JPA Query getResultList - use in a generic way

Question

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

User · Answer

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;
}

User · Answer

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

User · Answer

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 lt Object   gt

User · Answer

Since JPA 2 0 a TypedQuery can be used   TypedQuery lt SimpleEntity gt  q            em createQuery  select t from SimpleEntity t   SimpleEntity class    List lt SimpleEntity gt  listOfSimpleEntities   q getResultList    for  SimpleEntity entity   listOfSimpleEntities           do something useful with entity

User · Answer

Here is the sample on what worked for me   I think that put method is needed in entity class to map sql columns to java class attributes         simpleExample     Query query   em createNativeQuery   SELECT u name s something FROM user u   someTable s WHERE s user id   u id    NameSomething class       List list    List lt NameSomething class gt   query getResultList      Entity class        Entity     public class NameSomething             Id         private String name           private String something              getters setters                           Generic put method to map JPA native Query to this object                         param column             param value                     public void put Object column  Object value                if    String  column  equals  name                      setName String  value                 else if    String  column  equals  something                      setSomething  String  value

User · Answer

I had the same problem and a simple solution that I found was  List lt Object   gt  results   query getResultList    for  Object   result  results        SomeClass something    SomeClass result 1       something doSomething     I know this is defenitly not the most elegant solution nor is it best practice but it works  at least for me

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

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