JPA : How to convert a native query result set to POJO class collection

Question

I am using JPA in my project.

I came to a query in which I need to make join operation on five tables. So I created a native query which returns five fields.

Answer 1

I have found a couple of solutions to this.

Using Mapped Entities (JPA 2.0)

Using JPA 2.0 it is not possible to map a native query to a POJO, it can only be done with an entity.

For instance:

Query query = em.createNativeQuery("SELECT name,age FROM jedi_table", Jedi.class);
@SuppressWarnings("unchecked")
List<Jedi> items = (List<Jedi>) query.getResultList();

But in this case, Jedi, must be a mapped entity class.

An alternative to avoid the unchecked warning here, would be to use a named native query. So if we declare the native query in an entity

@NamedNativeQuery(
 name="jedisQry", 
 query = "SELECT name,age FROM jedis_table", 
 resultClass = Jedi.class)

Then, we can simply do:

TypedQuery<Jedi> query = em.createNamedQuery("jedisQry", Jedi.class);
List<Jedi> items = query.getResultList();

This is safer, but we are still restricted to use a mapped entity.

Manual Mapping

A solution I experimented a bit (before the arrival of JPA 2.1) was doing mapping against a POJO constructor using a bit of reflection.

public static <T> T map(Class<T> type, Object[] tuple){
   List<Class<?>> tupleTypes = new ArrayList<>();
   for(Object field : tuple){
      tupleTypes.add(field.getClass());
   }
   try {
      Constructor<T> ctor = type.getConstructor(tupleTypes.toArray(new Class<?>[tuple.length]));
      return ctor.newInstance(tuple);
   } catch (Exception e) {
      throw new RuntimeException(e);
   }
}

This method basically takes a tuple array (as returned by native queries) and maps it against a provided POJO class by looking for a constructor that has the same number of fields and of the same type.

Then we can use convenient methods like:

public static <T> List<T> map(Class<T> type, List<Object[]> records){
   List<T> result = new LinkedList<>();
   for(Object[] record : records){
      result.add(map(type, record));
   }
   return result;
}

public static <T> List<T> getResultList(Query query, Class<T> type){
  @SuppressWarnings("unchecked")
  List<Object[]> records = query.getResultList();
  return map(type, records);
}

And we can simply use this technique as follows:

Query query = em.createNativeQuery("SELECT name,age FROM jedis_table");
List<Jedi> jedis = getResultList(query, Jedi.class);

JPA 2.1 with @SqlResultSetMapping

With the arrival of JPA 2.1, we can use the @SqlResultSetMapping annotation to solve the problem.

We need to declare a result set mapping somewhere in a entity:

@SqlResultSetMapping(name="JediResult", classes = {
    @ConstructorResult(targetClass = Jedi.class, 
    columns = {@ColumnResult(name="name"), @ColumnResult(name="age")})
})

And then we simply do:

Query query = em.createNativeQuery("SELECT name,age FROM jedis_table", "JediResult");
@SuppressWarnings("unchecked")
List<Jedi> samples = query.getResultList();

Of course, in this case Jedi needs not to be an mapped entity. It can be a regular POJO.

Using XML Mapping

I am one of those that find adding all these @SqlResultSetMapping pretty invasive in my entities, and I particularly dislike the definition of named queries within entities, so alternatively I do all this in the META-INF/orm.xml file:

<named-native-query name="GetAllJedi" result-set-mapping="JediMapping">
    <query>SELECT name,age FROM jedi_table</query>
</named-native-query>

<sql-result-set-mapping name="JediMapping">
        <constructor-result target-class="org.answer.model.Jedi">
            <column name="name" class="java.lang.String"/>
            <column name="age" class="java.lang.Integer"/>
        </constructor-result>
    </sql-result-set-mapping>

And those are all the solutions I know. The last two are the ideal way if we can use JPA 2.1.

Answer 2

All you need is a DTO with a constructor :

public class User2DTO implements Serializable {

    /** pode ser email ou id do Google comecando com G ou F para Facebook */
    private String username;

    private String password;

    private String email;

    private String name;

    private Integer loginType;

    public User2DTO(Object...fields) {
        super();
        this.username = (String) fields[0];
        this.name = (String) fields[1];
        this.email = (String) fields[2];
        this.password = (String) fields[3];
        this.loginType = (Integer) fields[4];
    }

and call it :

EntityManager em = repo.getEntityManager();
        Query q = em.createNativeQuery("SELECT u.username, u.name, u.email, 'blabla' as passe, login_type as loginType FROM users u");
        List<Object[]> objList = q.getResultList();
        List<User2DTO> ooBj = objList.stream().map(User2DTO::new).collect(Collectors.toList());

Answer 3

Using Hibernate :

@Transactional(readOnly=true)
public void accessUser() {
EntityManager em = repo.getEntityManager();
    org.hibernate.Session session = em.unwrap(org.hibernate.Session.class);
    org.hibernate.SQLQuery q = (org.hibernate.SQLQuery) session.createSQLQuery("SELECT u.username, u.name, u.email, 'blabla' as passe, login_type as loginType FROM users u").addScalar("username", StringType.INSTANCE).addScalar("name", StringType.INSTANCE).addScalar("email", StringType.INSTANCE).addScalar("passe", StringType.INSTANCE).addScalar("loginType", IntegerType.INSTANCE)
        .setResultTransformer(Transformers.aliasToBean(User2DTO.class));

    List<User2DTO> userList = q.list();
}