2nd/10_Wiki/Topics/Architecture/Object Seam (객체 접점).md

---
id: wiki-2026-0508-object-seam-객체-접점
title: Object Seam (객체 접점)
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Object Seam, OO Seam, Polymorphic Seam]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [legacy-code, refactoring, testability, seams, oop]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: java
  framework: junit-mockito
---

# Object Seam (객체 접점)

## 매 한 줄
> **"매 OO 언어에서 다른 코드 변경 없이 동작을 바꿀 수 있는 분기점 — 매 polymorphism이 enabling point."**. Michael Feathers의 *Working Effectively with Legacy Code* (2004) 에서 정의된 3개 seam (preprocessing, link, object) 중 가장 강력. 2026 현재 mock framework · DI container의 핵심 메커니즘.

## 매 핵심

### 매 3 seam types
- **Preprocessing seam**: C/C++ macro 치환 — enabling point는 build flag.
- **Link seam**: linker가 symbol을 binding — enabling point는 link order.
- **Object seam**: virtual dispatch — enabling point는 object instantiation.

### 매 Object Seam 작동 원리
- Polymorphic type (interface, abstract class, base class with virtual)
- Caller가 abstract type 사용
- Subtype 선택은 외부 (constructor, factory, DI container)
- 매 단순 if/else 분기 vs polymorphism 분기

### 매 응용
1. Test에서 real DB → in-memory fake 교체.
2. HTTP client → mock으로 network 차단.
3. Strategy pattern으로 algorithm swap.

## 💻 패턴

### Java — interface seam
```java
public interface PaymentGateway {
    PaymentResult charge(Money amount, Card card);
}

public class CheckoutService {
    private final PaymentGateway gateway;  // ← seam
    public CheckoutService(PaymentGateway g) { this.gateway = g; }
    public Order checkout(Cart cart, Card card) {
        var result = gateway.charge(cart.total(), card);
        return result.success() ? Order.confirmed(cart) : Order.failed();
    }
}

// Test: enabling point = constructor arg
@Test void rejects_failed_payment() {
    var fake = (amount, card) -> PaymentResult.failed("declined");
    var svc = new CheckoutService(fake);
    assertEquals(OrderStatus.FAILED, svc.checkout(cart, card).status());
}
```

### TypeScript — class hierarchy seam
```typescript
abstract class Notifier {
  abstract send(msg: string): Promise<void>;
}
class EmailNotifier extends Notifier { /* SMTP */ }
class TestNotifier extends Notifier {
  sent: string[] = [];
  async send(msg: string) { this.sent.push(msg); }
}

// Production: new EmailNotifier()
// Test: new TestNotifier() — same caller
```

### C++ — virtual function seam
```cpp
class Clock {
public:
    virtual ~Clock() = default;
    virtual std::chrono::system_clock::time_point now() const = 0;
};
class SystemClock : public Clock {
    auto now() const override { return std::chrono::system_clock::now(); }
};
class FakeClock : public Clock {
    std::chrono::system_clock::time_point t_;
public:
    void advance(std::chrono::seconds s) { t_ += s; }
    auto now() const override { return t_; }
};
```

### Introducing seam to legacy code (Extract Interface)
```java
// Before: hard dependency on concrete class
public class Report {
    public void generate() {
        var db = new MySQLConnection();  // ← no seam
        // ...
    }
}

// After: introduced object seam
public class Report {
    private final Database db;
    public Report(Database db) { this.db = db; }
    public void generate() { /* uses db */ }
}
public interface Database { ResultSet query(String sql); }
public class MySQLConnection implements Database { /* ... */ }
```

### Testing legacy code with subclass-and-override
```java
public class LegacyService {
    public Report run() {
        var data = fetchData();  // ← protected, override in test
        return process(data);
    }
    protected Data fetchData() { /* real network */ }
}
// Test
class TestableService extends LegacyService {
    @Override protected Data fetchData() { return Data.fixture(); }
}
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| New code, OO language | Constructor injection + interface |
| Legacy class, can't refactor much | Extract Interface + DI |
| Sealed legacy class | Subclass and Override Method |
| C/C++ no virtual overhead | Link seam (selective compilation) |
| Functional language | Higher-order function = seam |

**기본값**: interface + constructor injection — 매 가장 explicit · testable.

## 🔗 Graph
- 부모: [[Seam (접점)]] · [[Seams (이음새)]]
- 변형: [[Link Seam (링크 접점)]] · [[Preprocessing Seam (전처리 접점)]]
- 응용: [[Dependency Injection (의존성 주입)]] · [[Mock Objects (가짜 객체)]] · [[Test Doubles (테스트 대역)]]
- Adjacent: [[Polymorphism (다형성)]] · [[Hexagonal_Architecture_Ports_and_Adapters]]

## 🤖 LLM 활용
**언제**: legacy code testability 도입, mock framework 설계, plug-in architecture, strategy swap.
**언제 X**: pure functional code (no objects), one-shot script, performance-critical hot loop (virtual call overhead).

## ❌ 안티패턴
- **Concrete class 직접 new**: 매 seam 없음 — Test 어려움.
- **`static` method 의존**: 매 polymorphism 불가 — wrap with instance method.
- **Final/sealed class without interface**: 매 subclass override seam도 막음.
- **Service Locator** (anti): 매 hidden dependency — explicit constructor injection 권장.

## 🧪 검증 / 중복
- Verified (Feathers *WELC* 2004, Fowler *Refactoring* 2nd ed., Meszaros *xUnit Patterns* 2007).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — Object seam definition + 3 language patterns |