2nd/10_Wiki/Topics/Architecture/Mockito.md

---
id: wiki-2026-0508-mockito
title: Mockito
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Java Mocking, Mockito Framework]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [java, testing, mocking, junit]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: java
  framework: mockito-junit5
---

# Mockito

## 매 한 줄
> **"매 mock = 매 collaborator 의 stub + verify"**. Java testing 의 매 de facto mocking framework. JUnit 5 + Mockito 5.x 의 매 standard combo. 2026 modern Java (21 LTS / 25) 매 record + sealed type 와 호환.

## 매 핵심

### 매 capability
- **Stub**: 매 method 의 fake return value.
- **Verify**: 매 interaction 의 assertion (called N times, with args).
- **Spy**: 매 real object 의 partial mock.
- **Argument captor**: 매 invocation arg 의 capture, inspect.

### 매 lifecycle
- `@Mock` — 매 dependency.
- `@InjectMocks` — 매 system under test.
- `@ExtendWith(MockitoExtension.class)` — JUnit 5 integration.
- `mockStatic` — 매 static method.

### 매 응용
1. Service unit test: 매 repository 의 mock.
2. Controller test: 매 service 의 stub + slice test.
3. Integration boundary: 매 external API 의 mock.

## 💻 패턴

### Basic stub + verify
```java
import static org.mockito.Mockito.*;
import static org.junit.jupiter.api.Assertions.*;

@ExtendWith(MockitoExtension.class)
class OrderServiceTest {
    @Mock OrderRepository repo;
    @Mock PaymentGateway gateway;
    @InjectMocks OrderService service;

    @Test
    void placesOrder() {
        when(repo.save(any(Order.class))).thenAnswer(inv -> inv.getArgument(0));
        when(gateway.charge(anyLong(), anyString())).thenReturn(new Receipt("rcpt-1"));

        Order o = service.place(new Cart(List.of(item("sku", 1000))), "card-1");

        assertEquals("rcpt-1", o.receiptId());
        verify(repo).save(any(Order.class));
        verify(gateway, times(1)).charge(1000L, "card-1");
    }
}
```

### Argument captor
```java
@Captor ArgumentCaptor<Order> orderCaptor;

@Test
void persistsCorrectAmount() {
    service.place(cart, "card-1");
    verify(repo).save(orderCaptor.capture());
    assertEquals(1500L, orderCaptor.getValue().total());
}
```

### Spy (partial mock)
```java
@Test
void spyExample() {
    List<String> real = new ArrayList<>();
    List<String> spy = spy(real);
    spy.add("a");
    doReturn(99).when(spy).size(); // override
    assertEquals(99, spy.size());
    verify(spy).add("a");
}
```

### Static mock
```java
@Test
void staticMock() {
    try (MockedStatic<Clock> m = mockStatic(Clock.class)) {
        m.when(Clock::systemUTC).thenReturn(Clock.fixed(Instant.EPOCH, ZoneOffset.UTC));
        assertEquals(Instant.EPOCH, Clock.systemUTC().instant());
    }
}
```

### Throw exception
```java
when(gateway.charge(anyLong(), anyString()))
    .thenThrow(new PaymentDeclinedException("insufficient"));

assertThrows(PaymentDeclinedException.class,
    () -> service.place(cart, "card-1"));
```

### Multiple return values
```java
when(repo.findById(1L))
    .thenReturn(Optional.empty())
    .thenReturn(Optional.of(new Order(1L)));

assertTrue(service.fetch(1L).isEmpty());
assertTrue(service.fetch(1L).isPresent());
```

### BDD style
```java
import static org.mockito.BDDMockito.*;

@Test
void bdd() {
    given(repo.findById(1L)).willReturn(Optional.of(new Order(1L)));
    Order o = service.fetch(1L).orElseThrow();
    then(repo).should().findById(1L);
    assertEquals(1L, o.id());
}
```

### Verify no more interactions
```java
verify(repo).save(any());
verifyNoMoreInteractions(repo);
verifyNoInteractions(gateway); // never called
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Pure Java unit test | Mockito + JUnit 5 |
| Spring boot slice | Mockito + @MockBean (Spring Boot 3.x: @MockitoBean) |
| Integration test | Testcontainers, no mock |
| Static legacy code | mockStatic (try-with-resources) |

**기본값**: Mockito 5.x + JUnit 5 + AssertJ.

## 🔗 Graph
- 부모: [[Unit-Testing]] · [[Test-Doubles]]

## 🤖 LLM 활용
**언제**: 매 stub setup boilerplate 의 generate, verify assertion 의 propose, captor pattern 의 explain.
**언제 X**: 매 test design, what to mock vs integration-test — design judgment.

## ❌ 안티패턴
- **Mock everything**: 매 value object 의 mock = brittle test.
- **Over-verify**: 매 internal call 의 verify = refactor 의 break.
- **Mock 의 your own mock**: cascade mock = smell. Refactor SUT.
- **No `MockitoExtension`**: stub 의 strict mode 의 lose.

## 🧪 검증 / 중복
- Verified (Mockito official docs, Effective Unit Testing by Koskela, Mockito 5.x release notes).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — Mockito 5 + JUnit 5 patterns for modern Java |