Using JPA Metamodel in criteria API throws NPE first time - jpa

Question

I am using JPA Criteria API with JPA meta model.
final CriteriaBuilder cb = em.getCriteriaBuilder();
final CriteriaQuery<T> criteria = cb.createQuery(User.class);
final Root<T> root = criteria.from(User.class);
criteria.select(root).where(cb.equal(root.get(User_.username), value));
return em.createQuery(criteria).getResultList();
here the problem is root.get method throws NPE first time after the server starts (i.e first execution after server start), but the same code works fine from second execution onwards. It doesn't matter when the first execution executed after server started. It may be executed after few seconds or after few minutes.
Following is the exception thrown:
java.lang.NullPointerException: null
at org.apache.openjpa.persistence.criteria.PathImpl.get(PathImpl.java:245) ~[openjpa-2.4.2.jar:2.4.2]
If you look at the source code of PathImpl.get
public <Y> Path<Y> get(SingularAttribute<? super X, Y> attr) {
if (getType() != attr.getDeclaringType()) {
attr = (SingularAttribute)((ManagedType)getType()).getAttribute(attr.getName());
}
return new PathImpl<X,Y>(this, (Members.SingularAttributeImpl<? super X, Y>)attr, attr.getJavaType());
}
it access attr.getDeclaringType()
If I look at the JPA meta model which is auto generated it looks as follows:
public class User_ {
public static volatile SingularAttribute<User,String> username;
...
}
username is definitely null, however the code works well except first time.
It appears during runtime JPA set value to User_.username.
so question is how do prevent throwing NPE first time!
Using TomEE 7.0.4