How to mock run() method of child thread using mockito-inline

Question

Code explanation: MainThread creates ChildThread based on the list of users - one Childthread per user. I am trying to write a unit test case for MainThread and I want to skip the implementation of ChildThread (a separate unit test case will be written for ChildThread). Below is the code snippet.

@Slf4j
public class MainThread implements Runnable {
 private static final UserService USER_SERVICE = ApplicationContextUtils.getApplicationContext().getBean("userService", UserService.class);
 private final String threadName;

 public MainThread(String threadName) {
    this.threadName = threadName;
 }

 public void run() {
    log.info("{} thread created at {}", threadName, LocalDateTime.now());
    List<UsersDTO> usersDTOs = USER_SERVICE.getUsers();
    ExecutorService executor = Executors.newFixedThreadPool(usersDTOs.size());
    usersDTOs.stream().map(ChildThread::new).forEach(executor::execute);
    executor.shutdown();
 }
}

@Slf4j
 public class ChildThread implements Runnable {
 private final UserDTO userDTO;

 public ChildThread(UserDTO userDTO) {
    this.userDTO = userDTO;
 }

 public void run() {
    log.info("Child thread created for user: {}", userDTO.getName());
    // some business logic
 }
}

@TestInstance(TestInstance.Lifecycle.PER_CLASS)
public class MainThreadTest {

 @Mock
 private ApplicationContext applicationContext;
 @Mock
 private UserService userService;

 @BeforeEach
 public void setUp() {
    MockitoAnnotations.openMocks(this);
    new ApplicationContextUtils().setApplicationContext(applicationContext);
 }

 @Test
 void test() {
    Mockito.when(applicationContext.getBean("userService", UserService.class)).thenReturn(userService);
    Mockito.when(userService.getUsers()).thenReturn(MockObjectHelper.getUsersList());

    ChildThread childThread = new ChildThread(MockObjectHelper.getUser());
    ChildThread spy = spy(childThread);
    doNothing().when(spy).run();

    MainThread mainThread = new MainThread("TestingThread");
    mainThread.run();

    verify(userService, times(1)).getUsers(any());
 }
}

Despite spying ChildThread, the run() method of ChildThread is executed. doNothing().when(spy).run(); is of no effect. For some reason, I cannot use PowerMockito. How to achieve this with mockito-inline (version 3.10.0) and java8?

Any help would be appreciated.

Answer 1

Instead of mocking ChildThread refactor MainThread so ExecutorService is injected in constructor.

Then mock ExecutorService and check if it is receiving correct ChildThread instances.