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

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

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

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

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)

// 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

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