映射到pojo列表的JPA命名查询的结果在从查询执行中未收到任何数据时不返回空列表

Question

I have a named query in jpa ( hibernate implementation ), the result of which is to be mapped to a list of custom pojo( myresult.java ). In one case the query doesn't return any values and I was expecting an empty list to be returned. Instead I am getting a list with one object of type " myresult ", with all the members initialized as null. I don't understand why this is happening...any help is much appreciated.. Thanks in advance.

Answer 1

I think the reason of your problem is explained in this link

Can javax.persistence.Query.getResultList() return null?

In JPA in place to return empty, returns a null, then in your case add null to the list to return, I worked with spring-data and return a empty list, i dont know why hibernate decide to use this approach in place of return an empty list

Answer 2

After quite a bit of trials I found that Jpa hibernate implementation has it that if only aggregated values are to be returned from a named query then in "no data" case, an empty list of pojos is not returned. Instead a list with a single object which has appropriate defaults set for attributes (null for sum, 0 for count etc) is returned. However, if only column values or column values along with aggregates are selected, then an empty list will be returned.