Spring JUnit: How to Mock autowired component in autowired component

Spring Boot 1.4 introduced testing annotation called @MockBean. So now mocking and spying on Spring beans is natively supported by Spring Boot.

You could use Mockito. I am not sure with PostConstruct specifically, but this generally works:

// Create a mock of Resource to change its behaviour for testing
@Mock
private Resource resource;

// Testing instance, mocked `resource` should be injected here 
@InjectMocks
@Resource
private TestedClass testedClass;

@Before
public void setUp() throws Exception {
    // Initialize mocks created above
    MockitoAnnotations.initMocks(this);
    // Change behaviour of `resource`
    when(resource.getSomething()).thenReturn("Foo");   
}