Java Spring JPA 查询返回 Null 而不是 List
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Spring JPA Query returns Null instead of List
提问by Vjeetje
I have an @Entity Videohaving a one-to-manyrelation with a List<Tag> tagsas one of its fields. I use the following @Repositoryusing Spring Datato get the most popular tags:
我与 a作为其字段之一@Entity Video具有一对多关系List<Tag> tags。我使用以下@Repository使用Spring Data来获取最受欢迎的标签：
@Repository
public interface TagRepository extends CrudRepository<Tag, Integer>{
@Query("SELECT t FROM Tag t WHERE (SELECT SUM(v.views) FROM Video v WHERE t MEMBER OF v.tags) > 0")
public List<Tag> findMostViewedTags(int maxTags);
}
The Queryis processed and considered valid by Spring, I tested the generated SQLvs my database locally and it returned 2 Tags. In my Code however, I receive the value Nullwhen I call the method findMostViewedTags(100).
该查询处理和认为是有效的春天，我测试生成的SQLVS我的数据库在本地，它返回2个标签。然而，在我的代码中，当我调用 findMostViewedTags(100) 方法时，我收到了Null值。
The Query lookup strategy is the default "CREATE_IF_NOT_FOUND".
查询查找策略是默认的“CREATE_IF_NOT_FOUND”。
- If there are no results found, should the method return an empty list or Null? My desired behavior is to receive an empty list.
- Why does the method call return
Nullinstead of aList<Tag>with size() 2?
- 如果没有找到结果，该方法应该返回一个空列表还是Null？我想要的行为是接收一个空列表。
- 为什么方法调用返回
Null而不是List<Tag>with size() 2？
采纳答案by Vjeetje
- The normal behavior is indeed returning an empty list if no results are found. If a
List<Object>is the return value of the method in the defined interface, the method should never returnNull. - The problem is that a parameter is given to the method and is not used anywhere in the Query. For some reason Springdecides to return a Nullin that case. Solution: remove the unused parameter or use the parameter in the Query.
- 如果没有找到结果，正常行为确实返回一个空列表。如果 a
List<Object>是已定义接口中方法的返回值，则该方法不应返回Null。 - 问题是该方法有一个参数，并没有在Query 中的任何地方使用。出于某种原因，Spring决定在这种情况下返回Null。解决方案：删除未使用的参数或使用Query 中的参数。
回答by user1747134
I have experienced similar problem. The cause was that I was using Mockito and have not correctly mocked the data with when().
我遇到过类似的问题。原因是我使用的是 Mockito 并且没有正确地使用 .mockito 模拟数据when()。
