Mockito Strict Stubbing | Java Fresher Interview

Direct Answer

Strict stubbing validates test setup at the end of the test lifecycle. With STRICT_STUBS, Mockito reports stubbings that were never used and can fail early when code calls a stubbed method with different arguments in situations covered by its mismatch detection. It also treats stubbed invocations as verified for no-more-interactions checks. It does not make every unstubbed method throw; ordinary unstubbed calls still use the mock's default answer.

Code Example

@ExtendWith(MockitoExtension.class)
class PaymentServiceTest {

    @Mock
    private PaymentGateway gateway;

    @InjectMocks
    private PaymentService service;

    @Test
    void strictStubbingCatchesUnusedStub() {
        // This stub is never called in the test
        when(gateway.authorize(100.0)).thenReturn(true);
        // Strict stubbing throws UnnecessaryStubbingException here

        // Test calls a different method or different arguments
        service.refund(50.0);  // doesn't use the stub above
    }

    @Test
    void strictStubbingCatchesWrongArguments() {
        when(gateway.authorize(100.0)).thenReturn(true);  // stubbed with 100.0

        service.processPayment(200.0);  // calls authorize(200.0) - different argument!
        // Strict stubbing detects the mismatch and fails the test
    }
}

Common Mistakes & Pitfalls

• Disabling strict stubs when seeing UnnecessaryStubbingException: Adding lenient() or switching to lenient mode instead of fixing the actual problem—removing the unused stub or adding the missing assertion. The exception is telling you the test is broken; silencing it just hides technical debt.
• Declaring stubs in @BeforeEach for optional scenarios: Setting up stubs that only some tests need in a shared setup method—strict stubbing fails tests that don't use those stubs. Move stubs into individual test methods instead.
• Copy-pasting test setup without cleanup: Duplicating a test method, changing one line, but leaving all the original stubs intact—strict stubbing catches the now-irrelevant stubs that the new test doesn't exercise.