Proxy Pattern: Trade-Offs and Alternatives
Understanding when to use Proxy versus alternatives is critical for making appropriate design decisions. The pattern introduces indirection which has both benefits and costs.
When to Use Proxy:
- Expensive object creation: Object initialization involves heavy I/O (file loading, database connections), network calls, or complex computations. Virtual Proxy defers this cost. However, if the object will definitely be used immediately in most scenarios, the indirection overhead is wasted.
- Access control requirements: You need to enforce permissions, validate preconditions, or implement security checks before allowing operations. Protection Proxy centralizes this logic. But if access rules are simple (single check) or change frequently across different contexts, embedding logic directly may be more maintainable.
- Cross-cutting concerns: You need logging, caching, or monitoring that applies to multiple methods uniformly. Proxy provides a single interception point. However, if concerns apply to many unrelated classes, an AOP (Aspect-Oriented Programming) framework or Decorator chain might be more scalable.
- Transparent substitution: Client code should not be modified to accommodate the proxy. If clients must be aware they are using a proxy (calling special methods like
initialize()orcheckPermission()), the pattern's transparency is lost and a simpler solution may suffice.
Trade-Offs:
• Adds functionality transparently
• Manages object lifecycle (lazy, pooling)
• Single Responsibility: subject stays focused
• Open/Closed: extend via new proxies
• Increases number of classes (complexity)
• Can hide performance issues (unclear when expensive operations occur)
• May complicate debugging (stack traces longer)
• Overuse creates "proxy hell" with nested proxies
Proxy vs Alternatives:
Proxy vs Decorator: Both wrap an object and share its interface. Use Decorator when adding responsibilities (new features) that can be stacked (logging + encryption + compression). Use Proxy when controlling access or managing lifecycle. A Decorator enhances what an object does. A Proxy controls whether and when it does it. In practice, if your wrapper checks permissions or delays creation, it is a Proxy. If it adds data transformation or additional behavior without access control, it is a Decorator. However, the distinction blurs when a Proxy also adds caching (enhancement), so focus on primary intent.
Proxy vs Facade: Facade provides a simplified interface to a complex subsystem with multiple classes. It does not implement the same interface as the subsystem. Proxy implements the same interface as a single subject. Use Facade to reduce coupling to a subsystem. Use Proxy to control access to one object. If you need to simplify interactions with a library or module, Facade is appropriate. If you need to add behavior around a specific object, Proxy is appropriate.
Proxy vs Direct Implementation: Embedding access checks or lazy initialization directly in the subject is simpler when you only have one variation. Use Proxy when you need multiple configurations (different access rules for different user types) or when the subject is from a third-party library you cannot modify. If you control the subject and have simple needs, direct implementation avoids unnecessary indirection. But if access logic may change independently of business logic, Proxy provides better separation.
When Proxy Is Overkill:
Better Approach: Use direct instantiation when needed. The Proxy's complexity (extra class, interface extraction, reference management) is not justified.
Scenario: You need per-method access control with different rules for each method (method A requires role X, method B requires role Y and permission Z).
Better Approach: Use method-level annotations or interceptors (AOP) rather than a Proxy that contains complex conditional logic. A Proxy with many conditionals becomes a maintenance burden.