Following example shows how to use interface based nested Projections.

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;
    .............
}

Defining Projections

public interface EmployeeInfo {
    String getName();
    int getSalary();
    DeptInfo getDepartment();

    interface DeptInfo {
        String getDeptName();
    }
}

In above class, EmployeeInfo is the main projection to get the selected attributes and DeptInfo is the nested projection to get department name only.

Creating Repository

public interface EmployeeRepository extends CrudRepository<Employee, Long> {
    List<EmployeeInfo> findBy();
    List<EmployeeInfo> findBySalaryBetween(int salaryMin, int salaryMax);
    List<EmployeeInfo> findByDepartmentLocation(String location);
}

The third query method in above example is nested derived query method which applies condition equivalent to JPQL 'where employee.department.location = ?'.

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();
        for (EmployeeInfo ei : list) {
            EmployeeInfo.DeptInfo di = ei.getDepartment();
            System.out.printf("Name: %s, Salary: %s, Dept: %s%n",
                    ei.getName(), ei.getSalary(), di.getDeptName());
        }

        System.out.println(" -- EmployeeInfo with salary between 3000 and 4000 --");
        list = repo.findBySalaryBetween(3000, 4000);
        for (EmployeeInfo ei : list) {
            EmployeeInfo.DeptInfo di = ei.getDepartment();
            System.out.printf("Name: %s, Salary: %s, Dept: %s%n",
                    ei.getName(), ei.getSalary(), di.getDeptName());
        }

        System.out.println(" -- EmployeeInfo with dept Location TX --");
        list = repo.findByDepartmentLocation("TX");
        for (EmployeeInfo ei : list) {
            EmployeeInfo.DeptInfo di = ei.getDepartment();
            System.out.printf("Name: %s, Salary: %s, Dept: %s%n",
                    ei.getName(), ei.getSalary(), di.getDeptName());
        }
    }

    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)
        );
    }
}

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 --
Name: Diana, Salary: 3000, Dept: Admin
Name: Mike, Salary: 35000, Dept: IT
Name: Rose, Salary: 4000, Dept: Sales
Name: Sara, Salary: 3500, Dept: Admin
Name: Joe, Salary: 3000, Dept: IT
Name: Charlie, Salary: 4500, Dept: IT
 -- EmployeeInfo with salary between 3000 and 4000 --
Name: Diana, Salary: 3000, Dept: Admin
Name: Rose, Salary: 4000, Dept: Sales
Name: Sara, Salary: 3500, Dept: Admin
Name: Joe, Salary: 3000, Dept: IT
 -- EmployeeInfo with dept Location TX --
Name: Mike, Salary: 35000, Dept: IT
Name: Sara, Salary: 3500, Dept: Admin
Name: Joe, Salary: 3000, 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