Mockito: using a method in "thenReturn" to return a mock doesn't work

Question

I have encountered what I assume might be a bug with Mockito, but was wondering if anyone else can shed light as to why this test doesn't work.

Basically, I have two objects, like this:

public class FirstObject {     private SecondObject secondObject;     public SecondObject getSecondObject() { return secondObject; } }  public class SecondObject {     private String name;     public String getName() { return name; } } 

The first object is mocked via annotation and the before method:

@Mock FirstObject mockedFirstObject;  @Before public void setup() {     MockitoAnnotations.initMocks(this); } 

The second object is mocked in a method:

public SecondObject setupMockedSecondObject() {     SecondObject secondObject = Mockito.mock(SecondObject.class);     Mockito.when(secondObject.getName()).thenReturn("MockObject");     return secondObject; } 

When thenReturn contains a direct call to this method to setup and obtain a mock of the second object, it fails:

@Test public void notWorkingTest() {     Mockito.when(mockedFirstObject.getSecondObject()).thenReturn(setupMockedSecondObject());     Assert.assertEquals(mockedFirstObject.getSecondObject().getName(), "MockObject"); } 

But, when the mock returned by the same method is assigned to a local variable, which is used in thenReturn, it works:

@Test public void workingTest() {     SecondObject mockedSecondObject = setupMockedSecondObject();     Mockito.when(mockedFirstObject.getSecondObject()).thenReturn(mockedSecondObject);     Assert.assertEquals(mockedFirstObject.getSecondObject().getName(), "MockObject"); } 

Are we doing something wrong or is this indeed a bug/limitation in Mockito? Is there a deliberate reason for this not working?

Answer 1

This is indeed a limitation of Mockito, and it is referenced in their FAQ:

Can I thenReturn() an inlined mock()?

Unfortunately you cannot do this:

when(m.foo()).thenReturn(mock(Foo.class)); //                         ^ 

The reason is that detecting unfinished stubbing wouldn't work if we allow above construct. We consider is as a 'trade off' of framework validation (see also previous FAQ entry). However you can slightly change the code to make it working:

//extract local variable and start smiling: Foo foo = mock(Foo.class); when(m.foo()).thenReturn(foo); 

The workaround, as mentioned, is to store the desired returned value in a local variable, like you have done.

The way I understand it is that Mockito validates the usage you make of it every time you call its methods. When another method is called during an on-going stubbing process, you are breaking its validation process.