Spring Data - Custom DTO Query with filtering - spring

Question

I have a complexe application and I need to retrieve and filter 1000~5000 object for an xls export. Each object having multiple eager relationship (I need them for the export).
If I retrieve all the objects and their relationship as it is, I got some stackoverflow error.
Generaly when I need to make a big export, in order to make it efficient I use a DTO object with an #Query like this :
public interface myRepository extends JpaRepository<Car, Long> {
#Query("SELECT new com.blabla.myCustomObject(p.name, p.surname, c.model, c.number ...) "
+ "FROM Car c "
+ "LEFT JOIN c.person p "
+ "WHERE ... ")
List<myCustomObject> getExportCustomObject();
}
The problem is that the #Query is static and I want to add dynamic filter to my Query (Specifications, Criteria or some other system...)
How to do it ?

Answer 1

Specification cannot be used because this is only the where clause.
But you can use Criteria API. Here's an example. The BasicTeacherInfo is the DTO:
CriteriaQuery<BasicTeacherInfo> query = cb.createQuery(BasicTeacherInfo.class);
Root<Teacher> teacher = query.from(Teacher.class);
query.multiselect(teacher.get("firstName"),teacher.get("lastName"));
List<BasicTeacherInfo> results = em.createQuery(query).getResultList();

Answer 2

You can use #Param annotation to pass dynamic values to HQL, something like:
#Query("SELECT new com.blabla.myCustomObject(p.name, p.surname, c.model, c.number ...) "
+ "FROM Car c "
+ "LEFT JOIN c.person p "
+ "WHERE c.status = :status AND p.name = :name")
List<myCustomObject> getExportCustomObject(
#Param("status") Integer status,
#Param("name") String name
);

Answer 3

Below is one of the possible way where you can try to add offset and limit into your query you can make it dynamic with the help off placeholders.
Below is an sample pseudo code for reference:
Dao Layer:
#Query(value="SELECT e FROM tablename e WHERE condition_here ORDER BY e.id offset :offset limit:limit ")
public returnType yourMethod(String name, int offset, int limit);
Service Layer:
long count = number of records in db.
int a = // number of records to be fetched on each iterations
int num_iterations = count % a ;
int additionalrecords = count / a;
int start= 0;
while(num_iterations>0)
{
dao.yourMethod(start,a);
start = start+a;
count--;
// write your data to excel here
}
dao.yourMethod(start,additionalrecords);
Hope it is helpful.