In Spring Data JPA, projects can also be specified during runtime by using generic repository methods. For example:

public interface EmployeeRepository extends CrudRepository<Employee, Long> {
   <T> List<T> findByDept(String dept, Class<T> type);
}

Example

Entity

@Entity
public class Employee{
    private @Id
    @GeneratedValue
    Long id;
    private String name;
    private String dept;
    private int salary;
    private String phone;
    .............
}

Repository

public interface EmployeeRepository extends CrudRepository<Employee, Long> {

   <T> List<T> findByDept(String dept, Class<T> type);
}

Projection classes

public class SalaryInfo {
    private String name;
    private int salary;

    public SalaryInfo(String name, int salary) {
        this.name = name;
        this.salary = salary;
    }
    .............
}

public class ContactInfo {
    private String name;
    private String phone;

    public ContactInfo(String name, String phone) {
        this.name = name;
        this.phone = phone;
    }
    .............
}

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(" -- finding SalaryInfo for IT dept --");
        List<SalaryInfo> list = repo.findByDept("IT", SalaryInfo.class);
        list.forEach(System.out::println);

        System.out.println(" -- finding ContactInfo for IT dept --");
        List<ContactInfo> list2 = repo.findByDept("IT", ContactInfo.class);
        list2.forEach(System.out::println);
    }

    private List<Employee> createEmployees() {
        return Arrays.asList(
                Employee.create("Diana", "Admin", 3000, "111-111-111"),
                Employee.create("Mike", "IT", 1000, "222-222-222"),
                Employee.create("Rose", "Admin", 4000, "333-333-333"),
                Employee.create("Sara", "Admin", 3500, "444-444-444"),
                Employee.create("Tanaka", "IT", 3000, "555-555-555"),
                Employee.create("Charlie", "IT", 450, "666-666-666")
        );
    }
}

Main class

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', dept='Admin', salary=3000}
Employee{id=2, name='Mike', dept='IT', salary=1000}
Employee{id=3, name='Rose', dept='Admin', salary=4000}
Employee{id=4, name='Sara', dept='Admin', salary=3500}
Employee{id=5, name='Tanaka', dept='IT', salary=3000}
Employee{id=6, name='Charlie', dept='IT', salary=450}
 -- finding SalaryInfo for IT dept --
SalaryInfo{name='Mike', salary=1000}
SalaryInfo{name='Tanaka', salary=3000}
SalaryInfo{name='Charlie', salary=450}
 -- finding ContactInfo for IT dept --
ContactInfo{name='Mike', phone='222-222-222'}
ContactInfo{name='Tanaka', phone='555-555-555'}
ContactInfo{name='Charlie', phone='666-666-666'}

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 5.3.6.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