Spring Data JPA: Query by Example?

Question

Using Spring Data JPA can I do a query by example where a particular entity instance is used as the search criteria?

For example (no pun intended), if I have a

Answer 1

Using Spring data's Specification interface I was able to approximate the use of query by example. Here's a PersonSpec class which implements Specification and requires an "example" person in order to setup the Predicate returned by the Specification:

public class PersonSpec implements Specification {

  private final Person example;

  public PersonSpec(Person example) {
    this.example = example;
  }

  @Override
  public Predicate toPredicate(Root root, CriteriaQuery cq, CriteriaBuilder cb) {
    List predicates = new ArrayList<>();

    if (StringUtils.isNotBlank(example.getLastName())) {
      predicates.add(cb.like(cb.lower(root.get(Person_.lastName)), example.getLastName().toLowerCase() + "%"));
    }

    if (StringUtils.isNotBlank(example.getFirstName())) {
      predicates.add(cb.like(cb.lower(root.get(Person_.firstName)), example.getFirstName().toLowerCase() + "%"));
    }

    if (example.getEmployed() != null) {
      predicates.add(cb.equal(root.get(Person_.employed), example.getEmployed()));
    }

    if (example.getDob() != null) {
      predicates.add(cb.equal(root.get(Person_.dob), example.getDob()));
    }

    return andTogether(predicates, cb);
  }

  private Predicate andTogether(List predicates, CriteriaBuilder cb) {
    return cb.and(predicates.toArray(new Predicate[0]));
  }
}

The repository is simply:

import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.data.jpa.repository.JpaSpecificationExecutor;

public interface PersonRepository extends JpaRepository, JpaSpecificationExecutor {}

Usage example:

Person example = new Person();
example.setLastName("James");
example.setEmployed(true);
PersonSpec personSpec = new PersonSpec(example);
List persons = personRepository.findAll(personSpec);