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。

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。

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：

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：

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。

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);}

本文作者： 钟潘
本文链接： http://zhongpan.tech/2020/07/20/035-a-trap-for-using-criteriaquery/

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