<servlet>
<servlet-name>JPATest</servlet-name>
<servlet-class>com.my.jpa.JPATestServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>JPATest</servlet-name>
<url-pattern>/jpatest</url-pattern>
</servlet-mapping>
<welcome-file-list>
<welcome-file>index.html</welcome-file>
</welcome-file-list>
</web-app>
persistence.xml
<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.0"
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_2_0.xsd">
<persistence-unit name="JPATest">
<provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
<class>com.my.jpa.Contact</class>
<properties>
<property name="javax.persistence.jdbc.driver" value="com.google.cloud.sql.Driver" />
<property name="javax.persistence.jdbc.url" value="jdbc:google:rdbms://instance-name/stock" />
<property name="javax.persistence.jdbc.user" value="" />
<property name="javax.persistence.jdbc.password" value="" />
<!-- EclipseLink should create the database schema automatically -->
<property name="eclipselink.ddl-generation" value="create-tables" />
<property name="eclipselink.ddl-generation.output-mode"
value="database" />
</properties>
</persistence-unit>
</persistence

Answer 1

Do you need to use final for EntityManagerFactory in EMF. Try to use Singleton Design Pattern for EMF. EntityManagerFactory class is thread-safe.
EMF.java
public final class EMF {
private EntityManagerFactory emfInstance;
private static EMF emf;
private EMF() {
}
public EntityManagerFactory get() {
if(emfInstance == null) {
emfInstance = Persistence.createEntityManagerFactory("JPATest");
}
return emfInstance;
}
public static EMF getInstance() {
if(emf == null) {
emf = new EMF();
}
return emf;
}
}
// usages
EntityManagerFactory emf = Emf.getInstance().get();
Here better way to use EntityManagerFactory in web applicaiton.