How do I write a JUnit test case to test threads and events

Question

I have a java code which works in one (main) thread. From the main thread, i spawn a new thread in which I make a server call. After the server call is done, I am doing some

Answer 1

Here is my solution to test asynchrone method which used thread.start:

public class MyClass {      
   public void doSomthingAsynchrone() {
      new Thread(() -> {
         doSomthing();
      }).start();
   }

   private void doSomthing() {
   }
}

@RunWith(PowerMockRunner.class)
@PrepareForTest(MyClass.class)
public class MyClassTest {
   ArgumentCaptor runnables = ArgumentCaptor.forClass(Runnable.class);

   @InjectMocks
   private MyClass myClass;

   @Test
   public void shouldDoSomthingAsynchrone() throws Exception {  

      // create a mock for Thread.class
      Thread mock = Mockito.mock(Thread.class);

      // mock the 'new Thread', return the mock and capture the given runnable
      whenNew(Thread.class).withParameterTypes(Runnable.class)
            .withArguments(runnables.capture()).thenReturn(mock);

      myClass.doSomthingAsynchrone();

      runnables.getValue().run();

      /**
       *  instead of 'runnables.getValue().run();' you can use a real thread.start
       *
       *   MockRepository.remove(Thread.class);
       *   Thread thread = new Thread(runnables.getValue());
       *   thread.start();
       *   thread.join();
       **/

      verify(myClass, times(1)).doSomthing();
   }
}