Return Stream based result from Query

Hibernate has already added a lot of alignments with Java 8, Stream and Optional are supported.

JPA 2.2 add official support for Stream result of query.

The following codes demonstrates query posts by keyword in JPA 2.1.

public List<Post> findByKeyword(String keyword) {
    CriteriaBuilder cb = this.entityManager.getCriteriaBuilder();

    CriteriaQuery<Post> q = cb.createQuery(Post.class);
    Root<Post> c = q.from(Post.class);

    List<Predicate> predicates = new ArrayList<>();

    if (null != keyword && "".equals(keyword.trim())) {
        predicates.add(
                cb.or(
                        cb.like(c.get(Post_.title), '%' + keyword + '%'),
                        cb.like(c.get(Post_.content), '%' + keyword + '%')
                )
        );
    }

    q.where(predicates.toArray(new Predicate[predicates.size()]));

    TypedQuery query = this.entityManager.createQuery(q);

    return query.getResultList();
}

This can be simplified in JPA 2.2. The following is the complete codes of PostRepository.

@Stateless
public class PostRepository extends AbstractRepository<Post, Long> {

    @PersistenceContext
    private EntityManager entityManager;

    public List<Post> findByKeyword(String keyword) {
        return stream().filter(p -> p.getTitle().contains(keyword) || p.getContent().contains(keyword))
                .collect(toList());
    }

    public long countByKeyword(String keyword) {
        return stream().filter(p -> p.getTitle().contains(keyword) || p.getContent().contains(keyword))
                .count();
    }

    @Override
    protected EntityManager entityManager() {
        return this.entityManager;
    }

}

And AbstractRepository is a generic Repository class can be reused for all entities.

public abstract class AbstractRepository<T extends AbstractEntity, ID> {

    protected abstract EntityManager entityManager();

    private Class<T> entityClass() {
        ParameterizedType parameterizedType = (ParameterizedType) this.getClass().getGenericSuperclass();
        return (Class<T>) parameterizedType.getActualTypeArguments()[0];
    }

    public List<T> findAll() {

        CriteriaBuilder cb = this.entityManager().getCriteriaBuilder();

        CriteriaQuery<T> q = cb.createQuery(entityClass());
        Root<T> c = q.from(entityClass());

        return entityManager().createQuery(q).getResultList();
    }

    public T save(T entity) {
        if (entity.getId() == null) {
            entityManager().persist(entity);

            return entity;
        } else {
            return entityManager().merge(entity);
        }
    }

    public T findById(Long id) {
        return entityManager().find(entityClass(), id);
    }

    public void delete(T entity) {
        T _entity = entityManager().merge(entity);
        entityManager().remove(_entity);
    }

    public Stream<T> stream() {
        CriteriaBuilder cb = this.entityManager().getCriteriaBuilder();
        CriteriaQuery<T> q = cb.createQuery(entityClass());
        Root<T> c = q.from(entityClass());

        return entityManager().createQuery(q).getResultStream();
    }

    public Optional<T> findOptionalById(Long id) {
       return Optional.ofNullable(findById(id));
    }

}

Grab the source codes from my GitHub account, and have a try.