As an alternative to projection interface having methods with SpEL, we can use Java 8 default methods for custom logic.

Quick Example

public interface EmployeeInfo {
  String getName();
  Department getDepartment();
  default String getDisplayString() { //custom logic
      return String.format("Name: %s, Dept: %s", getName(),
              getDepartment().getDeptName());
  }
}

Complete Example

Entities

@Entity
public class Employee {
    @Id
    @GeneratedValue
    private Integer id;
    private String name;
    @ManyToOne(cascade = CascadeType.ALL)
    private Department department;
    private int salary;
    .............
}

@Entity
public class Department {
    @Id
    @GeneratedValue
    private Integer id;
    private String deptName;
    private String location;
    .............
}

Projection interface using default method

public interface EmployeeInfo {
    String getName();
    Department getDepartment();
    default String getDisplayString() {
        return String.format("Name: %s, Dept: %s", getName(),
                getDepartment().getDeptName());
    }
}

Repository

public interface EmployeeRepository extends CrudRepository<Employee, Long> {
    List<EmployeeInfo> findBy();
}

Example Client

@Component
public class ExampleClient {

    @Autowired
    private EmployeeRepository repo;

    public void run() {
        List<Employee> employees = createEmployees();
        repo.saveAll(employees);

        System.out.println("-- finding all employees --");
        Iterable<Employee> all = repo.findAll();
        all.forEach(System.out::println);

        System.out.println("-- All EmployeeInfo --");
        List<EmployeeInfo> list = repo.findBy();
        System.out.println("-- display strings --");
        list.forEach(employeeInfo -> System.out.println(employeeInfo.getDisplayString()));
    }

    private List<Employee> createEmployees() {
        return Arrays.asList(
                Employee.of("Diana", Department.of("Admin", "NY"), 3000),
                Employee.of("Mike", Department.of("IT", "TX"), 35000),
                Employee.of("Rose", Department.of("Sales", "NC"), 4000),
                Employee.of("Sara", Department.of("Admin", "TX"), 3500),
                Employee.of("Joe", Department.of("IT", "TX"), 3000),
                Employee.of("Charlie", Department.of("IT", "NY"), 4500)
        );
    }
}

Main method

public class ExampleMain {

    public static void main(String[] args) {
        AnnotationConfigApplicationContext context =
                new AnnotationConfigApplicationContext(AppConfig.class);
        ExampleClient exampleClient = context.getBean(ExampleClient.class);
        exampleClient.run();
        EntityManagerFactory emf = context.getBean(EntityManagerFactory.class);
        emf.close();
    }
}

-- finding all employees --
Employee{id=1, name='Diana', department=Department{id=2, deptName='Admin', location='NY'}}
Employee{id=3, name='Mike', department=Department{id=4, deptName='IT', location='TX'}}
Employee{id=5, name='Rose', department=Department{id=6, deptName='Sales', location='NC'}}
Employee{id=7, name='Sara', department=Department{id=8, deptName='Admin', location='TX'}}
Employee{id=9, name='Joe', department=Department{id=10, deptName='IT', location='TX'}}
Employee{id=11, name='Charlie', department=Department{id=12, deptName='IT', location='NY'}}
-- All EmployeeInfo --
-- display strings --
Name: Diana, Dept: Admin
Name: Mike, Dept: IT
Name: Rose, Dept: Sales
Name: Sara, Dept: Admin
Name: Joe, Dept: IT
Name: Charlie, Dept: IT

Example Project

Dependencies and Technologies Used:

• spring-data-jpa 2.0.9.RELEASE: Spring Data module for JPA repositories.
  Uses org.springframework:spring-context version 5.0.8.RELEASE
• hibernate-core 5.3.5.Final: Hibernate's core ORM functionality.
  Implements javax.persistence:javax.persistence-api version 2.2
• h2 1.4.197: H2 Database Engine.
• JDK 1.8
• Maven 3.5.4