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使用Spring Data Jpa的CriteriaQuery一個(gè)陷阱

更新時(shí)間：2020年11月12日 08:35:38 作者：鐘潘

使用Spring Data Jpa的CriteriaQuery進(jìn)行動(dòng)態(tài)條件查詢時(shí)，可能會遇到一個(gè)陷阱，當(dāng)條件為空時(shí)，查詢不到任何結(jié)果，并不是期望的返回所有結(jié)果。這是為什么呢？

例如下述代碼，當(dāng)predicates為空時(shí)，返回結(jié)果總是為空。

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的注釋就明白了，因?yàn)榭諚l件總是為false，而and的空條件總是為true。所以，如果最后是and就沒有問題，只有or的時(shí)候有問題。

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);
}

所以正確的寫法應(yīng)該這樣：

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;
 }

如果條件為空則返回一個(gè)空conjunction，也就是空的and，總是為true。

公司項(xiàng)目的代碼中常見這種寫法：

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，但是其實(shí)沒有必要在toPredicate方法中調(diào)用where，toPredicate只需要返回條件，外層會調(diào)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);
}