Using @Mock as a Method Parameter with Mockito

As with many other Java developers, I heavily utilise Mockito as a mocking framework for unit testing.

I'll often end up with tests that look like this:

import org.mockito.Mock;

// ...

@Test
void getPagesReturnsValueFromDelegate() throws AnalyticsServiceException {
  // given
  LocalDate date = LocalDate.now();
  List<Page> mockList = mock(List.class);       // <--- this is what we're interested in
  when(delegate.getPages(any())).thenReturn(mockList);

  // when
  List<Page> actual = sut.getPages(date);

  // then
  assertThat(actual).isSameAs(mockList);
}

In this case, I want to create a List and check it's returned by the caching layer I'm testing.

However, this is a good example of where inlining our mock calls is bad, because IntelliJ warns us:

Unchecked assignment: 'java.util.List' to 'java.util.List<me.jvt.www.api.analytics.model.Page>'

This is quite annoying, and pollutes our codebase with warnings that we really want to avoid.

One solution would be to make it a field of the test class, and @Mock it there, but in this case it's a throwaway mock that is only required in this test, not in all tests (therefore wouldn't make as much sense to make a field variable).

Fortunately, we can utilise something my colleague Lewis taught me the other week, and use @Mock as a method parameter when using JUnit5.

This gives us the following test:

@Test
void getPagesReturnsValueFromDelegate(@Mock List<Page> mockList)
    throws AnalyticsServiceException {
  // given
  LocalDate date = LocalDate.now();
  when(delegate.getPages(any())).thenReturn(mockList);

  // when
  List<Page> actual = sut.getPages(date);

  // then
  assertThat(actual).isSameAs(mockList);
}

This would of course, make our tests more readable if we had many mocks being set up!

Note that you also need to make sure you're using Mockito's mock setup:

import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.junit.jupiter.MockitoExtension;

@ExtendWith(MockitoExtension.class)
class CachingAnalyticsServiceTest {
  // ...

This has been tested as of spring-boot-starter-test v2.2.4.RELEASE, and works in Mockito 2+ with JUnit5+.

I have raised a PR to add it to the Mockito 3.x documentation, but not 2.x as per request of the Mockito core team.