How to use should_get_handler method of org.mockito.internal.util.ObjectMethodsGuruTest class

PropertyDescriptor#getPropertyType() returns an object of Class<?>, where the ? means "this is a type, but I don't know what it is". Let's call this type "X". So when(mockDescriptor.getPropertyType()) creates an OngoingStubbing<Class<X>>, whose method thenReturn(Class<X>) can only accept objects of Class<X>. But the compiler doesn't know what type this "X" is, so it will complain about you passing in a Class of any type. I think this is the same reason the compiler complains about calling add(...) on a Collection<?>.

When you explicitly specify Class<?> for the type on the when method, you're not saying that mockDescriptor.getPropertyType() returns a Class<?>, you're saying that when returns an OngoingStubbing<Class<?>>. Then, the compiler checks to make sure whatever you're passing into when is of a type that matches Class<?>; since getPropertyType() returns the "Class<X>" I mentioned earlier, it of course matches the Class<?> you specified.

So basically

// the inferred type is Class<"some type">
Mockito.when(mockDescriptor.getPropertyType())

// the specified type is Class<"any type">
Mockito.<Class<?>>when(mockDescriptor.getPropertyType())

In my IDE, the error message for your original code is

The method thenReturn(Class<capture#1-of ?>) in the type OngoingStubbing<Class<capture#1-of ?>> is not applicable for the arguments (Class<String>)

That capture#1-of ? is the "X" I described above.

You need to have prepared the class calling the constructor, not the class on which the constructor is called, the following should fix you up:

@PrepareForTest(DummyTarget.class)

For more information check this page.

You want to set up an Answer object that does that. Have a look at the Mockito documentation, at https://static.javadoc.io/org.mockito/mockito-core/2.8.47/org/mockito/Mockito.html#answer_stubs

You might write something like

when(mockService.doAction(any(Request.class), any(Callback.class))).thenAnswer(
    new Answer<Object>() {
        Object answer(InvocationOnMock invocation) {
            ((Callback<Response>) invocation.getArguments()[1]).reply(x);
            return null;
        }
});

(replacing x with whatever it ought to be, of course)

The sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) - according to the stack trace you provided, mockito got a bit confused when reporting the error. It's not really related with your problem, but anyway that's the NativeMethodAccessorImpl.invoke0 which is native (see grep code). The SpringJUnit4ClassRunner runs it, it's how it works, nothing you should care about.

Now about your real problem, as I wrote in comment, It's because Spring wraps the mock object into a proxy. If you add

System.out.println(memberRepositoryMock.getClass());

you'll see in the console that it's not the mock but some proxy there. Mockito can only stub mocks, so that's why you recieve the error.

The question might be now, how to work this around. First thing is that your test is not actually an integration test as you are trying to mock the repository instead of really test how the data access behave. In this case I would resign from using the spring-test and simply go for simple MockitoJUnitRunner.

UPDATE:

OK, now knowing that it is actually the cache to be tested, I understand you need the mocked repository to be decorated with Spring cache and that your test IS an integration test indeed.

Here's how it could be done. A little variation of mentioned frant.hartm proposal but without use of static reference. Slightly more natural I think.

@ActiveProfiles(Profiles.TEST)
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(classes = { FullIntegrationTestConfiguration.class, BaseTestConfiguration.class, MemberCachingIntegrationTest.Config.class })
public class MemberCachingIntegrationTest {

    @Autowired
    private MemberRepository cachedRepository;

    @Autowired
    private MockProvider mockProvider;

    @After
    public void validate() {
        validateMockitoUsage();
    }

    @Test
    public void test() {
        when(mockProvider.get().findByEmail(anyString())).thenReturn("foo", "bar");

        String firstInvocation = cachedRepository.findByEmail("foo@foo.com");
        assertThat(firstInvocation, is("foo"));

        String secondInvocation = cachedRepository.findByEmail("foo@foo.com");
        assertThat(secondInvocation, is("foo"));

        verify(mockProvider.get(), times(1)).findByEmail("foo@foo.com");

        String thirdInvocation = cachedRepository.findByEmail("bar@bar.com");
        assertThat(thirdInvocation, is("bar"));

        verify(mockProvider.get(), times(1)).findByEmail("bar@bar.com");
    }

    @Configuration
    static class Config {

        private MemberRepository mockRepository = mock(MemberRepository.class);

        @Bean
        public MemberRepository cachedRepository() {
            return mockRepository;
        }

        @Bean
        public MockProvider mockProvider() {
            return new MockProvider(mockRepository);
        }

    }

    public static class MockProvider {

        private final MemberRepository repository;

        public MockProvider(MemberRepository repository) {
            this.repository = repository;
        }

        public MemberRepository get() {
            return this.repository;
        }

    }
}

Note: Repository returns Strings instead of Members for a more clear example

You need to have prepared the class calling the constructor, not the class on which the constructor is called, the following should fix you up:

@PrepareForTest(DummyTarget.class)

For more information check this page.

Can I do something else to set this field?

