Query dinamiche con le Specification

public interface UserRepository
    extends JpaRepository<User, Long>,
            JpaSpecificationExecutor<User> {
}

Specification<User> hasName(String name) {
    return (root, query, cb) ->
        cb.equal(root.get("name"), name);
}

List<User> users =
    userRepository.findAll(hasName("Alice"));

Specification<User> spec =
    hasName("Alice").and(isActive(true));
List<User> users = userRepository.findAll(spec);

Specification<User> spec = Specification.where(null);
if (name != null) {
    spec = spec.and(hasName(name));
}
if (active != null) {
    spec = spec.and(isActive(active));
}
return userRepository.findAll(spec);

Specification<User> nameLike(String part) {
    return (root, query, cb) ->
        cb.like(root.get("name"), "%" + part + "%");
}

Specification<User> olderThan(int age) {
    return (root, query, cb) ->
        cb.greaterThan(root.get("age"), age);
}

Specification<User> inCity(String city) {
    return (root, query, cb) ->
        cb.equal(root.join("address").get("city"), city);
}

Page<User> page = userRepository.findAll(
    spec, PageRequest.of(0, 20));

long matches = userRepository.count(hasName("Alice"));

public class UserSpecs {
    public static Specification<User> active() {
        return (root, q, cb) ->
            cb.isTrue(root.get("active"));
    }
}