JUnit Mockito: Testing a Static Method and Calling Another Stubbed Static Method Inside Not Working

Question

class A {
   public static int f1() {
      return 1;
   }
   
   public static int f2() {
      return A.f1();
   }
}

class ATest {
   @Test
   void testF2() {
      try (MockedStatic<A> aStatic = Mockito.mockStatic(A.class)) {
         aStatic.when(A::f1).thenReturn(2);
         int ret = A.f2(); // getting 0 here
         assertEquals(ret, 2);
      } catch(Exception e) {
      
      }
   }
}

In the testF2 I want to test static function A::f2().

And it internally calls another static function A::f1().

I did stub A::f1() to return 2 using "MockedStatic" and "when" way.

But it's not working, it's returning 0.

How to solve it?

Answer 1

When you mock a class with static methods, all static methods are mocked. If you only want to mock the behavior of only 1 method, you have to add Mockito.CALLS_REAL_METHODS argument to Mockito.mockStatic() as you can see in the following example.

 @Test
  void testF2() {
    try (MockedStatic<A> aStatic = Mockito.mockStatic(A.class, Mockito.CALLS_REAL_METHODS)) {
      aStatic.when(A::f1).thenReturn(2);
      int ret = A.f2(); // getting 2 here
      Assert.assertEquals(2, ret); // (expected, result)
    } catch(Exception e) {

    }
  }

This way only the f1 method invocation is mocked but f2 invocation calls the real code.

Answer 2

I think you miss to specify a mock behavior:

class ATest {
  @Test
  void testF2() {
    try (MockedStatic<A> aStatic = Mockito.mockStatic(A.class)) {
        aStatic.when(A::f1).thenReturn(2);
        aStatic.when(A::f2).thenReturn(A.f1()); // <- added this
        int ret = A.f2(); // getting 0 here
        Assertions.assertEquals(ret, 2);
    } catch (Exception e) {

    }
  }
}

by telling the mock what to do when A.f2() is invoked, test runs fine.

Update:
Mocks do what you tell them, if you don't tell what to do when a method is invoked they do nothing, that's why you have to mock f2 too.

You want to test A, then mock it is not your friend. I normally use a Mockito.spy() to partially mock my subject under test .You want to mock f1 but test f2, I don't think spy applies here because there is no instance to spy..

I suggest you to rearrange A avoiding static methods if possible or using parameters you can mock.