JPA Criteria API - Select only specific columns

Question

I would like to select only specific columns  ex  SELECT a FROM b   I have a generic DAO and what I came up with is   public List lt T gt  getAll boolean idAndVersionOnly        CriteriaBuilder builder   manager getCriteriaBuilder        CriteriaQuery lt T gt  criteria   builder createQuery entityClazz       Root lt T gt  root   criteria from entityClazz       if  idAndVersionOnly            criteria select root get  ID   get  VERSION        HERE IS ERROR       else           criteria select root             return manager createQuery criteria  getResultList        And the error is  The method select Selection lt   extends T gt   in the type CriteriaQuery lt T gt  is not applicable for the arguments  Path lt Object gt    How should I change that  I want to get a type T object that has only ID and VERSION fields  and all others are null   Type T extends AbstractEntity which has those 2 fields   entityClazz is T class

User · Accepted Answer

One of the JPA ways for getting only particular columns is to ask for a Tuple object.

In your case you would need to write something like this:

CriteriaQuery<Tuple> cq = builder.createTupleQuery();
// write the Root, Path elements as usual
Root<EntityClazz> root = cq.from(EntityClazz.class);
cq.multiselect(root.get(EntityClazz_.ID), root.get(EntityClazz_.VERSION));  //using metamodel
List<Tuple> tupleResult = em.createQuery(cq).getResultList();
for (Tuple t : tupleResult) {
    Long id = (Long) t.get(0);
    Long version = (Long) t.get(1);
}

Another approach is possible if you have a class representing the result, like T in your case. T doesn't need to be an Entity class. If T has a constructor like:

public T(Long id, Long version)

then you can use T directly in your CriteriaQuery constructor:

CriteriaQuery<T> cq = builder.createQuery(T.class);
// write the Root, Path elements as usual
Root<EntityClazz> root = cq.from(EntityClazz.class);
cq.multiselect(root.get(EntityClazz_.ID), root.get(EntityClazz_.VERSION));  //using metamodel
List<T> result = em.createQuery(cq).getResultList();

See this link for further reference.

User · Answer

cq select cb construct entityClazz class  root get  ID    root get  VERSION          HERE IS NO ERROR   https   wiki eclipse org EclipseLink UserGuide JPA Basic JPA Development Querying Criteria Constructors

User · Answer

First of all  I don t really see why you would want an object having only ID and Version  and all other props to be nulls  However  here is some code which will do that for you  which doesn t use JPA Em  but normal Hibernate  I assume you can find the equivalence in JPA or simply obtain the Hibernate Session obj from the em delegate Accessing Hibernate Session from EJB using EntityManager     List lt T gt  results   session createCriteria entityClazz       setProjection  Projections projectionList            add  Property forName  ID              add  Property forName  VERSION                setResultTransformer Transformers aliasToBean entityClazz         list      This will return a list of Objects having their ID and Version set and all other props to null  as the aliasToBean transformer won t be able to find them  Again  I am uncertain I can think of a situation where I would want to do that

User · Answer

You can do something like this  Session session   app factory openSession    CriteriaBuilder builder   session getCriteriaBuilder    CriteriaQuery query   builder createQuery    Root lt Users gt  root   query from Users class   query select root get  firstname     String name   session createQuery query  getSingleResult      where you can change  firstname  with the name of the column you want

[java] JPA & Criteria API - Select only specific columns

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