What Problem It Solves
You need to control access to an object. The proxy acts as a surrogate, doing something before/after delegating to the real object — lazy loading, access control, logging, caching, or remote communication.
Proxy vs Decorator
| Aspect | Proxy | Decorator |
|---|---|---|
| Purpose | Control access, manage lifecycle | Add behavior |
| Creates real object? | Usually (virtual proxy) | No (receives it) |
| Interface | Same as real subject | Same as wrapped component |
| Transparency | Client may or may not know it's a proxy | Client usually doesn't know |
Proxy Types
| Type | What It Does | LLD Use |
|---|---|---|
| Virtual Proxy | Lazy initialization — defers expensive object creation | Large images, DB connections |
| Protection Proxy | Controls access based on permissions | Auth/Access Control |
| Logging Proxy | Logs every call to the real object | Debugging, auditing |
| Caching Proxy | Caches results of expensive operations | Cache aside pattern |
| Remote Proxy | Represents an object in a different address space | RPC, gRPC stubs |
Implementation
// ─── EXAMPLE 1 ──────────────────────────────────────────────────────────────
// WHAT WE ARE IMPLEMENTING:
// A network video-loading gateway implementing an internal cache wrapper
// around a slow cloud storage SDK.
//
// WHERE THE PROXY FITS IN:
// VideoService is the Subject. RealVideoService is the Real Subject.
// ProxyVideoService is the Cache Proxy controlling access and caching
// results.
// ────────────────────────────────────────────────────────────────────────────
// --- Subject ---
interface Image {
void display();
}
// --- Real subject ---
class RealImage implements Image {
private final String filename;
public RealImage(String filename) {
this.filename = filename;
loadFromDisk(); // Expensive operation
}
private void loadFromDisk() {
System.out.println(" [RealImage] Loading " + filename + " from disk...");
try { Thread.sleep(500); } catch (InterruptedException ignored) {}
}
public void display() {
System.out.println(" [RealImage] Displaying " + filename);
}
}
// --- Proxy (virtual proxy — lazy load) ---
class ProxyImage implements Image {
private final String filename;
private RealImage realImage;
public ProxyImage(String filename) { this.filename = filename; }
public void display() {
if (realImage == null) {
realImage = new RealImage(filename); // Lazy initialization
}
realImage.display();
}
}
// --- Protection proxy ---
class ProtectedImageProxy implements Image {
private final ProxyImage proxyImage;
private final String userRole;
public ProtectedImageProxy(String filename, String userRole) {
this.proxyImage = new ProxyImage(filename);
this.userRole = userRole;
}
public void display() {
if (!userRole.equals("ADMIN")) {
System.out.println(" [Proxy] Access denied: " + userRole + " cannot view " + proxyImage);
return;
}
proxyImage.display();
}
}
public class Main {
public static void main(String[] args) {
System.out.println("--- Virtual Proxy (lazy load) ---");
Image img = new ProxyImage("photo.jpg");
System.out.println("Image created — no loading yet");
img.display(); // Loads only now
System.out.println("\n--- Protection Proxy ---");
Image protectedImg = new ProtectedImageProxy("confidential.png", "USER");
protectedImg.display(); // Denied
Image adminImg = new ProtectedImageProxy("confidential.png", "ADMIN");
adminImg.display(); // Allowed
}
}Review
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