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 listVmhostSpecWithRelationByPage(String name) {
 Specification spec = (root, cq, cb) -> {
 root.join("user", JoinType.LEFT);
 root.join("tenant", JoinType.LEFT);
 List predicates = new ArrayList<>();
 ......
 return cb.or(predicates.toArray(new javax.persistence.criteria.Predicate[0]));
 };
 PageRequest pagable = PageRequest.of(0, 5);
 Page 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 listVmhostSpecWithRelationByPage(String name) {
 Specification spec = (root, cq, cb) -> {
  root.join("user", JoinType.LEFT);
  root.join("tenant", JoinType.LEFT);
  List 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 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 listVmhostSpecWithRelationByPage(String name) {
 Specification spec = (root, cq, cb) -> {
 root.join("user", JoinType.LEFT);
 root.join("tenant", JoinType.LEFT);
 List 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 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 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 root, CriteriaQuery<?> query, CriteriaBuilder criteriaBuilder);
}