Test Callback Invocation using Junit5 and Mockito

In this article we will use thenAnswer method of Mockito to invoke a callback method passed to a mock. 

Let's consider following classes.

Here, ServiceToBeTested class is calling submitCallback() method of ServiceToBeMocked class and passing it a Callback object.

The value set in Callback object is latter returned by the method being tested.

ServiceToBeTested.java

public class ServiceToBeTested {

    private final ServiceToBeMocked serviceToBeMocked;

    public ServiceToBeTested(ServiceToBeMocked serviceToBeMocked) {
        this.serviceToBeMocked = serviceToBeMocked;
    }

    public String methodToBeTested() {
        Callback callback = new Callback();
        serviceToBeMocked.submitCallback(callback);

        return "Value Received after callback = " + callback.getValue();
    }
}

ServiceToBeMocked.java

public class ServiceToBeMocked {

    public void submitCallback(Callback callback) {
        //some logic that sets value back in callback.
        callback.setValue(null);
    }
}

Callback.java

/**
 * Simple class demonstrating callback events. The implementation could be much more complex.
 */
public class Callback {

    private String value;

    public String getValue() {
        return value;
    }

    public void setValue(String value) {
        this.value = value;
    }
}

In above scenario, thenReturn method of Mockito will be of no use. We can use thenAnswer() method in this case to achieve our test scenario.

ServiceToBeTestedTest.java

@ExtendWith(MockitoExtension.class)
class ServiceToBeTestedTest {

    @Mock
    private ServiceToBeMocked serviceToBeMocked;

    @InjectMocks
    private ServiceToBeTested serviceToBeTested;

    @Test
    void test_callback() {
        Mockito.doAnswer(it -> {
            Callback callback = it.getArgument(0);
            callback.setValue("Mocked Value");
            return null;
        }).when(serviceToBeMocked).submitCallback(Mockito.any(Callback.class));

        String returnValue = serviceToBeTested.methodToBeTested();

        Assertions.assertEquals("Value Received after callback = Mocked Value", returnValue);
    }
}

Full sample code can be found at our Github repository.