Spring Data JPA之动态查询
在日常工作中，Spring Data JPA的使用给我们带来了极大的方便，但是实际业务中很多场景需要支持动态查询。比如前端查询功能提供了很多查询条件，用户可以根据一部分条件进行查询，那么后端就需要支持可配置的查询服务。在使用mybatis等时，可以用动态查询的方式轻松搞定，但是对于初次使用Spring Data JPA的小白来说，着实有些困难。本人也是在实际工作中参考资料，一步步摸索，掌握了一定的方法，现跟大家分享。
通过了解发现，Spring Data JPA是支持动态查询的，需要我们的repo继承JpaSpecificationExecutor接口，使用的时候传入相应参数即可。先看一下JpaSpecificationExecutor接口：
public interface JpaSpecificationExecutor {
T findOne(Specification spec);
List findAll(Specification spec);
Page findAll(Specification spec, Pageable pageable);
List findAll(Specification spec, Sort sort);
long count(Specification spec);
} 其中，Pageable是分页查询用的，Sort是排序用的，比较简单，先不做介绍，本次主要介绍Specification。先看看它的定义：
public interface Specification {
Predicate toPredicate(Root root, CriteriaQuery query, CriteriaBuilder cb);
} Root是查询的根对象，CriteriaBuilder可以用来构建查询关系。下面以一个实例来介绍具体使用：
下面是一个保存学生信息的表t_student：
先构建一下查询条件的参数体：
public class StudentParam {
private String name; //姓名
private String address; //住址
private String school; //学校
private String birthday; //出生日期
//省略get、set方法。
}查询条件包含姓名、住址、学校、出生日期等。
看一下repo接口：
public interface StudentRepo extends CrudRepository, JpaSpecificationExecutor,
PagingAndSortingRepository {
} 业务实现代码：
public List queryStudent(final StudentParam studentParam) {
Specification specification = new Specification() {
public Predicate toPredicate(Root root, CriteriaQuery query, CriteriaBuilder cb) {
List predicates = new ArrayList();
if (studentParam.getName() != null && !"".equals(studentParam.getName())) {
predicates.add(cb.equal(root.get("name"), studentParam.getName()));
}
if (studentParam.getStartDate() != null && !"".equals(studentParam.getStartDate())) {
predicates.add(cb.greaterThan(root.get("birthday"), studentParam.getStartDate()));
}
if (studentParam.getEndDate() != null && !"".equals(studentParam.getEndDate())) {
predicates.add(cb.lessThan(root.get("birthday"), studentParam.getEndDate()));
}
return cb.and(predicates.toArray(new Predicate[predicates.size()]));
}
};
List studentEntities = studentRepo.findAll(specification);
List students = new ArrayList();
for (StudentEntity studentEntity : studentEntities) {
students.add(studentCopier.copy(studentEntity));
}
return students;
} 说明：以其中一行代码：predicates.add(cb.equal(root.
测试代码：
@Test
public void queryStudentTest() {
StudentParam studentParam = new StudentParam();
studentParam.setName("张三");
studentParam.setStartDate("2001-03-01");
studentParam.setEndDate("2002-03-01");
List students = studentService.queryStudent(studentParam);
for (Student student : students) {
info(student.toString());
}
} 执行结果：
Hibernate:
select
studentent0_.id as id1_0_,
studentent0_.address as address2_0_,
studentent0_.birthday as birthday3_0_,
studentent0_.name as name4_0_,
studentent0_.school as school5_0_,
studentent0_.tel as tel6_0_
from
t_student studentent0_
where
studentent0_.name=?
and studentent0_.birthday>?
and studentent0_.birthday在执行中，我们发现Spring Data JPA根据配置的查询条件自动生成了条件语句：where
studentent0_.name=?
and studentent0_.birthday>?
and studentent0_.birthday
上例中，将查询条件通过and关联起来，进一步，如果我们需要用的or，该如何实现？看下面：
public List queryStudent(final StudentParam studentParam) {
Specification specification = new Specification() {
public Predicate toPredicate(Root root, CriteriaQuery query, CriteriaBuilder cb) {
List predicates = new ArrayList();
if (studentParam.getName() != null && !"".equals(studentParam.getName())) {
predicates.add(cb.equal(root.get("name"), studentParam.getName()));
}
if (studentParam.getStartDate() != null && !"".equals(studentParam.getStartDate())) {
predicates.add(cb.greaterThan(root.get("birthday"), studentParam.getStartDate()));
}
if (studentParam.getEndDate() != null && !"".equals(studentParam.getEndDate())) {
predicates.add(cb.lessThan(root.get("birthday"), studentParam.getEndDate()));
}
//定义or的条件数组
List orPredicates = new ArrayList();
if (studentParam.getSchool() != null && !"".equals(studentParam.getSchool())) {
orPredicates.add(cb.equal(root.get("school"), studentParam.getSchool()));
}
if (studentParam.getAddress() != null && !"".equals(studentParam.getAddress())) {
orPredicates.add(cb.equal(root.get("address"), studentParam.getAddress()));
}
//生成or的查询表达式
Predicate orPredicate = cb.or(orPredicates.toArray(new Predicate[orPredicates.size()]));
//与and结合起来
predicates.add(orPredicate);
return cb.and(predicates.toArray(new Predicate[predicates.size()]));
}
};
List studentEntities = studentRepo.findAll(specification);
List students = new ArrayList();
for (StudentEntity studentEntity : studentEntities) {
students.add(studentCopier.copy(studentEntity));
}
return students;
} 测试代码：
@Test
public void queryStudentTest() {
StudentParam studentParam = new StudentParam();
studentParam.setName("张三");
studentParam.setStartDate("2001-03-01");
studentParam.setEndDate("2002-03-01");
studentParam.setAddress("清江小区");
studentParam.setSchool("西城中学");
List students = studentService.queryStudent(studentParam);
for (Student student : students) {
info(student.toString());
}
} 执行结果：
Hibernate:
select
studentent0_.id as id1_0_,
studentent0_.address as address2_0_,
studentent0_.birthday as birthday3_0_,
studentent0_.name as name4_0_,
studentent0_.school as school5_0_,
studentent0_.tel as tel6_0_
from
t_student studentent0_
where
studentent0_.name=?
and studentent0_.birthday>?
and studentent0_.birthdaySpring Data JPA 支持的动态查询也非常强大，实际工作中可以根据情况灵活配置。希望我的分享能给需要的朋友一点帮助，不足之处，敬请斧正。