Can a JPA Query return results as a Java Map?

Question

We are currently building a Map manually based on the two fields that are returned by a named JPA query because JPA 2.1 only provides a getResultList() method:

@NamedQuery{name="myQuery",query="select c.name, c.number from Client c"}

HashMap<Long,String> myMap = new HashMap<Long,String>();

for(Client c: em.createNamedQuery("myQuery").getResultList() ){
     myMap.put(c.getNumber, c.getName);
}

But, I feel like a custom mapper or similar would be more performant since this list could easily be 30,000+ results.

Any ideas to build a Map without iterating manually.

(I am using OpenJPA, not hibernate)

Answer 1

There is no standard way to get JPA to return a map.

see related question: JPA 2.0 native query results as map

Iterating manually should be fine. The time to iterate a list/map in memory is going to be small relative to the time to execute/return the query results. I wouldn't try to futz with the JPA internals or customization unless there was conclusive evidence that manual iteration was not workable.

Also, if you have other places where you turn query result Lists into Maps, you probably want to refactor that into a utility method with a parameter to indicate the map key property.