2nd/10_Wiki/Topics/Architecture/Mock Objects (가짜 객체).md

---
id: wiki-2026-0508-mock-objects-가짜-객체
title: Mock Objects (가짜 객체)
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Mock, Mocking, Stubbing, Test Doubles, 테스트 대역]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [testing, tdd, unit-test, mock]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: multi
  framework: jest-pytest-mockito
---

# Mock Objects (가짜 객체)

## 매 한 줄
> **"매 real collaborator 의 자리에 매 control 가능한 가짜를 끼워넣음."**. Mock object 는 매 unit test 에서 매 dependency 를 isolate 하기 위한 매 test double. Gerard Meszaros 의 매 분류 (Dummy / Stub / Spy / Mock / Fake) 가 표준 — 매 잘못 쓰면 매 test 가 매 implementation 에 coupling 된다.

## 매 핵심

### 매 Test Double 5종 (Meszaros)
- **Dummy**: 매 인자 자리만 채움, 매 호출 안됨.
- **Stub**: 매 hardcoded response 반환 (state verification).
- **Spy**: 매 호출 record (verify 가능한 stub).
- **Mock**: 매 expected 호출 정의, 매 위반 시 fail (behavior verification).
- **Fake**: 매 실제 working 구현 (in-memory DB 등).

### 매 언제 mock
- 매 외부 service (network, DB, FS).
- 매 nondeterministic (time, random, env).
- 매 expensive (slow, paid API).
- 매 hard to trigger (error case, race condition).

### 매 언제 mock 하지 말 것
- 매 own code 의 internal collaborator — 매 over-mocking → 매 fragile test.
- 매 third-party library 의 직접 mock — 매 own wrapper 만들고 매 그걸 mock.
- 매 value object — 매 그냥 real 사용.

### 매 London vs Detroit (TDD school)
- **London (mockist)**: 매 collaborator 다 mock, 매 interaction verify.
- **Detroit (classical)**: 매 real object 사용, 매 state verify, 매 boundary 만 mock.
- 매 2026 trend: 매 Detroit 우세 (mockist 의 fragile test 비판) — 매 boundary 에서만 mock.

## 💻 패턴

### Jest (JS) — module mock
```javascript
// userService.test.js
import { fetchUser } from './userService';
import * as api from './api';

jest.mock('./api');

test('fetchUser returns user data', async () => {
  api.get.mockResolvedValue({ id: 1, name: 'Alice' });

  const user = await fetchUser(1);

  expect(user.name).toBe('Alice');
  expect(api.get).toHaveBeenCalledWith('/users/1');
  expect(api.get).toHaveBeenCalledTimes(1);
});
```

### Jest — spy on real
```javascript
import { logger } from './logger';
import { processOrder } from './orders';

test('logs order processing', async () => {
  const spy = jest.spyOn(logger, 'info');
  await processOrder({ id: 'o1' });
  expect(spy).toHaveBeenCalledWith('processing', { id: 'o1' });
  spy.mockRestore();
});
```

### pytest — monkeypatch
```python
def test_fetch_user(monkeypatch):
    def fake_get(url):
        return {"id": 1, "name": "Alice"}

    import api
    monkeypatch.setattr(api, "get", fake_get)

    from user_service import fetch_user
    user = fetch_user(1)
    assert user["name"] == "Alice"
```

### pytest — unittest.mock
```python
from unittest.mock import patch, MagicMock

@patch('user_service.api.get')
def test_fetch_user(mock_get):
    mock_get.return_value = {"id": 1, "name": "Alice"}

    from user_service import fetch_user
    user = fetch_user(1)

    mock_get.assert_called_once_with('/users/1')
    assert user["name"] == "Alice"
```

