JPA:TypedQuery有时返回null而不是NoResultException

Question

通常，我使用NoResultException返回“空”对象，例如一个空的错误列表或新的BigInteger(“0”)(如果我从TypedQuery中没有得到任何结果)。现在事实证明，有时这是行不通的。突然getSingleResult()返回null而不是引起NoResultException，而且我不明白为什么。看这个例子:

public BigInteger pointsSumByAccountId(long accountId)
{
    try
    {
        TypedQuery<BigInteger> pointsQuery = entityManager.createNamedQuery(Points.SumByAccountId, BigInteger.class);
        pointsQuery.setParameter(Points.AccountIdParameter, accountId);

        return pointsQuery.getSingleResult();
    }
    catch (NoResultException e)
    {
        return new BigInteger("0");
    }
}

实体的重要组成部分...

@NamedQueries({@NamedQuery(name = "Points.sumByAccountId", query = "select sum(p.value) from Points p where p.validFrom <= current_timestamp() and p.validThru >= current_timestamp() and p.account.id = :accountId")})
public class Points
{
    private static final long serialVersionUID = -15545239875670390L;

    public static final String SumByAccountId = Points.class.getSimpleName() + ".sumByAccountId";
    public static final String AccountIdParameter = "accountId";
.
.
.

如果我使用没有结果的accountId，则将获得null而不是NoResultException。有任何想法为什么会这样吗？甚至TypedQuery的Javadoc都说它必须返回NoResultException:

/**
 * Execute a SELECT query that returns a single result.
 *
 * @return the result
 *
 * @throws NoResultException if there is no result
 * @throws NonUniqueResultException if more than one result
 * @throws IllegalStateException if called for a Java
 * Persistence query language UPDATE or DELETE statement
 * @throws QueryTimeoutException if the query execution exceeds
 * the query timeout value set and only the statement is
 * rolled back
 * @throws TransactionRequiredException if a lock mode has
 * been set and there is no transaction
 * @throws PessimisticLockException if pessimistic locking
 * fails and the transaction is rolled back
 * @throws LockTimeoutException if pessimistic locking
 * fails and only the statement is rolled back
 * @throws PersistenceException if the query execution exceeds
 * the query timeout value set and the transaction
 * is rolled back
 */
X getSingleResult();

Answer 1

对我来说，这似乎是正确的行为。

当不返回任何行时，将抛出NoResultException，但在您的情况下sum会返回仅一个带有null值的行。根据JPA 2.0规范:

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.

如果要获取 0而不是 null，请使用 coalesce:

select coalesce(sum(p.value), 0) ...