如何使用Spring Data在Spring Boot中从实体中选择几个字段？

Question

I have a use case where I want to display the contents of an entity but hide certain fields. My entity is as follows -

Entity

public class StudentDetail {

@Id
private Long ID;
private String firstName;
private String middleName;
private String lastName;

@JsonFormat(pattern="dd-MMM-yyyy", timezone="IST")
@Temporal(TemporalType.DATE)
private Date dateOfBirth;
}

It has many other properties as well which I am not showing here.

Repository -

@Repository
public interface StudentDetailsRepository extends JpaRepository<StudentDetail, Integer> {
@Query("select d from StudentDetail d where month(d.dateOfBirth) = ?1 ")
    List<StudentDetail> getStudentListBasedOnDateOfBirth(int month);

}

Service class -

public List<StudentDetail> getStudentBirthdayDetails(int month) {
        List<StudentDetail> StudentDetail = StudentDetailsRepository.getStudentListBasedOnDateOfBirth(month);
        return StudentDetail;
    }

And there is a controller class which calls the Service class with a month parameter to filter the data set.

What I want to do is modify the query in Repository class and include only the firstname , middleName and lastName property. The Repository class should hide the dateOfBirth field. I realise that the following query will return the filtered items -

select d.firstName, d.middleName, d.lastName from StudentDetail d where month(d.dateOfBirth) = ?1 

However, the return type of the Repository class is of Entity Type StudentDetail . Selecting only few fields from it will result in error. So, I want to know what changes should I make in the repo / service and controller class (assuming only return types of the classes will change ) ?

Answer 1

This is called projection , and Spring offers you two ways to accomplish it.
Keep in mind this exists in JPA terms, not only in Spring.

Taking your Repository as a starting point

@Repository
public interface StudentDetailsRepository extends JpaRepository<StudentDetail, Integer> {
   ...
}

we can use

1. interface -based projection
   simply create an interface which represents the result you want to have

---

public interface StudentDetailProjection {
   String getFirstName();
   String getMiddleName();
   String getLastName();
}

and add a method to your Repository

@Repository
public interface StudentDetailsRepository extends JpaRepository<StudentDetail, Integer> {
   StudentDetailProjection get...(...);
}

Spring will subclass that interface automatically and it will ask JPA to execute a query which will extract only the specified fields.

2. class -based projection
   works in almost the same way as interface-based projection, but no proxying and sub-classing is necessary, as you're offering Spring a concrete class.

---

public class StudentDetailProjection {
   private final String getFirstName;
   private final String getMiddleName;
   private final String getLastName;

   public StudentDetailProjection(
      final String getFirstName,
      final String getMiddleName,
      final String getLastName,
   ) {...}

   // Getters
}

Documentation goes more in depth.

Also, a must read is this blog post by Vlad Mihalcea, the master of JPA.

---

The method might look, approximately, like

@Query("select new your.package.StudentDetailProjection(d.firstName, d.middleName, d.lastName) from StudentDetail d where month(d.dateOfBirth) = ?1")
List<StudentDetailProjection> getStudentListBasedOnDateOfBirth(final int month);

This will go along the concrete class option (2), because a constructor is required.