Mock

Simple Mock

Mockito offers a one-size-fits-all mehtod to create mocks of (non-final) classes and interfaces.

Dependency mock = Mockito.mock(Dependency.class);

This creates a mock instance of Dependency regardless of whether Dependency is a interface or class.

It is then possible to stub method calls to that mock using the Mockito.when(x).thenReturn(y) notation.

Mockito.when(mock.possiblyBuggyMethod()).thenReturn("someString");

So that calls to Dependency.possiblyBuggyMethod() simply return "someString".

There is another notation that is discouraged in most use cases as it is not typesafe.

Mockito.doReturn("someString").when(mock).possiblyBuggyMethod()

Mock with defaults

While a simple mock returns null (or defaults for primitives) to every call, it is possible to change that behaviour.

Dependency mock = Mockito.mock(Dependency.class, new Answer() {

        @Override
        public Object answer(InvocationOnMock invocationOnMock) throws Throwable {
            return "someString";
        }
    });

or using lambdas:

Dependency mock = Mockito.mock(Dependency.class, (Answer) invocationOnMock -> "someString");

This examples return “someString” to every invocation but it is possible to define any logic in the answer-method.

Mocking a class using annotations

“Spy” for partial mocking

Set private fields in mocked objects

In your class that is under test, you may have some private fields that are not accessible even through constructor. In such cases you can use reflection to set such properties. This is a snippet from such JUnit test.

@InjectMocks
private GreetingsService greetingsService = new GreetingsService(); // mocking this class

@Before
public void setUp() {
    MockitoAnnotations.initMocks(this);
    String someName = "Some Name";
    ReflectionTestUtils.setField(greetingsService, // inject into this object
        "name", // assign to this field
        someName); // object to be injected
}

I’m using Sptring’s ReflectionTestUtils.setField(Object targetObject, String name, Object value) method here to simplify, but you can use plain old Java Reflection to do the same.