List of Classes and Methods of org.mockito.internal.stubbing package of Mockito Framework

You may refactor your code a little bit. In the "Working with Legacy Code" book by Martin Fowler, he describes a technique of separating the external API (think Java Mail API) from your own application code. The technique is called "Wrap and Skin" and is pretty simple.

What you should do is simply:

1. Create an interface MySession (which is your own stuff) by extracting methods from the javax.mail.Session class.
2. Implement that interface by creating a concrete class (looking a bit like the original javax.mail.Session)
3. In each method DELEGATE the call to equivalent javax.mail.Session method
4. Create your mock class which implements MySession :-)
5. Update your production code to use MySession instead of javax.mail.Session

Happy testing!

EDIT: Also take a look at this blog post: http://www.mhaller.de/archives/18-How-to-mock-a-thirdparty-final-class.html

I will answer the question in two parts. First, yes it is code smell to mock or spy the class under test. That does not mean that it cannot be done correctly but that it is risk prone and should be avoided whenever possible.

WRT your specific example, I would see how the spy could be correctly used but that would be predicated on the assertion that you have elsewhere fully unit tested getStringMetadata. This then begs the question, if you have fully unit tested getStringMetadata elsewhere then you must know how to test it and therefore why not test getNumLines without the spy.

All this being said, millhouse makes a good point but either way you have to unit test the expensive code somewhere. His suggestion goes a long way to help isolate the expensive code and ensure that you only have to test / exercise it once.

For the future I would recommend Eran Harel's answer (refactoring moving new to factory that can be mocked). But if you don't want to change the original source code, use very handy and unique feature: spies. From the documentation:

You can create spies of real objects. When you use the spy then the real methods are called (unless a method was stubbed).

Real spies should be used carefully and occasionally, for example when dealing with legacy code.

In your case you should write:

TestedClass tc = spy(new TestedClass());
LoginContext lcMock = mock(LoginContext.class);
when(tc.login(anyString(), anyString())).thenReturn(lcMock);

If you happen to be using Spring then it has a MockHttpServletResponse class.

@Test
public void myTest() {
    MockHttpServletResponse response = new MockHttpServletResponse();
    // Do test stuff here
    // Verify what was written to the response using MockHttpServletResponse's methods
    response.getContentAsString();
    response.getContentAsByteArray();
    response.getContentLength();
}

I had the following problem: I had a method that was returning a Page<...> ,but I can use doNothing() only on void method ,so the fix is easy:

    @Override
    public Page<ThreadServiceModel> findAll(Pageable pageable) {
        return this.findAll(ThreadServiceModel.class, pageable);
    }

And here we go:

        //Will fail and the actualy findAll will be invoked:
        when(this.threadServices.findAll(ThreadServiceModel.class,pageable)).thenReturn(null);

        //Will fail ,cuz you cannot call doNothing() on something that returns values

        doNothing().when(this.threadServices).findAll(ThreadServiceModel.class,pageable);

        //This is the solution ,and when you use doReturn() the actual method will be NOT invoked,
        //+In my case i dont need value , but if i want I cant use...
        doReturn(null).when(this.threadServices).findAll(ThreadServiceModel.class,pageable);

So the solution is simple ,use the last method on the code that I have showed you :)

Fail the test if an exception is caught.

@Test
public void testGetBalanceForPerson() {

   // creating mock person
   Person person1 = mock(Person.class);
   when(person1.getId()).thenReturn("mockedId");

   // calling method under test
   try {
      myClass.getBalanceForPerson(person1);
   } catch(Exception e) {
      fail("Should not have thrown any exception");
   }
}

There's no way to use anyMapOf to do this. It's designed to help with the simple case of mapping simple classes to simple classes in Java 7, and yours is more complex than that.

Java 8 parameter inference improved, so in Java 8, you can just use any().

verify(dataProcessor).process(Matchers.any());

Barring that, the best way to make this look is either like you wrote above:

verify(dataProcessor).process(Matchers.<Map<String, Set<String>>>any());

Or by extracting the matcher to a static function, which gives Java just enough information it needs to infer the type on its own:

@Test public void yourTest() {
  // ...
  verify(dataProcessor).process(anyStringSetMap());
}

private static Map<String, Set<String>> anyStringSetMap() {
  return any();
}

(Caveat: Note that the return value of anyStringSetMap() is null; it's the side-effect of calling any that you're looking for. The extracted method is just to inform the Java compiler of the expected return type; beware that doing anything fancier will probably break in really-quite-interesting ways.)

OK. So, the contract of the method is the following: Parse the input string as JSON, and send back BAD_REQUEST if it's invalid. If it's valid, create an entity in the datastore with various properties (you know them), and send back OK.

And you need to verify that this contract is fulfilled by the method.

Where does Mockito help here? Well, if you test this method without Mockito, you need a real DataStoreService, and you need to verify that the entity has been created correctly in this real DataStoreService. This is where your test is not a unit test anymore, and this is also where it's too complex to test, too long, and too hard to run because it needs a complex environment.

Mockito can help by mocking the dependency on the DataStoreService: you can create a mock of DataStoreService, and verify that this mock is indeed called with the appropriate entity argument when you call your initialize() method in your test.

To do that, you need to be able to inject the DataStoreService into your object under test. It can be as easy as refactoring your object in the following way:

public class MyRestService {
    private DataStoreService dataStoreService;

    // constructor used on the server
    public MyRestService() {
        this.dataStoreService = DatastoreServiceFactory.getDatastoreService();
    }

    // constructor used by the unit tests
    public MyRestService(DataStoreService dataStoreService) {
        this.dataStoreService = dataStoreService;
    }

    public Response initialize(String DatabaseSchema) {
         ...
         // use this.dataStoreService instead of datastore
    }
}

And now in your test method, you can do:

@Test
public void testInitializeWithGoodInput() {
    DataStoreService mockDataStoreService = mock(DataStoreService.class);
    MyRestService service = new MyRestService(mockDataStoreService);
    String goodInput = "...";
    Response response = service.initialize(goodInput);
    assertEquals(Response.Status.OK, response.getStatus());

    ArgumentCaptor<Entity> argument = ArgumentCaptor.forClass(Entity.class);
    verify(mock).put(argument.capture());
    assertEquals("the correct kind", argument.getValue().getKind());
    // ... other assertions
}

It is because of Java/Mockito compatibility. For example, if you are using Java 11 with Mockito 2.17.0, it will throw the same error (Upgrading to Mockito 2.22.0 or later will solve it). Aside from digging into their release artifact on Github, I am yet to see a comprehensive document that spells out the compatibility matrix. Here is a helpful compatibility Matrix (based on quick test)

• Mockito * - 2.17.0 => Java 8

(footnote: * - dont know how far back)

• Mockito 2.18.0 - 3.2.4 => Java 11