JPA:TypedQuery有时返回null而不是NoResultException
通常我使用NoResultException返回一个“空”对象,例如空错误列表或新的BigInteger(“0”),如果我从TypedQuery中没有得到任何结果。现在,事实证明,这有时是行不通的。突然,getSingleResult()返回null而不是导致NoResultException,我不明白为什么。看看这个例子:JPA:TypedQuery有时返回null而不是NoResultException,jpa,hql,jpa-2.0,Jpa,Hql,Jpa 2.0,通常我使用NoResultException返回一个“空”对象,例如空错误列表或新的BigInteger(“0”),如果我从TypedQuery中没有得到任何结果。现在,事实证明,这有时是行不通的。突然,getSingleResult()返回null而不是导致NoResultException,我不明白为什么。看看这个例子: public BigInteger pointsSumByAccountId(long accountId) { try { TypedQu
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");
}
}
这对我来说似乎是正确的行为 当没有返回任何行时,将抛出
NoResultException
,但在您的案例中,sum
只返回一行值为null
的行。来自JPA 2.0规范:
如果使用了SUM、AVG、MAX或MIN,并且没有可用于聚合函数的值
应用时,聚合函数的结果为空
如果要获取0
而不是null
,请使用coalesce
:
select coalesce(sum(p.value), 0) ...
我知道空集的平均值、最大值和最小值未定义,但空集的总和为0。我想知道为什么它们会失败。对于非空集,SUM、AVG、MAX或MIN也可以是0。返回null将确认空集。
/**
* 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();
select coalesce(sum(p.value), 0) ...