Proxy Design Pattern: Controlled Access with Structural Elegance

🛡️ Proxy Design Pattern: Controlled Access with Structural Elegance

The Proxy Design Pattern is a structural pattern that provides a surrogate or placeholder for another object to control access to it. Whether you're optimizing performance, enforcing security, or managing remote resources, the proxy pattern offers a clean and extensible solution.

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🎯 Intent and Motivation

The core idea is to create a proxy object that implements the same interface as the real object. The client interacts with the proxy, which then delegates requests to the actual object—optionally adding logic like access control, caching, or lazy initialization.

“The Proxy pattern suggests that you create a new proxy class with the same interface as an original service object. Then you update your app so that it passes the proxy object to all of the original object’s clients.” — Refactoring Guru

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🧱 Key Components

Component	Description
Subject	Interface shared by both the proxy and the real object
RealSubject	The actual object that performs the operations
Proxy	Controls access to the RealSubject, adding extra behavior

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🧪 Example: Image Viewer with Lazy Loading

Imagine an image viewer that loads high-resolution images. To avoid loading all images upfront, we use a proxy.

interface Image {
    void display();
}

class RealImage implements Image {
    private String filename;

    public RealImage(String filename) {
        this.filename = filename;
        loadFromDisk();
    }

    private void loadFromDisk() {
        System.out.println("Loading " + filename);
    }

    public void display() {
        System.out.println("Displaying " + filename);
    }
}

class ProxyImage implements Image {
    private String filename;
    private RealImage realImage;

    public ProxyImage(String filename) {
        this.filename = filename;
    }

    public void display() {
        if (realImage == null) {
            realImage = new RealImage(filename);
        }
        realImage.display();
    }
}

Here, ProxyImage delays the creation of RealImage until display() is called—saving memory and startup time.

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🧠 Types of Proxies

Type	Purpose
Virtual Proxy	Lazy loading of expensive resources
Protection Proxy	Access control and authorization
Remote Proxy	Representing objects in different address spaces
Smart Proxy	Adding extra behavior like logging or reference counting

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✅ When to Use

• When object creation is expensive and can be deferred
• When access to the real object needs to be controlled
• When working with remote or distributed systems
• When adding cross-cutting concerns like logging or caching

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❌ When Not to Use

• When direct access is simpler and sufficient
• When the overhead of proxy logic outweighs its benefits
• When the system doesn’t require abstraction or control

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🔍 Real-World Applications

• Credit Cards: Act as proxies for bank accounts
• Web Browsers: Use image proxies for lazy loading
• Security Systems: Use protection proxies to restrict access
• Distributed Systems: Use remote proxies for inter-process communication

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🔄 Comparison with Decorator Pattern

Feature	Proxy Pattern	Decorator Pattern
Intent	Control access to an object	Add responsibilities dynamically
Structure	Acts as a substitute	Wraps the original object
Example	Lazy-loaded image	Logging wrapper around a service

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🧠 Summary

The Proxy Design Pattern is a powerful tool for managing access, optimizing performance, and extending behavior without modifying the original object. It’s a prime example of how thoughtful abstraction can lead to cleaner, more maintainable systems.