Unit Test Spring Service Mockito | Java Fresher Interview

To write a unit test for a Spring service using Mockito, you start by annotating the test class with at-ExtendWith MockitoExtension dot class. That's the JUnit 5 way to activate Mockito. Then you create mocks for the service's dependencies using at-Mock. For example, if your OrderService depends on an OrderRepository, you'd write at-Mock private OrderRepository orderRepository. Next, you annotate the service itself with at-InjectMocks, which tells Mockito to create a real OrderService instance and inject the mock repository into it. Inside your test method, you use when and thenReturn to define the mock's behavior. Say when orderRepository dot save is called with any Order, return a saved Order with an ID. Then you call your service method and use JUnit assertions to verify the result. The key thing here is this runs without Spring—no application context, no database connection, nothing. It's pure Java object testing. That's why it's so fast. You can run hundreds of these tests in a couple seconds. One mistake I see freshers make is adding at-SpringBootTest to these tests. That loads the entire Spring application, which takes several seconds per test. For unit testing service logic, you don't need Spring at all. Save at-SpringBootTest for end-to-end integration tests where you're testing the whole application flow.