Tutorials – Design Patterns

The Proxy Pattern is a structural design pattern that provides a surrogate or placeholder for another object to control access to it. It is used to add an extra layer of control and indirection to objects, making it useful for a variety of scenarios, including remote objects, lazy initialization, access control, and more. In this chapter, we will explore the Proxy Pattern, understand its intent, structure, implementation, and real-world use cases.

Introduction to the Proxy Pattern

The Proxy Pattern is one of the Gang of Four (GoF) design patterns, and it falls under the category of structural patterns. It involves creating a proxy object that acts as a substitute or placeholder for another object to control access to it.

Intent: The intent of the Proxy Pattern is to provide a surrogate or placeholder for another object to control access to it. It can be used to add an extra layer of control, such as lazy loading, access control, or monitoring.

Structure of the Proxy Pattern

The Proxy Pattern consists of the following key components:

1. Subject: This is an interface or an abstract class that defines the common interface for the RealSubject and Proxy to ensure they are interchangeable.
2. RealSubject: This is the real object that the Proxy represents. It implements the Subject interface and contains the actual business logic.
3. Proxy: The Proxy is an object that provides the same interface as the RealSubject. It maintains a reference to the RealSubject and controls access to it. The Proxy can also add additional behavior or control, such as lazy loading, access control, or caching.

Implementation of the Proxy Pattern

To implement the Proxy Pattern, follow these steps:

1. Define the common interface (Subject) that both the RealSubject and Proxy implement.
2. Create a RealSubject class that implements the Subject interface and contains the actual business logic.
3. Create a Proxy class that also implements the Subject interface. The Proxy should maintain a reference to the RealSubject.
4. In the Proxy class, implement methods that delegate calls to the RealSubject. The Proxy can add additional behavior or control, as needed.
5. Clients interact with the Proxy object, which controls access to the RealSubject.

Use Cases of the Proxy Pattern

The Proxy Pattern is applicable in various scenarios, including:

1. Lazy Loading: In scenarios where loading an object is expensive, a proxy can be used to delay the loading until it is actually needed. This is often used in database access and image loading.
2. Access Control: Proxies can enforce access control rules, allowing or denying access to the RealSubject based on certain conditions, such as user permissions.
3. Remote Proxies: When dealing with remote objects, a proxy can represent the remote object and handle communication between the client and the remote server.
4. Caching: Proxies can cache the results of expensive operations, such as database queries or network requests, to improve performance.
5. Monitoring and Logging: Proxies can log and monitor calls to the RealSubject, providing insights into the object’s usage.
6. Virtual Proxies: Virtual proxies provide placeholders for objects that are expensive to create. They create the real object only when it is needed.

Real-World Examples of the Proxy Pattern

Let’s explore a few real-world examples to understand how the Proxy Pattern is used:

Example 1: Virtual Proxy for Images

In a graphics application, loading high-resolution images can be time-consuming. A virtual proxy can be used to create a placeholder for the image and load it only when it needs to be displayed.

Example 2: Access Control Proxy

In a document management system, access to sensitive documents can be controlled using a proxy. The proxy checks the user’s permissions before allowing access to the document.

Benefits of the Proxy Pattern

The Proxy Pattern offers several advantages:

1. Control: It provides control over access to the RealSubject, allowing for additional behavior, such as lazy loading, access control, or caching.
2. Security: Proxies can enhance security by enforcing access control rules.
3. Efficiency: Lazy loading and caching can improve efficiency by loading objects only when needed and reusing results.
4. Simplified Interfaces: Proxies provide a consistent interface to clients, even when dealing with complex or remote objects.

Drawbacks of the Proxy Pattern

While the Proxy Pattern is a valuable design pattern, it also has limitations:

1. Complexity: Adding proxy classes can introduce complexity to the system, which might not be necessary for simple cases.
2. Performance Overhead: In some scenarios, proxies can introduce a performance overhead, especially when additional checks or logging are involved.
3. Tight Coupling: Clients may become tightly coupled to proxy classes, making it harder to switch between different proxy implementations.

Conclusion

The Proxy Pattern is a versatile design pattern that allows for controlled and indirect access to objects. It is a valuable tool for scenarios where access control, lazy loading, caching, or monitoring is required. Understanding how and when to apply the Proxy Pattern is essential for designing flexible and efficient software systems.