How to mock database operations to unit test Spring Boot REST API?

[Jf Okeeffe]

I have a REST API written in Spring Boot.

Now I want to create unit tests with JUnit and Mockito that mock the database operations.

How to do that?

Here is the endpoint that inserts a new customer to the db.

@RestController
public class CustomerController {

    @Autowired
    private CustomerRepository customerRepository;    
    
    @PostMapping(value="/customers", produces = "application/json")
    public ResponseEntity<Customer> addCustomer(@RequestBody Customer customer){
        
        Customer newCustomer = customerRepository.save(customer);       
        
        return new ResponseEntity<Customer>(newCustomer, HttpStatus.OK) ;
    }


    public void setCustomerRepositoryMock(CustomerRepository customerRepositoryMock) {
	this.customerRepository = customerRepositoryMock;
    }

Test:

public class CustomerControllerTest {

@Mock
private CustomerRepository mockedCustomerRepository;
	
@Test
public void testAddCustomer() {
		
	Customer customer = new Customer();
	customer.setCustomerId(1L);		
	customer.setName("Pete");
	customer.setAge(35);
	customer.setEmail("[e-mail]");
		
	CustomerController customerController = new CustomerController();
	customerController.setCustomerRepositoryMock(mockedCustomerRepository);		
		
	when(mockedCustomerRepository.save(any(Customer.class))).thenReturn(customer);

	ResponseEntity<Customer> respCustomer = customerController.addCustomer(customer);
		
	assertTrue(respCustomer.getBody().getIdCustomer() != null);
}

}

I'm getting a null pointer exception on the following line:

when(mockedCustomerRepository.save(any(Customer.class))).thenReturn(customer);

[Rob Spoor]

@Mock doesn't do anything by itself. You need to have something to activate it:

* In JUnit 4, you would annotate your test class with @RunWith(MockitoJUnitRunner.class).
* In JUnit 5, you would annotate your test class with @ExtendWith(MockitoExtension.class). You need this additional dependency.
* If you can't use either, explicitly call MockitoAnnotations.initMocks(this) in your test class. This would preferably be done in an @Before / @BeforeEach method.

[Jf Okeeffe]

Hi Rob,

Thanks for your answer.

I'm using org.junit.jupiter.api that comes with a Spring Boot new install.
Guess that is JUnit 5. Not sure.

Anyway, I could solve the issue with the following code.
Didn't need to add the class level annotations you mentioned.

I had tried with initMocks(this) but didn't work.
Found out that is deprecated.

This code is passing the tests.
Please take a look and let me know if you think it's a good solution.

Thank you.

public class CustomerControllerTest {
	
	private CustomerRepository mockedCustomerRepository;
	private CustomerController customerController;
	private Customer customer;
	
	@BeforeEach
	void setup() {

		customer = new Customer();
		customer.setName("Pete");
		customer.setAge(35);
		customer.setEmail("[e-mail]");
			
		mockedCustomerRepository = mock(CustomerRepository.class);
				
		customerController = new CustomerController();
		customerController.setCustomerRepositoryMock(mockedCustomerRepository);
		
	}
	
	@Test
	void testAddCustomer() {
			
		when(mockedCustomerRepository.save(any(Customer.class))).thenReturn(customer);		
		
		ResponseEntity<Customer> respCustomer = customerController.AddCustomer(customer);
		
		assertTrue(respCustomer.getBody().getName() != null);
		
	}

[Himai Minh]

Hi, how about doing this:

@MockBean
private CustomerRepository mockedCustomerRepository;
     
@Test
public void testAddCustomer() {  ...}

Here is an example:
https://stackoverflow.com/questions/41168352/unit-test-post-with-webmvctest-mockbean-service-returns-null

[Jf Okeeffe]

Hi Himai,
Thanks for your suggestion.
How does it improve my solution?

[Himai Minh]

Hi, Jf ,
Nowadays, I see people use @MockBean to inject beans.
Injecting beans is a practice of inversion of control.