How to select multiple rows without using JPQL? - jpa

Question

I am using JPA, so to get one row of data, I could do:
Person p = this.em.find(Person.class, 123);
// Note: 'em' is the EntityManager.
But how can I get multiple rows? How can I get something like SQL's SELECT * FROM people WHERE age>18;?
I know how to use JPQL to get multiple rows (in this case, all rows):
TypedQuery<Person> q = em.createQuery("SELECT p FROM Person p", Person.class);
List<Person> results = q.getResultList();
But is there a way to do this without having to write the JPQL (i.e. SELECT p FROM Person p) at all?
Note: This is a learning exercise. I want to stick with Jakarta EE only (no Spring or any other API that is outside Jakarta EE).

Answer 1

CriteriaQuery is the way to go, however another "standard" approach is:
// imports
#Entity
#Table(name = "Person")
#NamedQuery(
name = "Person.queryAll",
query = "SELECT p FROM Person p")
public class Person implements Serializable {
...
}
The benefit of using the criteria is that errors can be detected earlier. Compile time vs Run time. However, a lot of readers/devs find JPQL easier to use and understand.
Using CriteriaQuery will look like below:
//skipping imports & config
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Person> q = cb.createQuery(Person.class);
Root<Person> c = q.from(Person.class);
q.select(c);

Answer 2

Example of CriteriaQuery API:
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Person> criteria = cb.createQuery(Person.class);
Root<Person> root = criteria.from(Person.class);
criteria.select(root);
criteria.where(cb.equal(root.get("id"), 123));
List<Person> people = em.createQuery(criteria).getResultList();
official docs

Answer 3

You can use the Criteria API as an alternative to JPQL:
// Imports.
import java.util.List;
import javax.persistence.TypedQuery;
import javax.persistence.criteria.CriteriaBuilder;
import javax.persistence.criteria.CriteriaQuery;
import javax.persistence.criteria.Root;
// Example usage of the Criteria API to get a list of all Persons in the database.
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Person> cq = cb.createQuery(Person.class);
Root<Person> root = cq.from(Person.class);
cq.select(root);
TypedQuery<Person> q = em.createQuery(cq);
List<Person> allPeople = q.getResultList();
where em is a javax.persistence.EntityManager.
Note: Most people use JPQL instead of Criteria API because JPQL (e.g. SELECT p FROM Person p) is far more concise than the code above. But the Criteria API has an advantage over JPQL in terms of type safety.
Further reading
Tutorial from Oracle: Using the Criteria API to Create Queries
Examples on Wikibooks: Java Persistence/Criteria
Javadoc for javax.persistence.criteria