Mockito thenReturn always null , Spring-Boot + Junit5 + Mockito

Question

I'm trying to implement Mockito to test a particular method but the .thenReturn(...) seems to always be returning a null object instead of what I intended:

I am using mockito 2.22.0 and Junit-5 and spring-boot 2.0.4.RELEASE

User resultUser=userService.save(userDto); The above line return always nullpointer exception.not sure why resultUser always retuning null,

My Test class

 @SpringBootTest
    public class UserServiceTests {
    
    @InjectMocks
    private UserService userService =new UserServiceImpl();
    
    @Mock
    private UserRepository userRepository;
    
    @Mock
    BCryptPasswordEncoder passwordEncoder;
    
    @BeforeEach
    void setUp() throws Exception{
        MockitoAnnotations.initMocks(this);
        
    }
    
    @DisplayName("Test saveRegistrationTest method making call to Repository")
    @Test
    public void saveRegistrationTest() {
        
        User user =new User();
        user.setFirstName("TestFN");
        user.setLastName("TestLN");
        user.setEmail("test@test.com");
        user.setPassword("7asdf7asdfi");
        
        User testUser =new User();
        testUser.setFirstName("TestFN");
        
        UserRegistrationDto userDto = new UserRegistrationDto();
        userDto.setFirstName("TestFN");
        
        when(userRepository.save(testUser )).thenReturn(user);
        
        **User resultUser=userService.save(userDto);**
        
        String firstName=(userService.save(userDto)).getFirstName();
        String lastName=(userService.save(userDto)).getLastName();
        String resultEmailFromRepo=(userService.save(userDto)).getEmail();
        String password=(userService.save(userDto)).getPassword();
        
        assertNotNull(user);
        assertEquals("TestLN", firstName);
        assertEquals("TestLN", lastName);
        assertEquals("test@test.com", resultEmailFromRepo);
        assertNotNull(password);
        assertEquals("7asdf7asdfi", password);
    }

User Entity

 @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    private String firstName;
    private String lastName;
    private String email;
    private String password;
    
    @OneToMany(cascade = CascadeType.ALL)
    private List<Todo> todoList = new ArrayList<>();

    @ManyToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
    @JoinTable(
            name = "users_roles",
            joinColumns = @JoinColumn(
                    name = "user_id", referencedColumnName = "id"),
            inverseJoinColumns = @JoinColumn(
                    name = "role_id", referencedColumnName = "id"))
    private Collection<Role> roles;

UserDTO

public class UserRegistrationDto {

    @NotEmpty
    private String firstName;

    @NotEmpty
    private String lastName;

    @NotEmpty
    private String password;

    @NotEmpty
    private String confirmPassword;

    @Email
    @NotEmpty
    private String email;

    @Email
    @NotEmpty
    private String confirmEmail;

    @AssertTrue
    private Boolean terms;

Userserviceimpl class

   public User save(UserRegistrationDto registration){
        User user = new User();
        user.setFirstName(registration.getFirstName());
        user.setLastName(registration.getLastName());
        user.setEmail(registration.getEmail());
        user.setPassword(passwordEncoder.encode(registration.getPassword()));
        user.setRoles(Arrays.asList(new Role("TEST_USER")));
        user.setTodoList(null);
        return userRepository.save(user);
    }

Answer 1

Don't

@InjectMocks
private UserService userService = new UserServiceImpl();

This "object" is never managed by spring!

---

Best

@SpringBootTest
class UserServiceTest {
  @Autowired
  private UserService testee;

  @MockBean
  private UserRepository userRepoMock; // done!
  // ... many test methods, like: when(userRepoMock.xxx).thenYYY
  // testee.someMethod()
  // verify(userRepoMock...)
  // ...
}

where the service looks like:

@Service // or some other "spring managed bean"
public class UserService {
  @Autowired
  private UserRepository userRepository;
 ...
}

---

And as johannes commented:

@BeforeEach

is not needed (but not harmful??!) in this setup.

---

Core Problem

When we use a concrete object instance as "argument matcher" for mockito:

when(userRepository.save(someObject)).thenReturn(someMock);

, we should ensure, that when (during test):

userRepository(someOtherObject)

..is called,

someObject.equals(someOtherObject)

... is true!("Equality Matcher")

Otherwise mockito can'/won't match that invocation.

We can resolve by, implementing (custom) equals (and hashCode) methods:

// e.g. with lombok:
@EqualsAndHashCode(onlyExplicitlyIncluded = true)
// ...
class UserRegistrationDto {
  @EqualsAndHashCode.Include
  String name;
}

EqualsAndHashCode(Recommended, but do it well! (lombok small print, or manual!))

---

Or by using a "wider Argument Matcher", like:

when(userRepository.save(any(UserRegistrationDto.class)).thenReturn(myMockResponse);

..or even:

when(userRepository.save(any(/*java.lang.Object*/))...

ArgumentMatchers javadoc