🛡Proxy Pattern

Provide a surrogate or placeholder for another object to control access.

📌 Key IdeaClient talks to Proxy implementing the same interface as RealSubject. Proxy may defer creation (virtual), enforce access (protection), hide remote calls (RMI style), or add reference counting / logging.

🖼

Visual

🛡 Same interface, controlled access

  interface Image { void draw(); }

  class ProxyImage implements Image {
      private RealImage real;  // lazy create
      void draw() {
          if (real == null) real = new RealImage(path);
          real.draw();
      }
  }

Virtual proxy: expensive object created only when first needed.

✅

When to Use

Lazy loading — delay heavy resource until first use.
Access control — permission checks before delegating.
Remote object — local proxy marshals network calls.
Logging / caching around expensive operations.

🚫

When NOT to Use

You need to add features transparently in layers — often Decorator intent.
Proxy indirection hurts latency when always hitting real subject anyway.

🚨

Anti-Pattern (What NOT to Do)

Eager load on startupBadjava

class Page {
    private final HeavyChart chart = new HeavyChart(); // always paid for
}

User may never open the chart tab.

✅

The Right Way

Virtual proxy — create on first useGoodjava

class ChartProxy implements Chart {
    private HeavyChart real;

    public void draw() {
        if (real == null) real = new HeavyChart();
        real.draw();
    }
}

Same interface as real subject; Spring/JPA often enhance this with bytecode.

⚖️

Pros & Cons

👍 Pros

Separates concerns of access control / caching from subject logic.
Can protect real subject from misuse.

👎 Cons

Extra indirection — debugging one more hop.
Easy to confuse with Decorator — document intent.

🔀

Common Confusions Cleared Up

Proxy vs Decorator

Proxy controls access to often one heavy or remote subject. Decorator stacks features on an object. Same composition structure — intent differs (GoF stress this interview trap).

🎤

Interview Q&A

🔮

Likely Follow-up Questions

Dynamic proxy InvocationHandler in Java — how it maps to Proxy pattern?

🧾

Quick Cheat Sheet

IntentSurrogate controls access to real object.

vs DecoratorAccess vs add behaviour.

SpringAOP proxies around beans.

🔗