Spring Data JPA DTO Projection Example

Using DTO (Data Transfer Object) projections in Spring Data JPA is a powerful way to select specific columns from a database, reducing the overhead of selecting unnecessary data. Here's a step-by-step guide on how to set up DTO projections using an Employee entity as an example.

1. Setting up the project

Ensure you have the necessary dependencies in your pom.xml:


2. Define the Entity

public class Employee {
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String firstName;
    private String lastName;
    private String department;
    private Double salary;

    // getters, setters, etc.

3. Create a DTO Projection

Suppose you want to retrieve only the firstName, lastName, and department:
public class EmployeeInfoDto {
    private final String firstName;
    private final String lastName;
    private final String department;

    public EmployeeInfoDto(String firstName, String lastName, String department) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.department = department;

    // getters
Make sure the fields are final and initialized via a constructor. 

4. Create the Repository

Define a custom query using JPQL in the repository:
public interface EmployeeRepository extends JpaRepository<Employee, Long> {

    @Query("SELECT new com.yourpackage.EmployeeInfoDto(e.firstName, e.lastName, e.department) FROM Employee e")
    List<EmployeeInfoDto> findAllEmployeeInfo();
The @Query selects the desired fields and maps them to the EmployeeInfoDto

5. Use the Repository in a Service

Now, utilize the repository method in your service layer:
public class EmployeeService {

    private EmployeeRepository employeeRepository;

    public List<EmployeeInfoDto> getAllEmployeeInfo() {
        return employeeRepository.findAllEmployeeInfo();

6. Test the Service

Set up a test to ensure your DTO projections work correctly:
public class EmployeeServiceTest {

    private EmployeeService employeeService;

    public void testFetchEmployeeInfo() {
        List<EmployeeInfoDto> employeeInfo = employeeService.getAllEmployeeInfo();
        for (EmployeeInfoDto info : employeeInfo) {

7. Why Use DTO Projections? 

Performance: By selecting only the needed columns from a database, there's less data to transfer, process, and map. 

Avoiding Fetching Sensitive Data: If an entity has sensitive attributes (like passwords or private keys), using DTOs can avoid accidentally fetching or exposing them. 

Flexibility: DTOs allow for shaping data differently based on the context. The same entity can have multiple DTOs based on different use cases.