Persistence.createEntityManagerFactory hangs during execution - jpa-2.0

Question

When I attempt to contact http://localhost:8080/jax/users, I get an error that reads:
javax.servlet.ServletException: java.lang.NoClassDefFoundError: Could not initialize class services.DaoHelper
When debugging, the code hangs on the first line of the DaoHelper's contructor:
EntityManagerFactory factory = Persistence.createEntityManagerFactory("databaseU");
I am able to browse the database in Netbeans' database services panel.
What am I missing? Perhaps a missing element in the persistence.xml file?
trace:
java.lang.NoClassDefFoundError: Could not initialize class services.DaoHelper
services.UserResource.<init>(UserResource.java:23)
sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
java.lang.reflect.Constructor.newInstance(Constructor.java:513)
com.sun.jersey.server.spi.component.ResourceComponentConstructor._construct(ResourceComponentConstructor.java:191)
com.sun.jersey.server.spi.component.ResourceComponentConstructor.construct(ResourceComponentConstructor.java:179)
com.sun.jersey.server.impl.resource.PerRequestFactory$PerRequest._getInstance(PerRequestFactory.java:182)
com.sun.jersey.server.impl.resource.PerRequestFactory$AbstractPerRequest.getInstance(PerRequestFactory.java:144)
com.sun.jersey.server.impl.application.WebApplicationContext.getResource(WebApplicationContext.java:238)
com.sun.jersey.server.impl.uri.rules.ResourceClassRule.accept(ResourceClassRule.java:83)
com.sun.jersey.server.impl.uri.rules.RightHandPathRule.accept(RightHandPathRule.java:147)
com.sun.jersey.server.impl.uri.rules.RootResourceClassesRule.accept(RootResourceClassesRule.java:84)
com.sun.jersey.server.impl.application.WebApplicationImpl._handleRequest(WebApplicationImpl.java:1469)
com.sun.jersey.server.impl.application.WebApplicationImpl._handleRequest(WebApplicationImpl.java:1400)
com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1349)
com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1339)
com.sun.jersey.spi.container.servlet.WebComponent.service(WebComponent.java:416)
com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:537)
com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:699)
javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
services.UserResource:
#Path("/users")
public class UserResource {
//per Chris
//private EntityManager mgr = DaoHelper.getInstance().getEntityManager();
#GET
#Produces("application/xml")
public Collection<User> index(){
//per Chris
DaoHelper helper = DaoHelper.getInstance();
EntityManager mgr = helper.getEntityManager();
TypedQuery<User> query = mgr.createQuery("SELECT u FROM User u", User.class);
return query.getResultList();
}
}
services.DaoHelper:
public class DaoHelper {
private static DaoHelper instance = new DaoHelper();
//per Chris
//private final EntityManager mgr;
private DaoHelper(){
//per Chris
//EntityManagerFactory factory = Persistence.createEntityManagerFactory("databasePU");
//mgr = factory.createEntityManager();
}
public EntityManager getEntityManager(){
//per Chris
EntityManagerFactory factory = Persistence.createEntityManagerFactory("databasePU");
return factory.createEntityManager();
//return mgr;
}
public static DaoHelper getInstance(){
return instance;
}
}
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="databasePU" transaction-type="RESOURCE_LOCAL">
<provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
<class>entities.User</class>
<exclude-unlisted-classes>false</exclude-unlisted-classes>
<properties>
<property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/my_db"/>
<property name="javax.persistence.jdbc.password" value="password"/>
<property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
<property name="javax.persistence.jdbc.user" value="user"/>
<!-- logging -->
<property name="eclipselink.logging.file" value="output.log"/>
<property name="eclipselink.logging.logger" value="JavaLogger"/>
<property name="eclipselink.logging.level" value="FINE"/>
<!-- /logging -->
</properties>
</persistence-unit>
</persistence>
** edit 0 **
I have based my project on this sample: [Create rich data-centric web applications using JAX-RS, JPA, and Dojo][1]
** edit 1 **
javax.persistence.PersistenceException: Exception [EclipseLink-4003] (Eclipse Persistence Services - 2.3.0.v20110604-r9504): org.eclipse.persistence.exceptions.DatabaseException
Exception Description: Configuration error. Class [com.mysql.jdbc.Driver] not found.
org.eclipse.persistence.internal.jpa.EntityManagerSetupImpl.deploy(EntityManagerSetupImpl.java:501)
org.eclipse.persistence.internal.jpa.EntityManagerFactoryDelegate.getDatabaseSession(EntityManagerFactoryDelegate.java:188)
org.eclipse.persistence.internal.jpa.EntityManagerFactoryDelegate.createEntityManagerImpl(EntityManagerFactoryDelegate.java:277)
org.eclipse.persistence.internal.jpa.EntityManagerFactoryImpl.createEntityManagerImpl(EntityManagerFactoryImpl.java:290)
org.eclipse.persistence.internal.jpa.EntityManagerFactoryImpl.createEntityManager(EntityManagerFactoryImpl.java:268)
services.DaoHelper.getEntityManager(DaoHelper.java:28)
services.UserResource.index(UserResource.java:29)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
java.lang.reflect.Method.invoke(Method.java:597)
com.sun.jersey.spi.container.JavaMethodInvokerFactory$1.invoke(JavaMethodInvokerFactory.java:60)
com.sun.jersey.server.impl.model.method.dispatch.AbstractResourceMethodDispatchProvider$TypeOutInvoker._dispatch(AbstractResourceMethodDispatchProvider.java:185)
com.sun.jersey.server.impl.model.method.dispatch.ResourceJavaMethodDispatcher.dispatch(ResourceJavaMethodDispatcher.java:75)
com.sun.jersey.server.impl.uri.rules.HttpMethodRule.accept(HttpMethodRule.java:288)
com.sun.jersey.server.impl.uri.rules.ResourceClassRule.accept(ResourceClassRule.java:108)
com.sun.jersey.server.impl.uri.rules.RightHandPathRule.accept(RightHandPathRule.java:147)
com.sun.jersey.server.impl.uri.rules.RootResourceClassesRule.accept(RootResourceClassesRule.java:84)
com.sun.jersey.server.impl.application.WebApplicationImpl._handleRequest(WebApplicationImpl.java:1469)
com.sun.jersey.server.impl.application.WebApplicationImpl._handleRequest(WebApplicationImpl.java:1400)
com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1349)
com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1339)
com.sun.jersey.spi.container.servlet.WebComponent.service(WebComponent.java:416)
com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:537)
com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:699)
javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
[1]: http://www.ibm.com/developerworks/web/library/wa-datawebapp/

Answer 1

JPA is not implementation It is a specification so you need to add one of the implementation of jpa(hibernate ,openjpa,eclipselink ..) to your class path.In your case eclipselink( <provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>).
.In addition,your code trying to use same entitymanager instance but this instances not thread safe so look this how to use entitymanager and entitymanagerfactory.