Mockito Setup Spring Boot | Java Fresher Interview

Direct Answer

Mockito is a Java mocking library. It creates test doubles for collaborators such as repositories or HTTP clients, lets a test define their replies, and records calls for verification. In a Spring Boot project, declaring spring-boot-starter-test normally supplies Mockito and its JUnit Jupiter integration. Without that starter, add the Mockito dependencies with test scope. A mock can then be created directly with Mockito.mock(...) or through @Mock in a test managed by MockitoExtension.

Code Example

@ExtendWith(MockitoExtension.class)
class PriceServiceTest {
    @Mock ExchangeRateClient rates;

    @Test
    void convertsEurosToRupees() {
        when(rates.eurToInr()).thenReturn(new BigDecimal("90.00"));
        var service = new PriceService(rates);

        assertEquals(new BigDecimal("900.00"), service.inRupees(new BigDecimal("10.00")));
        verify(rates).eurToInr();
    }
}

The mock avoids a real network call. The stub supplies a known exchange rate, the assertion checks the service result, and verify confirms the required collaboration.

Common Mistakes & Pitfalls

• Assuming Mockito is automatically present: verify the declared test dependencies.
• Starting Spring for an ordinary unit test: construct the subject directly when only Java behavior is under test.
• Mocking the class under test: mock its external collaborators and exercise the real subject.
• Verifying calls without checking the result: assert the observable behavior first; verify only meaningful interactions.
• Mixing version management: prefer Boot-managed versions unless the build has a deliberate, tested override.