Yes you can annotate your field with @Spy too as next:

@Spy
@InjectMocks
A a;

Then you will be able to do Mockito.when(a.getString()).thenReturn("STRING");

Indeed when it is annotated with @InjectMocks only ,Mockito doesn't mock it, it creates a normal instance of the class A by default, such that you cannot use Mockito.when() on it. As workaround, you can ask Mockito to partially mock it using @Spy.

The exception that you actually get is due to the case #2 provided in the error message:

inside when() you don't call method on mock but on some other object.

import org.springframework.test.util.ReflectionTestUtils;

@RunWith(MockitoJUnitRunner.class)
public CustomServiceTest{

@InjectMocks
private CustomServiceImpl customService;

@Mock
private DataService dataService;

@Before
public void setup() {
    ReflectionTestUtils.setField(customService, "url", "http://someurl");
}    
...
}

You don't want to actually connect to a real server in a unit test. If you want to actually connect to a real server, that is technically an integration test.

Since you are testing the client code, you should use a unit test so you don't need to connect to a real server. Instead you can use mock objects to simulate a connection to a server. This is really great because you can simulate conditions that would be hard to achieve if you used a real server (like the connection failing in the middle of a session etc).

Unit testing with mocks will also make the tests run faster since you don't need to connect to anything so there is no I/O delay.

Since you linked to another question, I will use that code example (repasted here for clarity) I made a class called MyClass with a method foo() that connects to the URL and returns true or false if the connection succeeded. As the linked question does:

public class MyClass {

private String url = "http://example.com";

public boolean foo(){
    try {
           HttpURLConnection.setFollowRedirects(false);
           HttpURLConnection con = (HttpURLConnection) new URL(url).openConnection();
           con.setRequestMethod("HEAD");

           con.setConnectTimeout(5000); //set timeout to 5 seconds

           return (con.getResponseCode() == HttpURLConnection.HTTP_OK);
        } catch (java.net.SocketTimeoutException e) {
           return false;
        } catch (java.io.IOException e) {
           return false;
        }

    }
}

I will use Mockito to make the mock objects since that is one of the more popular mock object libraries. Also since the code creates a new URL object in the foo method (which isn't the best design) I will use the PowerMock library which can intercept calls to new. In a real production code, I recommend using dependency injection or at least method extraction for creating the URL object to a factory method so you can override it to ease testing. But since I am keeping with your example, I won't change anything.

Here is the test code using Mockito and Powermock to test timeouts:

import java.net.HttpURLConnection;
import java.net.SocketTimeoutException;    
import java.net.URL;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.Mockito;
import org.powermock.api.mockito.PowerMockito;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;

import static org.junit.Assert.*;

@RunWith(PowerMockRunner.class)
//This tells powermock that we will modify MyClass.class in this test 
//- needed for changing the call to new URL
@PrepareForTest(MyClass.class) 
public class ConnectionTimeOutTest {

String url = "http://example.com";
@Test
public void timeout() throws Exception{
    //create a mock URL and mock HttpURLConnection objects
    //that will be our simulated server
    URL mockURL = PowerMockito.mock(URL.class);
    HttpURLConnection mockConnection = PowerMockito.mock(HttpURLConnection.class);

    //powermock will intercept our call to new URL( url) 
    //and return our mockURL object instead!
    PowerMockito.whenNew(URL.class).withArguments(url).thenReturn(mockURL);
    //This tells our mockURL class to return our mockConnection object when our client
    //calls the open connection method
    PowerMockito.when(mockURL.openConnection()).thenReturn(mockConnection);



    //this is our exception to throw to simulate a timeout
    SocketTimeoutException expectedException = new SocketTimeoutException();

    //tells our mockConnection to throw the timeout exception instead of returnig a response code
    PowerMockito.when(mockConnection.getResponseCode()).thenThrow(expectedException);

    //now we are ready to actually call the client code
    // cut = Class Under Test
    MyClass cut = new MyClass();

    //our code should catch the timeoutexception and return false
    assertFalse(cut.foo());

   // tells mockito to expect the given void methods calls
   //this will fail the test if the method wasn't called with these arguments
   //(for example, if you set the timeout to a different value)
    Mockito.verify(mockConnection).setRequestMethod("HEAD");
    Mockito.verify(mockConnection).setConnectTimeout(5000);

}
}

This test runs in less than a second which is much faster than having to actually wait for over 5 seconds for a real timeout!

You cannot mock a local variable. What you could do, however, is extract its creation to a protected method and spy it:

public class A {
  public void methodOne(int argument) {
    //some operations
    methodTwo(int argument);
    //some operations
  }

  private void methodTwo(int argument) {
    DateTime dateTime = createDateTime();
    //use dateTime to perform some operations
  }

  protected DateTime createDateTime() {
    return new DateTime();
  }
}

public class ATest {
  @Test
  public void testMethodOne() {
    DateTime dt = new DateTime (/* some known parameters... */);
    A a = Mockito.spy(new A());
    doReturn(dt).when(a).createDateTime();
    int arg = 0; // Or some meaningful value...
    a.methodOne(arg);
    // assert the result
}