jpa2.0支持了Criteria查询,对应的接口是JpaSpecificationExecutor,我们可以利用Criteria实现动态查询
我们查看JpaSpecificationExecutor接口,发现这个接口里面的方法都需要Specification,JpaSpecificationExecutor接口方法代码如下:
T findOne(Specification<T> spec);
List<T> findAll(Specification<T> spec);
Page<T> findAll(Specification<T> spec, Pageable pageable);
List<T> findAll(Specification<T> spec, Sort sort);
long count(Specification<T> spec);
然后Specification却只有一个方法:
Predicate toPredicate(Root<T> root, CriteriaQuery<?> query, CriteriaBuilder cb);
参数解释:
CriteriaQuery接口:代表一个specific的顶层查询对象,它包含着查询的各个部分,比如:select 、from、where、group by、order by等
Root接口:代表Criteria查询的根对象,Criteria查询的查询根定义了实体类型,能为将来导航获得想要的结果,它与SQL查询中的FROM子句类似
CriteriaBuilder是Predicate实例的工厂,通过调用CriteriaBuilder 的条件方法( equal,notEqual, gt, ge,lt, le,between,like等)创建Predicate对象。
下面说几个简单的例子:
demo1:
//代码:
Specification<User> spec = new Specification<User>() {
@Override
public Predicate toPredicate(Root<User> root, CriteriaQuery<?> query, CriteriaBuilder cb) {
List<Predicate> list = new ArrayList<Predicate>(1);
list.add(cb.equal(root.get(User_.id),1));
Predicate[] p = new Predicate[list.size()];
return cb.and(list.toArray(p));
}
};
return spec;
}
//sql:
select * from user where id = 1
demo2:
//代码
public Specification<User> select(String userName){
Specification<User> spec = new Specification<User>() {
@Override
public Predicate toPredicate(Root<User> root, CriteriaQuery<?> query, CriteriaBuilder cb) {
List<Predicate> list = new ArrayList<Predicate>(1);
list.add(cb.equal(root.get(User_.id),1));
if(!StringUtils.isEmpty(userName)){
list.add(cb.equal(root.get(User_.userName),userName));
};
Predicate[] p = new Predicate[list.size()];
return cb.and(list.toArray(p));
}
};
return spec;
}
}
//sql
当userName不为空 select * from user where id = 1 and user_name = :userName
当userName为空 select * from user where id = 1
demo3:
//代码
public Specification<User> select(List<Integer> status ){
Specification<User> spec = new Specification<User>() {
@Override
public Predicate toPredicate(Root<User> root, CriteriaQuery<?> query, CriteriaBuilder cb) {
List<Predicate> list = new ArrayList<Predicate>(1);
list.add(cb.equal(root.get(User_.id),1));
Subquery<Role> subquery = query.subquery(Role.class);
Root<Role> subRoot = subquery.from(Role.class);
subquery.select(subRoot);
Predicate p = cb.equal(subRoot.get(Role_.id), root.get(User.role_id));
Predicate p2 = subRoot.get(Role_.status).in(status);
subquery.where(p,p2);
list.add(cb.not(cb.exists(subquery)));
}
Predicate[] p = new Predicate[list.size()];
return cb.and(list.toArray(p));
}
};
return spec;
}
}
//sql
select * from user a where id = 1 and not exists( select * from role b where b.id = a.role_id )