How to substitute Mockito in place to stub a class in JUnit Test Java

Question

I am trying to substitute the correct use of Mockito, during a test session by JUnit, in place of to stub a class. Unfortunately on web there are a lot of tutorials regarding Mockito but, less for the stub approach, and I would like to learn this technique.

This test is made by Mockito:

 @Test
public void addWrongNewUserSpaceInUsername() throws Exception {

    when(userValidator.isValidUsername(user.getUsername())).thenReturn(false);

    try {
        mockMvc.perform(
                post("/register")
                        .contentType(MediaType.APPLICATION_JSON)
                        .content(asJsonString(user)
                        ));
    } catch (Exception e) {
        Assert.assertTrue(e.getCause() instanceof UsernameNotValidException);
    }
}

To clarify these are the classes involved:

1) Controller

   @RestController
public class UserController {

    @Autowired
    RepositoryUserDB repositoryUserDB;

    @Autowired
    UserValidator userValidator;

    @RequestMapping(value = "/register", method = RequestMethod.POST)
    public User createUser(@RequestBody User user) {


        if (userValidator.isValidUsername(user.getUsername())) {

            if(!userValidator.isValidPassword(user.getPassword())){
                throw new PasswordNotValidException();
            }

            if(userValidator.isValidDateOfBirth(user.getDateOfBirth()) == false){
                throw new DOBNotValidException();
            }

            // se lo user e' gia' presente
            if (repositoryUserDB.getUserByUsername(user.getUsername()) == null) {
                return repositoryUserDB.createUser(user);
            }
            throw new UsernameAlreadyExistException();
        } else throw new UsernameNotValidException();

    }
}

2)Interface of repo:

public interface RepositoryUserDB {

    User getUserByUsername(String username);

    User createUser(User user);
}

3)Repo:

@Component

    public class MemoryUserDB implements RepositoryUserDB{

        Map<String, User> repo;

        public MemoryUserDB() {
            this.repo = new HashMap<>();
        }

        @Override
        public  User getUserByUsername(String username) {
            return repo.get(username);
        }

        @Override
        public User createUser(User user) {
            repo.put(user.getUsername(),user);
            return repo.get(user.getUsername());
        }
    }

4) Validator:

   @Component
public class UserValidator {

    public boolean isValidUsername(String username) {
        return username.matches("^[a-zA-Z0-9]+$");
    }

    public boolean isValidPassword(String pwd) {
        if (pwd == null)
            return false;
        return pwd.matches("^(?=.*[a-z])(?=.*[A-Z])(?=.*\\d){4,}.+$");
    }

    public boolean isValidDateOfBirth(String DOB) {
        return DOB.matches("^(?:(?:(?:0?[13578]|1[02])(\\/|-|\\.)31)\\1|(?:(?:0?[1,3-9]|1[0-2])(\\/|-|\\.)(?:29|30)\\2))(?:(?:1[6-9]|[2-9]\\d)?\\d{2})$|^(?:0?2(\\/|-|\\.)29\\3(?:(?:(?:1[6-9]|[2-9]\\d)?(?:0[48]|[2468][048]|[13579][26])|(?:(?:16|[2468][048]|[3579][26])00))))$|^(?:(?:0?[1-9])|(?:1[0-2]))(\\/|-|\\.)(?:0?[1-9]|1\\d|2[0-8])\\4(?:(?:1[6-9]|[2-9]\\d)?\\d{2})$");
    }
}

5)ResEntityExceptionHandler

    @ControllerAdvice
public class RestEntityExceptionHandler {

    @ExceptionHandler(UsernameNotValidException.class)
    @ResponseStatus(value = HttpStatus.FORBIDDEN, reason = "username wrong")
    public void handleUsernameException() {
    }

    @ExceptionHandler(UsernameAlreadyExistException.class)
    @ResponseStatus(value = HttpStatus.BAD_REQUEST, reason = "the username is already presents")
    public void handleUsername() {
    }

    @ExceptionHandler(PasswordNotValidException.class)
    @ResponseStatus(value = HttpStatus.FORBIDDEN, reason = "password wrong")
    public void handlePasswordException() {
    }

    @ExceptionHandler(DOBNotValidException.class)
    @ResponseStatus(value = HttpStatus.FORBIDDEN, reason = "Date Of Birth wrong")
    public void handleDOBException(){
    }

}

Answer 1

Theoretically, for you use case, the stub approach should be rather simple to do.
But as you rely on a Spring Boot Test that uses the Spring beans container, things are really harder to setup as you should find a way to inject the mocked bean in the container to replace the actual bean : UserValidator.
Mocks in Spring Boot test rely usually on Spring Boot MockBean.
It is not a mockito mock but not very far.
To understand differences with Mockito mocks, you may refer to this question.

Using a framework gives many features out of the box, but also limits yourself as you want to bypass the framework features.

---

Supposing that you didn't perform an integration test with Spring Boot, but a real unit test (so without the Spring boot container), things could be performed in this way.

Instead of mocking UserValidator.isValidUsername(), you define a custom implementation of UserValidator that stubs the method return as expected in your test.
Finally, what Mockito or any mock framework does for you.

So here is the stub class :

public class UserValidatorStub extends UserValidator {

    private String expectedUsername;
    private boolean isValidUsername;

    public UserValidatorStub(String expectedUsername, boolean isValidUsername){
          this.expectedUsername = expectedUsername;
          this.isValidUsername = isValidUsername;
    }
    public boolean isValidUsername(String username) {
        if (username.equals(expectedUsername)){
           return isValidUsername;
        }
        // as fallback, it uses the default implementation but you may return false or null as alternative
       return super.isValidUsername(username);
    }

}

It accepts a constructor to store the expected arguments passed to the stubbed method and the stubbed result to return.

Now, here how your test could be written :

 @Test
public void addWrongNewUserSpaceInUsername() throws Exception {

    // inject the mock in the class under test
    UserController userController = new UserController(new UserValidatorStub(user.getUsername(), false));
    try {
        userController.createUser(user);
    } catch (Exception e) {
        Assert.assertTrue(e.getCause() instanceof UsernameNotValidException);
    }
}

Note that the example relies on constructor injection in UserController to set the dependencies.