Unit testing callback based on Consumer functional interface

Question

I'm trying to unit test code that runs as callback in a Consumer functional interface.

@Component
class SomeClass {

  @Autowired
  private SomeInteface toBeMockedDependency;

  public method() {
     toBeMockedDependency.doSomething(message -> {
        // Logic under test goes here 
        //             (implements java.util.function.Consumer interface)
        ...
     });
  }
}

@RunWith(MockitoJUnitRunner.class)
public class SomeClassTest {
  @InjectMocks
  private SomeClass someClass;
  @Mock
  private SomeInteface toBeMockedDependency;

  @Test
  public void testMethod() {
    ...
    someClass.method();
    ...
  }
}

Essentially I want to provide the tested code some tested "message" via "toBeMockedDependency".

How can the "toBeMockedDependency" be mocked to provide a predefined message?
Is it the right approach?

Where does `message` come from? Is it a field of `SomeClass`, parameter of `method()` or something else? — noscreenname, Aug 18 '16 at 12:54
In the production code, the dependency produces the message as follows: `toBeMockedDependency.doSomething(Consuner consumer) { ... String message = "... .."; consumer.accept(message); ... }` — Boris, Aug 18 '16 at 13:01
can you provide full code of when and where from `consumer.accept(message)` is called? — noscreenname, Aug 18 '16 at 13:09

score 8 · Accepted Answer · answered Aug 18 '16 at 17:46

Don't try to make toBeMockedDependency automatically call your functional interface. Instead, use a @Captor to capture the anonymous functional interface, and then use your test to manually call it.

@RunWith(MockitoJUnitRunner.class)
public class SomeClassTest {
  @InjectMocks
  private SomeClass someClass;
  @Mock
  private SomeInteface toBeMockedDependency;
  @Captor
  private ArgumentCaptor<Consumer<Message>> messageConsumerCaptor;

  @Test
  public void testMethod() {
    someClass.method();
    verify(toBeMockedDependency).doSomething(messageConsumerCaptor.capture());
    Consumer<Message> messageConsumer = messageConsumerCaptor.getValue();

    // Now you have your message consumer, so you can test it all you want.
    messageConsumer.accept(new Message(...));
    assertEquals(...);
  }
}

Unit testing callback based on Consumer functional interface

1 Answers1