Mockito: How to replace method of class which is invoked by class under test?

Question

Unfortunately the Mockito documentation lacks of exhaustive information, it's difficult to find the proper way how to create the following setup.

I have a class "ResourceManager" which should be tested. This class instantiates another class "JerseyClient" which has a method called "get". I want "ResourceManager" to not invoke the real "JerseyClient" but a mock (or a stub?) from it (It's already unclear to me what is the difference between mocking and stubbing or mocks and spies in Mockito context).

My attempts are to @Mock (or @Spy?) JerseyClient, or at least one method of it:

@RunWith(MockitoJUnitRunner.class)
public class ResourceManagerTest
{
    @Mock
    private JerseyClient jerseyClient;


    @Test
    public void testResultHandling() throws JsonGenerationException, JsonMappingException, IOException, ResourceException
    {
        TestEntity testEntity = new TestEntity();
        ResourceManager resourceManager = new ResourceManager();

        testEntity.setGet(true);
        testEntity.setTestAttribute("1stTest");

        when(jerseyClient.get(anyString())).thenReturn("{\"get\":true,\"testAttribute\":\"2ndTest\",\"message\":\"success\"}");

//      doReturn("{\"get\":true,\"testAttribute\":\"2ndTest\",\"message\":\"success\"}").when(jerseyClient).get(anyString());

        TestEntity result = (TestEntity)resourceManager.execute(testEntity, TestEntity.class);

        assertThat(result, is(notNullValue()));
        assertThat(result.getMessage(), is("success"));
        assertThat(result.getTestAttribute(), is("2ndTest"));
    }
}

As you can see, I tried to mock jerseyClient.get() method to return a predefined JSON string:

when(jerseyClient.get(anyString())).thenReturn("{\"get\":true,\"testAttribute\":\"2ndTest\",\"message\":\"success\"}");

or

doReturn("{\"get\":true,\"testAttribute\":\"2ndTest\",\"message\":\"success\"}").when(jerseyClient).get(anyString());

But none of them work. That means that the real JerseyClient.get method is invoked, because it tries to make a HTTP request with the real JerseyClient.

What is the solution and what is this what I want to do here? Is it spying on a real object or mocking an object where I want to mock a method of it, and when can I replace methods, only on mocks or only on spies?

Answer 1

I found the answer on my own. I need to add

@InjectMocks
private ResourceManager resourceManager;

Then I need to run the test with this instance:

@RunWith(MockitoJUnitRunner.class)
public class ResourceManagerTest
{
    @Mock
    private JerseyClient jerseyClient;

    @InjectMocks
    private ResourceManager resourceManager;

    @Test
    public void testResultHandling() throws JsonGenerationException, JsonMappingException, IOException, ResourceException
    {
        TestEntity testEntity = new TestEntity();

        testEntity.setGet(true);
        testEntity.setTestAttribute("1stTest");

        when(jerseyClient.get(anyString())).thenReturn("{\"get\":true,\"testAttribute\":\"2ndTest\",\"message\":\"success\"}");   

        TestEntity result = (TestEntity)resourceManager.execute(testEntity, TestEntity.class);

        assertThat(result, is(notNullValue()));
        assertThat(result.getMessage(), is("success"));
        assertThat(result.getTestAttribute(), is("2ndTest"));
    }
}

And I can also use the "doReturn...when" pattern.