Mockito Expert

Mockito Expert

Mockito specialist for precise, maintainable test doubles in Java unit tests.

Role Definition

Designs and implements Mockito-based test doubles that isolate units from collaborators without over-specifying implementation details. Applies stubbing, verification, capturing, and custom answering strategies with strict discipline to keep tests meaningful and refactor-safe.

When to Use This Skill

• Isolating a class under test from its dependencies
• Simulating error conditions and edge cases from collaborators
• Capturing arguments passed to a dependency for detailed assertion
• Verifying interaction contracts without testing return values
• Replacing partial implementations with spies
• Configuring Mockito for JUnit 5 projects

Dependency Setup

Maven

<dependency>
  <groupId>org.mockito</groupId>
  <artifactId>mockito-core</artifactId>
  <version>5.12.0</version>
  <scope>test</scope>
</dependency>

<!-- JUnit 5 integration -->
<dependency>
  <groupId>org.mockito</groupId>
  <artifactId>mockito-junit-jupiter</artifactId>
  <version>5.12.0</version>
  <scope>test</scope>
</dependency>

Gradle

dependencies {
    testImplementation("org.mockito:mockito-core:5.12.0")
    testImplementation("org.mockito:mockito-junit-jupiter:5.12.0")
}

JUnit 5 Integration

Always use @ExtendWith(MockitoExtension.class) instead of manual MockitoAnnotations.openMocks().

@ExtendWith(MockitoExtension.class)
class OrderServiceTest {

    @Mock
    private OrderRepository repository;

    @Mock
    private PaymentGateway paymentGateway;

    @InjectMocks
    private OrderService orderService;  // Mockito injects @Mock fields

    @Captor
    private ArgumentCaptor<Order> orderCaptor;

    @Test
    void shouldSaveOrderAfterPayment() {
        // stub
        when(paymentGateway.charge(any())).thenReturn(PaymentResult.success("txn-1"));

        // act
        orderService.place(Order.of("item-1", 2));

        // verify
        verify(repository).save(orderCaptor.capture());
        assertThat(orderCaptor.getValue().status()).isEqualTo(OrderStatus.ACCEPTED);
    }
}

Mock Creation

Annotation-based (preferred)

@Mock
private UserRepository userRepository;

@Spy
private AuditLogger auditLogger = new AuditLogger();  // real object, partial mock

@InjectMocks
private UserService userService;

Programmatic (when annotations are unavailable)

UserRepository userRepository = mock(UserRepository.class);
AuditLogger auditLogger = spy(new AuditLogger());
UserService userService = new UserService(userRepository, auditLogger);

Mocking with settings

// with a name (improves error messages)
UserRepository repo = mock(UserRepository.class, "userRepository");

// lenient mock (disables strict stubbing for this mock only)
UserRepository repo = mock(UserRepository.class, withSettings().lenient());

// mock with default answer
UserRepository repo = mock(UserRepository.class, RETURNS_DEEP_STUBS);

Stubbing

when(...).thenReturn(...) — return a value

when(repository.findById("user-1")).thenReturn(Optional.of(alice));
when(repository.findById("missing")).thenReturn(Optional.empty());

// chain for consecutive calls
when(clock.now())
    .thenReturn(Instant.parse("2024-01-01T00:00:00Z"))
    .thenReturn(Instant.parse("2024-01-02T00:00:00Z"));

when(...).thenThrow(...) — simulate exceptions

when(paymentGateway.charge(any())).thenThrow(new PaymentException("card declined"));

// throw checked exceptions
when(fileStore.read("missing.txt")).thenThrow(IOException.class);

when(...).thenAnswer(...) — dynamic response

when(idGenerator.next()).thenAnswer(inv -> UUID.randomUUID().toString());

when(repository.findAll()).thenAnswer(inv -> {
    // simulate delay or side effects
    return List.of(alice, bob);
});

doReturn / doThrow / doAnswer — for void methods and spies

Use do* form when stubbing void methods or when the method is called on a @Spy.

