Simplify mocking dependencies with @InjectMocks

In my Spring Boot app, I have some tests that look like this

@ExtendWith(MockitoExtension.class)
class InviteServiceTests {

    @Mock
    private UserService userService;

    @Mock
    private EmailSendingService emailSendingService;

    @InjectMocks
    private InviteService inviteService;

    @Test
    void testSomething() {
        inviteService.inviteUser("user@example.com");
        verify(emailSendingService).sendEmail("user@example.com");
    }
}

I have to declare userService because it's a dependency of inviteService that is called during inviteService.inviteUser. However I don't need to mock or verify any userService methods, so IntelliJ marks this as an unused field.

I'm aware I could tell IntelliJ to ignore any used fields annotated with @Mock, but what I'd prefer is to not have to declare this field at all. In other words, when a dependency of InviteService is found that does not have a corresponding @Mock field, Mockito will automatically create a default mock for the dependency.

Currently, if the userService field is removed, the dependency is set to null, which causes a NullPointerException when the test runs.

To achieve this behavior, it seems that there is no out-of-the-box solution in Mockito. However, you can write your own custom extension that inspects your test class, finds a field annotated with @InjectMocks, examines its fields, and if any of them are null, instantiates them with mock objects. It could look something like this:

import org.junit.jupiter.api.extension.BeforeEachCallback;
import org.junit.jupiter.api.extension.ExtensionContext;
import org.mockito.InjectMocks;
import org.mockito.Mockito;
import org.mockito.exceptions.misusing.InjectMocksException;
import org.springframework.util.ReflectionUtils;

import java.lang.reflect.Modifier;
import java.util.Arrays;

public class AutoMockitoExtension implements BeforeEachCallback {

    @Override
    public void beforeEach(ExtensionContext context) throws Exception {
        var testInstance = context.getRequiredTestInstance();

        var injectMocksField = Arrays.stream(testInstance.getClass().getDeclaredFields())
            .filter(field -> field.isAnnotationPresent(InjectMocks.class))
            .findFirst()
            .orElseThrow(() -> new IllegalStateException("@InjectMocks field not found"));

        injectMocksField.setAccessible(true);
        var injectMocksObject = injectMocksField.get(testInstance);

        if (injectMocksObject == null) {
            throw new IllegalStateException("""
                @InjectMocks field is null.
                You may need to add MockitoExtension.class to the beginning of the @ExtendWith extensions list""");
        }
        // Assign a mock object to any null instance field of the @InjectMocks object
        ReflectionUtils.doWithFields(injectMocksObject.getClass(),
            field -> field.set(injectMocksObject, Mockito.mock(field.getType())),
            field -> {
                var isInstanceField = !Modifier.isStatic(field.getModifiers());
                field.setAccessible(true);
                try {
                    return isInstanceField && field.get(injectMocksObject) == null;
                } catch (IllegalAccessException e) {
                    throw new InjectMocksException("Failed to access field: " + field, e);
                }
            });
    }
}

And then test:

@ExtendWith({MockitoExtension.class, AutoMockitoExtension.class})
class InviteServiceTests {

    @Mock
    private EmailSendingService emailSendingService;

    @InjectMocks
    private InviteService inviteService;

    @Test
    void testSomething() {
        inviteService.inviteUser("user@example.com");
        verify(emailSendingService).sendEmail("user@example.com");
    }
}

In this example, I haven’t accounted for all the possible details and pitfalls that might need to be considered, but it works.