What does JPA EntityManager.getSingleResult() return for a COUNT query?

What does EntityManager.getSingleResult() return for a COUNT query?

So.. what is the precise runtime type of foo?

Object foo = em.createQuery("SELECT COUNT(t) FR开发者_开发问答OM com.company.Thing t WHERE prop = :param")
       .setParameter("param", value).getSingleResult();

As per the JPA specification, COUNT returns a Long:

4.8.4 Aggregate Functions in the SELECT Clause The result of a query

may be the result of an aggregate function applied to a path expression.

The following aggregate functions can be used in the SELECT clause of a query: AVG, COUNT, MAX, MIN, SUM.

For all aggregate functions except COUNT, the path expression that is the argument to the aggregate function must terminate in a state-field. The path expression argument to COUNT may terminate in either a state-field or a association-field, or the argument to COUNT may be an identification variable.

Arguments to the functions SUM and AVG must be numeric. Arguments to the functions MAX and MIN must correspond to orderable state-field types (i.e., numeric types, string types, character types, or date types).

The Java type that is contained in the result of a query using an aggregate function is as follows:

• COUNT returns Long.
• MAX, MIN return the type of the state-field to which they are applied.
• AVG returns Double.
• SUM returns Long when applied to state-fields of integral types (other than BigInteger); Double when applied to state-fields of floating point types; BigInteger when applied to state-fields of type BigInteger; and BigDecimal when applied to state-fields of type BigDecimal.

If SUM, AVG, MAX, or MIN is used, and there are no values to which the aggregate function can be applied, the result of the aggregate function is NULL.

If COUNT is used, and there are no values to which COUNT can be applied, the result of the aggregate function is 0.

The argument to an aggregate function may be preceded by the keyword DISTINCT to specify that duplicate values are to be eliminated before the aggregate function is applied.

Null values are eliminated before the aggregate function is applied, regardless of whether the keyword DISTINCT is specified.

NB : there's a difference between JQPL and Native query

Query query = em.createQuery("SELECT COUNT(p) FROM PersonEntity p " );

query.getSingleResult().getClass().getCanonicalName() --> java.lang.Long

Query query = em.createNativeQuery("SELECT COUNT(*) FROM PERSON " );

query.getSingleResult().getClass().getCanonicalName() --> java.math.BigInteger

Native queries can return different objects for a COUNT based on the database driver; however, all those objects extend java.lang.Number, which implements the longValue() method.

AFAIK COUNT function returns Long

COUNT(t) specifically returns java.lang.Long. When its appears on its own in this context it is returned as-is.

(In other contexts the Long generated by COUNT may be wrapped, but not today.)

Obviously, a number which is the count from the query:) Its type by specification is Long.

You can return directly integer, rather than a long.

return query.getSingleResult() != null ? Integer.parseInt(query.getSingleResult().toString()) : 0;

return query.getSingleResult() != null ? Integer.parseInt(query.getSingleResult().toString()) : 0;

Second call query.getSingleResult() will throw org.hibernate.SessionException: Session is closed!

To avoid the org.hibernate.SessionException: Session is closed! use as below

Object obj = query.getSingleResult();
    if (obj != null) {
        Integer result = Integer.parseInt(obj.toString());
        return result;
    }

    return Integer.valueOf(0);

Count returns long.

Query query = em.createQuery("SELECT COUNT(*) FROM Mandate " );

return (Long) query.getSingleResult();

Mapped your entity with the Query.