Deep mocking with Mockito and Spring Boot

I need to mock (with Mockito) the "A" class used by "B" class however my test case is done to "D" class.

See my example:

@Service
public class Aclass {
    public Long sumTwoNumbers(Long a, Long b){
       return a + b;
    }
}

@Service
public class Bclass {
     @Autowired    
     private  Aclass classA;

     public Long sunThreeNumbers(Long a, Long b, Long c){
           return classA.sumTwoNumbers(a, b) + c;
     }
}

@Service
public class Cclass {
     @Autowired    
     private  Bclass classB;

     public Long sunFourNumbers(Long a, Long b, Long c, Long d){
           return classB.sunThreeNumbers(a, b, c) + d;
     }
}

@Service
public class Dclass {
     @Autowired    
     private  Cclass classC;

     public Long sunFourNumbers(Long a, Long b, Long c, Long d){
           return classC.sunFourNumbers(a, b, c, d);
     }
}

My test is :

@ExtendWith(MockitoExtension.class)
public class DclassTest{

     @InjectMocks    
     private  Dclass classD;

     @Mock    
     private  Aclass classA; 
     /* Look , I need to mock Aclass , not Bclass or Cclass. */
     
     @Test
     public void testSunFourNumbers(){
        Mockito.when(classA.sumTwoNumbers(any(Long.class), any(Long.class))).thenReturn(0L);

        Long sum = classC.sunFourNumbers(1L, 1L, 1L, 1L);

        Assert.assertEquals(2L, sum);        
     } 
}

It was a simple example of what I need.

When I run this I received NullPointerException in Dclass.

Is it possible ?

I Think You should Test Your class Bclass with Mockito Like this :

public class BclassTest {

        Long a = 0L;
        Long b = 0L;
        Long c = 1L;
         
         @InjectMocks
         private Bclass bclass;

         @Mock
         private Aclass aclass;
       
         //You Should initialize your Mock Objects in @Before method.
         @Before
         public void setup() {
         MockitoAnnotations.initMocks(this);

         @Test
         public void testSunThreeNumbers(){             
         Mockito.when(aclass.sumTwoNumbers(anyLong(),anyLong())).thenReturn(0L);
         assertEquals(Long.valueOf(1), bclass.sunThreeNumbers(Long a, Long b, Long c));
                }
         }
       }

As per the example code, its pretty clear that below services dependent as below

1. DClass depends on Cclass
2. Cclass on Bclass and
3. Bclass on Aclass

When we write unit test cases for Dclass (code as below),

@Service
public class Dclass {
     @Autowired    
     private  Cclass classC;

     public Long sunFourNumbers(Long a, Long b, Long c, Long d){
           return classC.sunFourNumbers(a, b, c, d);
     }
}

Here Dclass is directly dependent on Cclass, so mocking Cclass is enough in this case. Unit testing is all about testing a small unit by mocking its direct dependents.

 @InjectMocks Dclass classD; // It has all 3 mocks now (but A and B mocks not needed)
 @Mock Aclass classA;        //not needed (Dclass not dependent on Aclass)
 @Mock Bclass classB;        //not needed (Dclass not dependent on Bclass)    
 @Mock Cclass classC;        // **Mandatory** - Because Dclass is directly dependent on Cclass
 
 As per your code - you have a mock for Aclass and injected mock into Dclass.
 When(...).then(...) written for Aclass mock.

 @Test
 public void testSunFourNumbers(){

    Mockito.when(classA.sumTwoNumbers(any(Long.class), any(Long.class))).thenReturn(0L);

    //Since you don't have mock for 'classC' in your code, it will go as null and result in nullpointer
    Long sum = classC.sunFourNumbers(1L, 1L, 1L, 1L);

    Assert.assertEquals(2L, sum);        
 }