0%

CriteriaQuery使用的一个陷阱

使用Spring Data Jpa的CriteriaQuery进行动态条件查询时,可能会遇到一个陷阱,当条件为空时,查询不到任何结果,并不是期望的返回所有结果。这是为什么呢?

例如下述代码,当predicates为空时,返回结果总是为空。

1
2
3
4
5
6
7
8
9
10
11
12
public Page<VmhostWithRelationPO> listVmhostSpecWithRelationByPage(String name) {
Specification<VmhostWithRelationPO> spec = (root, cq, cb) -> {
root.join("user", JoinType.LEFT);
root.join("tenant", JoinType.LEFT);
List<javax.persistence.criteria.Predicate> predicates = new ArrayList<>();
......
return cb.or(predicates.toArray(new javax.persistence.criteria.Predicate[0]));
};
PageRequest pagable = PageRequest.of(0, 5);
Page<VmhostWithRelationPO> page = vmhostSpecWithRelationDao.findAll(spec, pagable);
return page;
}

看下or的注释就明白了,因为空条件总是为false,而and的空条件总是为true。所以,如果最后是and就没有问题,只有or的时候有问题。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
public interface CriteriaBuilder {

/**
* Create a conjunction of the given restriction predicates.
* A conjunction of zero predicates is true.
* @param restrictions zero or more restriction predicates
* @return and predicate
*/
Predicate and(Predicate... restrictions);


/**
* Create a disjunction of the given restriction predicates.
* A disjunction of zero predicates is false.
* @param restrictions zero or more restriction predicates
* @return or predicate
*/
Predicate or(Predicate... restrictions);
}

所以正确的写法应该这样:

1
2
3
4
5
6
7
8
9
10
11
12
public Page<VmhostWithRelationPO> listVmhostSpecWithRelationByPage(String name) {
Specification<VmhostWithRelationPO> spec = (root, cq, cb) -> {
root.join("user", JoinType.LEFT);
root.join("tenant", JoinType.LEFT);
List<javax.persistence.criteria.Predicate> predicates = new ArrayList<>();
......
return predicates.isEmpty() ? cb.conjunction() : cb.or(predicates.toArray(new javax.persistence.criteria.Predicate[0]));
};
PageRequest pagable = PageRequest.of(0, 5);
Page<VmhostWithRelationPO> page = vmhostSpecWithRelationDao.findAll(spec, pagable);
return page;
}

如果条件为空则返回一个空conjunction,也就是空的and,总是为true。

公司项目的代码中常见这种写法:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
public Page<VmhostWithRelationPO> listVmhostSpecWithRelationByPage(String name) {
Specification<VmhostWithRelationPO> spec = (root, cq, cb) -> {
root.join("user", JoinType.LEFT);
root.join("tenant", JoinType.LEFT);
List<javax.persistence.criteria.Predicate> predicates = new ArrayList<>();
......
if (predicates.isEmpty()) {
cq.where();
} else {
cq.where(cb.or(predicates.toArray(new javax.persistence.criteria.Predicate[0])));
}
return cq.getRestriction();
};
PageRequest pagable = PageRequest.of(0, 5);
Page<VmhostWithRelationPO> page = vmhostSpecWithRelationDao.findAll(spec, pagable);
return page;
}

也能正常工作,但是其实没有必要在toPredicate方法中调用where,toPredicate只需要返回条件,外层会调用where。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
public interface Specification<T> extends Serializable {


/**
* Creates a WHERE clause for a query of the referenced entity in form of a {@link Predicate} for the given
* {@link Root} and {@link CriteriaQuery}.
*
* @param root must not be {@literal null}.
* @param query must not be {@literal null}.
* @param criteriaBuilder must not be {@literal null}.
* @return a {@link Predicate}, may be {@literal null}.
*/
@Nullable
Predicate toPredicate(Root<T> root, CriteriaQuery<?> query, CriteriaBuilder criteriaBuilder);
}
-------------本文结束感谢您的阅读-------------