Mockito

Introduction

Mockito creates test doubles — mocks that record interactions and stubs return values. Combined with JUnit 5, it lets you unit-test a class in isolation without spinning up Spring or a database.

Informative example

Mock, stub, verify:

@ExtendWith(MockitoExtension.class)
class UserServiceTest {
    @Mock UserRepository repo;
    @Mock EmailService email;
    @InjectMocks UserService service;

    @Test
    void registersUser() {
        var req = new RegisterRequest("ada@example.com", "secret");
        when(repo.existsByEmail("ada@example.com")).thenReturn(false);
        when(repo.save(any())).thenAnswer(i -> i.getArgument(0));

        User user = service.register(req);

        assertThat(user.email()).isEqualTo("ada@example.com");
        verify(email).sendWelcome("ada@example.com");
        verify(repo, never()).delete(any());
    }
}

Best practices

• Mock external dependencies only — not the class under test.
• Use @InjectMocks for the SUT; @Mock for collaborators.
• Prefer verify over assert on mock return values when testing interactions.

Purpose of this lesson

Master Mockito so you can apply it confidently in production Java code, technical interviews, and code reviews.

Step-by-step explanation

1. Understand the core idea behind Mockito.
2. Walk through the runnable example and tweak it in the playground.
3. Apply the pattern in a small Spring Boot or CLI exercise of your own.
4. Re-read the common mistakes and interview Q&A to lock the concept in.

Interactive workflow diagram

Useful template

when(repo.findById(1L)).thenReturn(Optional.of(user));
verify(repo, times(1)).save(any());
verify(email, never()).send(any());

Debugging tips

• UnnecessaryStubbingException? Remove unused when() stubs or use @MockitoSettings(strictness = LENIENT).

Optimization strategies

• Profile before optimizing — JFR (Java Flight Recorder) and async-profiler reveal real hotspots.
• Prefer immutable data and stream pipelines over hand-rolled loops when readability matters.
• Reach for the right JDK collection (ArrayList vs LinkedList vs ArrayDeque) before writing custom data structures.

Enterprise example

Teams at Netflix, Uber and Goldman Sachs apply Mockito daily — usually wrapped behind Spring Boot services with observability hooks (Micrometer + OpenTelemetry).

Interview questions & answers

Summary

In this lesson you learned Mockito — the concept, syntax, a runnable example, and the production pitfalls to avoid. Apply it in the playground before moving on.