Using an interface for mocking purposes in unit tests for Java with Spring

I have a problem trying to mock interfaces in my API based on Spring. I am getting this error:

Description:

Parameter 0 of constructor in bm.app.services.Service required a bean of type 'bm.app.services.DiscountProvider' that could not be found.


Action:

Consider defining a bean of type 'bm.app.services.DiscountProvider' in your configuration.

And I just don't understand it. I am adding an interface (DiscountProvider) as a parameter for a constructor of my service class (Service) in order to use Mockito for testing purposes.

The beginning of my Service (I think the only relevant part here) looks like this:

@org.springframework.stereotype.Service
public class Service {

    DiscountProvider discountProvider;

    public Service(DiscountProvider discountProvider) {
        this.discountProvider = discountProvider;
    }

The interface looks like this:

import org.springframework.stereotype.Component;

import java.math.BigDecimal;

@Component
public interface DiscountProvider {

    BigDecimal getThePriceOfTheProduct();
}

And my test class looks like that:

@Component
public class ServiceTests {

    Service service;
    DiscountProvider discountProvider;

    @Test
    @DisplayName("Should return the value higher by ten percent.")
    void shouldReturnValueHigherByTenPercent() {
        //given
        Mockito.when(discountProvider.getThePriceOfTheProduct()).thenReturn(new BigDecimal("20.00"));
        AdditionalMiniProduct product = new AdditionalMiniProduct(UUID.randomUUID(), new BigDecimal("300.00"), "Yellow Loan");
        //when
        BigDecimal result = service.increaseByGivenAmount(product.getNetPrice(), discountProvider.getThePriceOfTheProduct());
        //then
        assertThat(result).isEqualTo("315.00");
    }

    @Test
    void shouldReturnACorrectRiskLevelValue() {
        int months = 18;
        assertEquals(5, calculateRiskLevelByNumberOfMonths(months));
    }

    @Test
    void shouldReturnACorrectEnumValueBasedOnNumberOfMonths() {
        int months = 26;
        assertEquals(RiskLevel.HIGH, giveTheRiskNameBasedOnNumberOfMonths(months));
    }

    @Test
    void shouldReturnAPriceOfAProvidedFinancialProduct() {
        BigDecimal productPrice = new BigDecimal(1200);
        String productName = "SilverLoan";
        assertEquals(productPrice,findAProductPriceByGivenName(productName));
    }

    @BeforeEach
    void setUp() {
        service = new Service(discountProvider);
        discountProvider = Mockito.mock(DiscountProvider.class);
    }


}

Now, I am fairly new to testing, so I do expect to have made some rookie mistakes. The thing is, that's my first attempt to have more "complex" unit tests run with one of my Spring apps and I just don't get it. It refuses to acknowledge the @Component (or any other I managed to find) annotation.

If you are unit testing your Service class, you should mock all external dependencies of this class under test.

Assuming you are using JUnit 5, a possible test setup can look like the following

@ExtendWith(MockitoExtension.class)
public class ServiceTest {

  @Mock
  private DiscountProvider discountProvider;

  @InjectMocks
  private Service service;

  @Test
  @DisplayName("Should return the value higher by ten percent.")
  void shouldReturnValueHigherByTenPercent() {
    //given
    Mockito.when(discountProvider.getThePriceOfTheProduct()).thenReturn(new BigDecimal("20.00"));
    AdditionalMiniProduct product = new AdditionalMiniProduct(UUID.randomUUID(), new BigDecimal("300.00"), "Yellow Loan");
     
    //when
    BigDecimal result = service.increaseByGivenAmount(product.getNetPrice(), discountProvider.getThePriceOfTheProduct());

    //then
    assertThat(result).isEqualTo("315.00");
  }

}

In addition, you don't need @Component for your test classes. This annotation is used for your production code to mark Spring beans.