Proxy Pattern - Object Structural Pattern

Intent

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

Structure

Participants:

• Proxy: Maintains a reference that lets the proxy access the real subject. It provides an interface identical to Subject’s so that a proxy can be substituted for the real subject. It controls access to the real subject and may be responsible for creating and deleting it.
• Subject: Defines the common interface for RealSubject and Proxy so that a Proxy can be used anywhere a RealSubject is expected.
• RealSubject: Defines the real object that the proxy represents.

Applicability

The Proxy pattern is useful when you need a more versatile or sophisticated reference to an object than a simple pointer. Common scenarios include:

• Remote Proxy: Provides a local representative for an object in a different address space.
• Virtual Proxy: Creates expensive objects on demand.
• Protection Proxy: Controls access to the original object. Useful when objects should have different access rights.
• Smart Reference: A replacement for a bare pointer that performs additional actions when an object is accessed. Typical uses include:
  • Reference counting for an object so it can be automatically freed when there are no more references.
  • Loading a persistent object into memory when it’s first referenced.
  • Checking that the real object is locked before it’s accessed to ensure no other object can change it.

Example

An agent proxy for buying items.

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public class ProxyPattern {
    public static void main(String[] args) {
        RealSubject realSubject = new RealSubject();
        Proxy proxy = new Proxy(realSubject);

        proxy.buy();
    }
}

interface Subject{
    public void buy();
}

class Proxy implements Subject{
    protected RealSubject realSubject;

    public Proxy(RealSubject realSubject){
        this.realSubject = realSubject;
    }

    @Override
    public void buy() {
        System.out.println("prepare");
        realSubject.buy();
        System.out.println("complete");
    }
}

class RealSubject implements Subject{

    @Override
    public void buy() {
        System.out.println("Money");
    }
}