Question

JPQL queries can return custom result objects with the NEW operator:

  SELECT NEW myPackage.MyVO(e.fieldX, e.relationshipX.fieldY)
  FROM MyEntity AS e

This is very useful to feed VOs. The problem is, you have to create constructors that exactly match the number of arguments, order and types of your query projection. This starts to get messy when you use a lot of projections for the same VO… Either you have one big constructor in your VO and resort to a lot of NULL literals on your query, or your VO must have a lot of different constructors.

So my question is: Is there a way in JPQL to set result object fields through mutators instead of constructors?

To people with .NET background, I’m looking to a equivalent of LINQ + object initializers.

Answer 1

DataNucleus JPA certainly supports two ways of instantiating result objects using no non-standard annotations or calls, primarily driven by the fact that it also supports JDO and that has the requirement for it :-

1. Result type with argumented constructor (as you say)
2. Result type with default constructor, and with setters

Such as

TypedQuery<MyResultType> q = em.createQuery("SELECT x AS field1, y AS field2 FROM ...", MyResultType.class);

where MyResultType has setters “setField1”, “setField2”.