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 items = (List) 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 query = em.createNamedQuery("jedisQry", Jedi.class);
List 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 map(Class type, Object[] tuple){
   List> tupleTypes = new ArrayList<>();
   for(Object field : tuple){
      tupleTypes.add(field.getClass());
   }
   try {
      Constructor 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  List map(Class type, List records){
   List result = new LinkedList<>();
   for(Object[] record : records){
      result.add(map(type, record));
   }
   return result;
}

public static  List getResultList(Query query, Class type){
  @SuppressWarnings("unchecked")
  List 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 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 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:

    SELECT name,age FROM jedi_table



        
            
            
        
    

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