用springboot写一个条件查询

假设要查询一个学生表，根据不同的条件查询不同的结果，可以按照以下步骤进行：

1. 定义实体类

@Entity
@Table(name = "student")
public class Student {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;

    private String name;

    private Integer age;

    private String gender;

    // getter/setter方法省略
}

2. 定义Repository接口

@Repository
public interface StudentRepository extends JpaRepository<Student, Integer>, JpaSpecificationExecutor<Student> {
}

3. 定义查询条件

public class StudentSpecification {
    public static Specification<Student> nameLike(String name) {
        return (root, criteriaQuery, criteriaBuilder) -> criteriaBuilder.like(root.get("name"), "%" + name + "%");
    }

    public static Specification<Student> ageGreaterThanOrEqualTo(Integer age) {
        return (root, criteriaQuery, criteriaBuilder) -> criteriaBuilder.greaterThanOrEqualTo(root.get("age"), age);
    }

    public static Specification<Student> genderEqual(String gender) {
        return (root, criteriaQuery, criteriaBuilder) -> criteriaBuilder.equal(root.get("gender"), gender);
    }
}

4. 编写Controller层代码

@RestController
@RequestMapping("/students")
public class StudentController {
    @Autowired
    private StudentRepository studentRepository;

    @GetMapping
    public List<Student> query(@RequestParam(required = false) String name,
                                @RequestParam(required = false) Integer age,
                                @RequestParam(required = false) String gender) {
        Specification<Student> specification = Specification.where(null);
        if (StringUtils.isNotBlank(name)) {
            specification = specification.and(StudentSpecification.nameLike(name));
        }
        if (age != null) {
            specification = specification.and(StudentSpecification.ageGreaterThanOrEqualTo(age));
        }
        if (StringUtils.isNotBlank(gender)) {
            specification = specification.and(StudentSpecification.genderEqual(gender));
        }
        return studentRepository.findAll(specification);
    }
}

以上代码实现了根据不同的条件查询不同的结果，当查询条件为空时返回所有的结果。可以通过访问`http://localhost:8080/students?name=张三&age=18&gender=男`来获取符合条件的学生信息。