Mocking static objects under completable future supply async

Question

Consider below code snippet:

class XYZ {
  public static getFieldABC(){
    return new ABC();
  }
}

...
CompletableFuture.supplyAsync(() -> XYZ.getFieldABC())
...

I need to unit test this piece of code.

I have a static method getFieldABC which returns an object of a particular class ABC. I want to mock this static method call using Mockito.mockStatic and then return mockObject using when().

The catch here is that this method call is under CompletableFuture.supplyAsync. Since, its running in a different thread, the call to the static method, instead of calling the mock, its invoking the actual method call.

It sounds like you should redesign the code you're testing to make it easier to test. — Louis Wasserman, Jul 12 '23 at 13:49

score 0 · Answer 1 · answered Jul 25 '23 at 10:16

Assuming the class you want to test looks like this

class ABCSupplier {

    void supply() {

        CompletableFuture.supplyAsync(XYZ::getFieldABC);
    }
}

You can mock statics with Mockito like this:

@ExtendWith(MockitoExtension.class)
class ABCSupplierTest {

    @InjectMocks
    ABCSupplier abcSupplier;

    @Captor
    private ArgumentCaptor<Supplier<ABC>> supplierArgumentCaptor;

    @Test
    void it_should_supply_ABC_field() {

        final ABC abc = new ABC();

        try (var completableFutureMockedStatic = Mockito.mockStatic(CompletableFuture.class);
            var xyzMockedStatic = Mockito.mockStatic(XYZ.class)) {

            xyzMockedStatic.when(XYZ::getFieldABC)
                .thenReturn(abc);

            this.abcSupplier.supply();

            completableFutureMockedStatic.verify(() -> CompletableFuture.supplyAsync(this.supplierArgumentCaptor.capture()));

            Assertions.assertThat(this.supplierArgumentCaptor.getValue().get())
                .isEqualTo(abc);
        }
    }
}

Mocking static objects under completable future supply async

1 Answers1