JPA Query.getResultList() - use in a generic way

I'm creating a complex query with multiple tables and need to list the result. Usually, I'm using the EntityManager and map the result to the JPA-Representation:

UserEntity user = em.find(UserEntity.class, "5");

Then I can access all values as the user UserEntity class defines it. But how can I access the field-values returned from a native, multiple-table query? What I get is a List of Objects. That's fine so far, but what "is" that Object? Array? Map? Collection? ...

//simpleExample
Query query = em.createNativeQuery("SELECT u.name,s.something FROM user u, someTable s WHERE s.user_id = u.id");
List list = query.getResultList();

//do sth. with the list, for example access "something" for every result row.

I guess the answer is quite simple, but most examples out there just show the usage when directly casting to a targetClass.

PS: In the example I could use the class-mappings of course. But in my case someTable is not managed by JPA, and therefore I don't have the entity nor do I have a class-representation of it, and since I'm joining like 20 tables, I don't want to create all the classes just to access the values.

Solution 1:

General rule is the following:

• If select contains single expression and it's an entity, then result is that entity
• If select contains single expression and it's a primitive, then result is that primitive
• If select contains multiple expressions, then result is Object[] containing the corresponding primitives/entities

So, in your case list is a List<Object[]>.

Solution 2:

Since JPA 2.0 a TypedQuery can be used:

TypedQuery<SimpleEntity> q = 
        em.createQuery("select t from SimpleEntity t", SimpleEntity.class);

List<SimpleEntity> listOfSimpleEntities = q.getResultList();
for (SimpleEntity entity : listOfSimpleEntities) {
    // do something useful with entity;
}

Solution 3:

If you need a more convenient way to access the results, it's possible to transform the result of an arbitrarily complex SQL query to a Java class with minimal hassle:

Query query = em.createNativeQuery("select 42 as age, 'Bob' as name from dual", 
        MyTest.class);
MyTest myTest = (MyTest) query.getResultList().get(0);
assertEquals("Bob", myTest.name);

The class needs to be declared an @Entity, which means you must ensure it has an unique @Id.

@Entity
class MyTest {
    @Id String name;
    int age;
}

Solution 4:

The above query returns the list of Object[]. So if you want to get the u.name and s.something from the list then you need to iterate and cast that values for the corresponding classes.