// void method stubbing
doNothing().when(emailService).send(any());
doThrow(new MailException("server down")).when(emailService).send(any());

// spy — prevents real method from executing before stub
doReturn(42).when(spyService).computeScore();

// doAnswer for void methods with side effects
doAnswer(inv -> {
    Order order = inv.getArgument(0);
    order.markProcessed();
    return null;
}).when(repository).save(any(Order.class));

Stubbing void methods with lambdas

doAnswer(invocation -> {
    Consumer<String> callback = invocation.getArgument(1);
    callback.accept("processed");
    return null;
}).when(asyncProcessor).process(eq("job-1"), any());

Argument Matchers

Matchers make stubs flexible and verifications readable.

import static org.mockito.ArgumentMatchers.*;

// any type
when(repo.findById(anyString())).thenReturn(Optional.of(user));
when(repo.findAll(anyList())).thenReturn(users);

// specific equality
when(repo.findById(eq("user-42"))).thenReturn(Optional.of(alice));

// type-based
when(processor.process(any(Order.class))).thenReturn(result);

// null / non-null
when(service.lookup(isNull())).thenThrow(IllegalArgumentException.class);
when(service.lookup(notNull())).thenReturn(result);

// string matchers
when(repo.search(startsWith("order-"))).thenReturn(orders);
when(repo.search(endsWith("-v2"))).thenReturn(drafts);
when(repo.search(contains("2024"))).thenReturn(yearOrders);
when(repo.search(matches("ORD-\\d{6}"))).thenReturn(formattedOrders);

// custom predicate
when(repo.findBy(argThat(spec -> spec.minAmount().compareTo(BigDecimal.TEN) > 0)))
    .thenReturn(highValueOrders);

Rule: When you use any matcher in an invocation, ALL arguments must be matchers.

// WRONG
verify(service).transfer("acc-1", anyString());

// CORRECT
verify(service).transfer(eq("acc-1"), anyString());

Verification

Basic verification

// called exactly once (default)
verify(repository).save(any(Order.class));

// exact count
verify(emailService, times(2)).send(any());

// never called
verify(auditLogger, never()).logFailure(any());

// at least / at most
verify(cache, atLeast(1)).evict(any());
verify(rateLimiter, atMost(3)).acquire();

// no more interactions
verifyNoMoreInteractions(repository);

// zero interactions at all
verifyNoInteractions(emailService);

Verification order

InOrder inOrder = inOrder(repository, emailService);
inOrder.verify(repository).save(any());
inOrder.verify(emailService).send(any());

Verify with timeout (async code)

verify(eventBus, timeout(500)).publish(any(OrderPlacedEvent.class));
verify(eventBus, timeout(500).times(2)).publish(any());

Argument Captors

Capture arguments for detailed assertion when matchers alone are insufficient.

@Captor
private ArgumentCaptor<Order> orderCaptor;

@Captor
private ArgumentCaptor<EmailMessage> emailCaptor;

@Test
void shouldSendConfirmationWithCorrectContent() {
    orderService.place(Order.of("item-1", 5));

    verify(emailService).send(emailCaptor.capture());
    EmailMessage sent = emailCaptor.getValue();

    assertThat(sent.to()).isEqualTo("customer@example.com");
    assertThat(sent.subject()).startsWith("Order confirmed");
    assertThat(sent.body()).contains("item-1", "5");
}

@Test
void shouldSaveAllItems() {
    orderService.placeAll(List.of(order1, order2, order3));

    verify(repository, times(3)).save(orderCaptor.capture());
    List<Order> saved = orderCaptor.getAllValues();

    assertThat(saved).extracting(Order::status)
        .containsOnly(OrderStatus.ACCEPTED);
}

Spy — Partial Mocking

Spies wrap a real object. Only explicitly stubbed methods are overridden.

@Spy
private AuditLogger auditLogger = new AuditLogger();

@Test
void shouldAuditOrderPlacement() {
    // real send() still runs unless stubbed
    orderService.place(order);
    verify(auditLogger).log(contains("ORDER_PLACED"));
}

// Stub one method, keep others real
doReturn("mocked-id").when(auditLogger).generateCorrelationId();

Warning: Do not use when(spy.method()).thenReturn(...) — it calls the real method first. Always use doReturn(...).when(spy).method().

Strict Stubbing

MockitoExtension enforces strict stubbing by default:

• Fails tests with unnecessary stubs (stubbed but never called)
• Fails with stubbing argument mismatch (stubbed with A, called with B)

This prevents test rot from accumulating dead stubs.

// This will fail if someMethod() is never called in the test
when(mock.someMethod()).thenReturn("value");

To selectively relax:

// Option 1: per-mock lenient setting
@Mock(lenient = true)
private UserRepository repository;

// Option 2: per-stub lenient wrapping
lenient().when(repository.findById(any())).thenReturn(Optional.empty());

// Option 3: class-level (use sparingly)
@MockitoSettings(strictness = Strictness.LENIENT)
class LegacyTest { ... }

Mocking Static Methods (Mockito 3.4+)

Requires mockito-inline dependency (included in mockito-core 5+).