### Mockito (Java)
```java
import static org.mockito.Mockito.*;

@Test
void fetchUser_returnsUser() {
    ApiClient api = mock(ApiClient.class);
    when(api.get("/users/1")).thenReturn(new User(1, "Alice"));

    UserService svc = new UserService(api);
    User u = svc.fetchUser(1);

    assertEquals("Alice", u.getName());
    verify(api).get("/users/1");
    verify(api, times(1)).get(anyString());
}
```

### Fake (in-memory DB)
```typescript
// Fake user repo — real working impl, no DB
class InMemoryUserRepo implements UserRepo {
  private users = new Map<string, User>();
  async save(u: User) { this.users.set(u.id, u); }
  async find(id: string) { return this.users.get(id) ?? null; }
}

test('UserService.create persists user', async () => {
  const repo = new InMemoryUserRepo();
  const svc = new UserService(repo);
  await svc.create({ name: 'Alice' });
  const all = await repo.find('1');
  expect(all?.name).toBe('Alice');
});
```

### MSW (Mock Service Worker) — HTTP-level mock
```typescript
import { setupServer } from 'msw/node';
import { http, HttpResponse } from 'msw';

const server = setupServer(
  http.get('/api/users/:id', ({ params }) =>
    HttpResponse.json({ id: params.id, name: 'Alice' })
  )
);

beforeAll(() => server.listen());
afterEach(() => server.resetHandlers());
afterAll(() => server.close());

test('component shows user', async () => {
  render(<UserCard id="1" />);
  expect(await screen.findByText('Alice')).toBeInTheDocument();
});
```

### Time mocking
```javascript
// Jest fake timers
jest.useFakeTimers();
jest.setSystemTime(new Date('2026-05-10T00:00:00Z'));

const result = formatRelative(new Date('2026-05-10T00:00:30Z'));
expect(result).toBe('30 seconds ago');

jest.useRealTimers();
```

## 매 결정 기준
| 상황 | Double type |
|---|---|
| 외부 HTTP API | MSW (network-level) > module mock |
| DB | Fake (in-memory) > Mock |
| 시간 / random | jest.useFakeTimers, freeze_time |
| 로깅 / metrics | Spy (verify 호출) |
| 단순 인자 채우기 | Dummy |
| Behavior 검증 (rare) | Mock |
| State 검증 (default) | Stub / Fake |

**기본값**: 매 boundary 만 mock, 매 internal 은 real. 매 state verify > behavior verify.

## 🔗 Graph
- 부모: [[Unit Testing]] · [[TDD]]
- 변형: [[Stub]] · [[Spy]] · [[Fake]] · [[Mockito]] · [[MSW]]
- 응용: [[Test Pyramid]] · [[Contract Testing]]
- Adjacent: [[Dependency Injection]] · [[Hexagonal Architecture]] · [[Mocking_Framework]]

## 🤖 LLM 활용
**언제**: unit test 작성 시 매 외부 dependency isolation, 매 deterministic test, 매 fast feedback.
**언제 X**: integration test — 매 real component 가 매 정답. 매 LLM 호출은 매 record/replay (VCR) 가 더 적합.

## ❌ 안티패턴
- **Over-mocking**: 매 internal 까지 mock → 매 refactor 시 매 test 다 깨짐.
- **Mock 의 Mock**: 매 mock 이 mock 을 반환 → 매 test 가 매 reality 와 무관.
- **Implementation coupling**: 매 `verify(api).callMethod(exact_args)` 의 남발 → 매 fragile.
- **Real time / random**: 매 flaky test 의 가장 흔한 원인.
- **No teardown**: 매 mock 이 매 다음 test 로 leak — 매 reset 필수.

## 🧪 검증 / 중복
- Verified (Meszaros "xUnit Test Patterns", Martin Fowler "Mocks Aren't Stubs", Jest/pytest/Mockito docs, MSW docs).
- 신뢰도 A.
- 매 [[Mocking and Stubbing (테스트 대역)]] 는 매 alias redirect.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — Meszaros 5종 + Jest/pytest/Mockito/MSW 패턴 |