Java Hibernate Count Rows

Question

I am using Java8 with Hibernate5 and JPA2. I would like to count the number of rows in a result set. I have the following code that works, however, I would like to know if there's a more efficient way of doing it? I think the code below first queries the entire result set and the counts the rows.

    final EntityManagerFactory entityManagerFactory = entityManager.getEntityManagerFactory();
    final CriteriaBuilder criteriaBuilder = entityManagerFactory.getCriteriaBuilder();
    CriteriaQuery<Rating> criteria = criteriaBuilder.createQuery(Rating.class);
    Root<Rating> root = criteria.from(Rating.class);
    ParameterExpression<Job> param = criteriaBuilder.parameter(Job.class);

    TypedQuery<Rating> queryRating = entityManager.createQuery(criteria);
    queryRating.setParameter(param, job);
    int results = queryRating.getResultList().size();

Is there a way of rather making the SQL do a count(*)?

UPDATE

Thanks to @chsdk below, I have a revised version of code:

    CriteriaBuilder qb = entityManager.getCriteriaBuilder();
    CriteriaQuery<Long> cq = qb.createQuery(Long.class);
    cq.select(qb.count(cq.from(Rating.class)));
    cq.where(/*your stuff*/);
    return entityManager.createQuery(cq).getSingleResult();

Question

How do I set the where clause with the Job parameter?

More info:

+--------+   +------------+    +-----+
| rating |   | rating_job |    | job |
+--------+   +------------+    +-----+
|   ID   |   |   RAT_ID   |    |  ID |
|        |   |   JOB_ID   |    |     |
+--------+   +------------+    +-----+

Rating.java

@ManyToOne(fetch=FetchType.EAGER)
@JoinTable
(
    name="rating_job",
    joinColumns={ @JoinColumn(name="RAT_ID", referencedColumnName="ID") },
    inverseJoinColumns={ @JoinColumn(name="JOB_ID", referencedColumnName="ID") }
)
private Job job;

UPDATE

Thanks to @chsdk, here is my version that works:

    final EntityManagerFactory entityManagerFactory = entityManager.getEntityManagerFactory();
    final CriteriaBuilder criteriaBuilder = entityManagerFactory.getCriteriaBuilder();
    CriteriaQuery<Long> criteria = criteriaBuilder.createQuery(Long.class);
    Root<Rating> root = criteria.from(Rating.class);
    ParameterExpression<Job> param = criteriaBuilder.parameter(Job.class);
    criteria.select(criteriaBuilder.count(root)).where(criteriaBuilder.equal(root.get("job"), param));
    TypedQuery<Long> queryRating = entityManager.createQuery(criteria);
    queryRating.setParameter(param, job);
    Long results = queryRating.getSingleResult();

    return results;

Answer 1

In JPA2 with CriteriaQuery you can do it this way:

final CriteriaBuilder criteriaBuilder = entityManagerFactory.getCriteriaBuilder();
CriteriaQuery<Long> criteria = qb.createQuery(Long.class);
Root<Country> root = criteria.from(Rating.class);
criteria.select(criteriaBuilder.count(root));

ParameterExpression<Job> param = criteriaBuilder.parameter(Job.class);
criteria.where(criteriaBuilder.equal(root.get("job"), param));

Make a CriteriaQuery<Long> instead of CriteriaQuery<Rating> so it gives you a row count when you count the rows of your criteria.from(Rating.class) result.

Edit:

I edited the answer code to include testing over Job given parameter in the query, respecting your entities mapping.

Then to execute your query you will need to write:

TypedQuery<Country> query = em.createQuery(criteria);
query.setParameter(param, yourJobObject);
Long resultsCount = query.getSingleResult();

Note that you need to wrap the query.getSingleResult() in a try ..catch block because it may throw an error.

Refernce:

Please check the answer here and this JPA Criteria API Queries tutorial for further reading.

Answer 2

Just an example, I use that to check the existence of entity.

CriteriaBuilder builder = em.getCriteriaBuilder();
CriteriaQuery<Long> cQuery = builder.createQuery(Long.class);
Root<Tag> from = cQuery.from(Tag.class);
cQuery.where(from.get("ID").in(ids));

CriteriaQuery<Long> nbTags = cQuery.select(builder.count(from));

return em.createQuery(nbTags).getSingleResult();

It just send me an 0 if their isn't any tags, otherwise the number of existing tags.