i have a project using spring data jpa .now i have a bussiness: i will find all data from table order by some filed,i user jpa Specification ,but failure.

code as :
@Entity
@Table(name = "GSOFT_USER")
public class GsoftUser implements java.io.Serializable {

private static final long serialVersionUID = -8681507674322020665L;

@GenericGenerator(name = "generator", strategy = "assigned")
@Id
@GeneratedValue(generator = "generator")
/**
* 主键
*/
private String uuid;
@NotEmpty(message = "请输入登录名")
private String loginName;

private String authType;

private String token;

private long tryMax;

private String pwd;

private int orderNum;

@Type(type="true_false")
private boolean enabled = true;

@Type(type="true_false")
private boolean sys;

private String depId;

@Transient
private String deptName;
........
}

dao interface:
public interface UserPersistence extends jpaRepository<GsoftUser, String>{
public List<GsoftUser> findByOrderByOrderNumDesc(Pageable pageable);
}
This interface ,i want find All data only sort by OrderNum field. when i run junit test,it throws Null point error, why?
i use spring data jpa,i could not track the daoImpl code, i do not know how spring data jpa reflect the interface name to search sentence? i feel this simple function cost my more time using spring data jpa.

who can help me?