@Test
void shouldUseCurrentTime() {
    try (MockedStatic<Instant> mockedInstant = mockStatic(Instant.class)) {
        var fixedTime = Instant.parse("2024-06-01T12:00:00Z");
        mockedInstant.when(Instant::now).thenReturn(fixedTime);

        var result = service.createTimestampedRecord("data");

        assertThat(result.createdAt()).isEqualTo(fixedTime);
    }
}

Caution: Prefer injecting Clock or Supplier<Instant> over mocking static Instant.now() in production design.

Mocking Constructors (MockedConstruction)

@Test
void shouldInitializeConnectorWithCorrectConfig() {
    try (MockedConstruction<DatabaseConnector> mocked = mockConstruction(
            DatabaseConnector.class,
            (mock, ctx) -> when(mock.isConnected()).thenReturn(true)
    )) {
        service.initialize();

        assertThat(mocked.constructed()).hasSize(1);
        verify(mocked.constructed().get(0)).connect(any());
    }
}

Custom Answers

// Return first argument
when(formatter.format(anyString())).thenAnswer(AdditionalAnswers.returnsFirstArg());

// Return argument at index
when(mapper.map(any(), anyInt())).thenAnswer(AdditionalAnswers.returnsArgAt(1));

// Delegate to real implementation
when(service.compute(any())).thenAnswer(AdditionalAnswers.delegatesTo(realService));

// Custom Answer
when(cache.get(anyString())).thenAnswer(inv -> {
    String key = inv.getArgument(0);
    return backingMap.get(key);
});

Reset and Clear

Avoid reset() — it indicates the test is doing too much. Split into smaller tests instead.

// Only if absolutely necessary (e.g., shared mock across @Nested classes)
reset(repository);
clearInvocations(repository); // clears recorded calls but keeps stubs

Common Pitfalls and Solutions

Mocking Final Classes

Create file src/test/resources/mockito-extensions/org.mockito.plugins.MockMaker:

mock-maker-inline

Or use Mockito 5 which enables this by default via the subclass MockMaker.

Reference Guide

Constraints

MUST DO

• Use @ExtendWith(MockitoExtension.class) for JUnit 5 integration
• Keep stubs close to the test that uses them (in @BeforeEach only for shared behavior)
• Assert on captured arguments rather than overusing any() in verifications
• Prefer state verification over interaction verification when both are possible

MUST NOT DO

• Do not mock types you don't own (mock an adapter/wrapper instead)
• Do not mock value objects or entities — construct them directly
• Do not mock everything — isolate only external dependencies
• Do not use reset() as a substitute for splitting large tests
• Do not use RETURNS_DEEP_STUBS outside of fluent API chains — it hides design problems

Output Templates

## Mock Design Review
- Class under test: [ClassName]
- Mocked collaborators: [list with rationale]
- Stubbing strategy: [thenReturn / thenAnswer / thenThrow]
- Verification targets: [list]
- ArgumentCaptor usage: [yes/no with justification]
- Strict stubbing exceptions: [none / lenient mocks with reason]

// Standard Mockito test template
@ExtendWith(MockitoExtension.class)
class TargetServiceTest {

    @Mock  RepositoryType repository;
    @Mock  GatewayType    gateway;
    @InjectMocks TargetService sut;
    @Captor ArgumentCaptor<DomainType> captor;

    @Test
    void shouldBehaviorWhenCondition() {
        // Arrange
        when(repository.findById(eq("id-1"))).thenReturn(Optional.of(entity));
        when(gateway.process(any())).thenReturn(successResult);

        // Act
        sut.execute("id-1");

        // Assert
        verify(repository).save(captor.capture());
        assertThat(captor.getValue().status()).isEqualTo(ExpectedStatus.DONE);
        verifyNoMoreInteractions(repository);
    }
}

Knowledge Reference

mockito 5, mockito-junit-jupiter, @mock, @spy, @injectmocks, @captor, @extendwith, when thenreturn, thenthrow, thenanswer, doreturn dothrow, argument matchers, any eq startswith, verify times never, verifynointeractions, verifynomore interactions, inorder verification, argumentcaptor, spy partial mock, strict stubbing, lenient, unnecessarystubbingexception, mockedstatic, mockedconstruction, mock final class, mock-maker-inline, custom answer, returnsFirstArg, delegatesTo, mockitosettings

Related Skills

• junit-expert
• testcontainers-expert
• java-expert
• code-reviewer