JPA 2.1 / Eclipselink: Reuse Criteria Queries as Named Queries - jpa

Question

i am trying to reuse a dynamic query as a named query as described here:
https://wiki.eclipse.org/EclipseLink/Release/2.5/JPA21#Add_Named_Query
the goal is to build the criteria-query only once and then reuse it as a namedquery if parameter did not change.
public static List<User>getUserByParameter(ParameterMap parameter){
EntityManager em = getEntityManager();
CriteriaBuilder builder = em.getCriteriaBuilder();
CriteriaQuery<User> criteriaQuery = builder.createQuery(User.class);
Root<User> user = criteriaQuery.from(User.class);
List<Predicate> predicates = new ArrayList<Predicate>();
//...build up the query depending on parameter
if (null != parameter.getStatus()){
predicates.add(builder.equal(user.<Integer>get("status"), parameter.getStatus()));
}
//etc.
criteriaQuery.select(user).where(predicates.toArray(new Predicate[]{}));
Query query = em.createQuery(criteriaQuery);
//now register this query as a namedQuery
em.getEntityManagerFactory().addNamedQuery("userByParameter", query);
return query.getResultList();
}
i thought about something like:
public static List<User>getUserByParameter(ParameterMap parameter){
Query userByParameter = em.createNamedQuery("userByParameter");
if (null != userByParameter){
return userByParameter.getResultList();
}else {
//build the dynamic query as above
}
}
this results in a nullpointer as the namedQuery doesn't exist the first time.
how can i reuse the query in the same method or in other words, how can i check in a clean way (without using try-catch) if a namedquery exists?

Answer 1

I'm not sure I understand the problem you are looking to solve. The getUserByParameter method is something that should be built on the EntityManagerFactory, when it is first initialized or obtained. Feel free to add properties to your factory if you wish to keep track of what you have added already, but these should be done only once, upfront during initialization.
What is confusing is that you are expecting the query results to be reused - named queries are designed to help reduce the cost of parsing and preparing queries. EclipseLink has a query cache feature that can return the results for you if the same parameters are used, without you needing to cache the query, its parameters and the results yourself.