Scenario

Problem

• Don't want to change the functionality of the action()
• But you may want to change some of the non-functional features of it
• Adding additional features means changing the Subject
• Want to do so flexibly, i.e., opportunistically add additional features on how the functionality is delivered
• This may mean caching, protection, etc.

Proxy

Proxy Pattern

Proxy Pattern

• Changes the behavior of the service but preserves the interface
• Delegate action to another class
• Client uses the proxy just like it was the real thing (in general)

Adapter vs. Proxy

Examples

Examples

• Virtual Proxy Provide a placeholder for an expensive object and delay its instantiation until it is needed
• Remote Proxy Represent an object in a different address space, e.g., access a remote database through a remote proxy
• Protection Proxy Control access to the original object based on specific permissions or authentication, e.g., check user access rights before granting access to a secured system
• Cache Proxy Store the result of an expensive operation and returns cached results, e.g., cache frequently accessed web pages
• Firewall Proxy Protects a network from unauthorized access by filtering incoming and outgoing traffic, e.g., filter packets
• Smart Reference Proxy Adds extra behavior when an object is accessed, like reference counting, e.g., keep track of the number of times an object is accessed
• Logging Proxy Add logging to an object for debugging or auditing, e.g., log requests made to a web service
• Synchronization Proxy Provide thread-safe access to a shared resource, e.g., ensure only one thread can access a shared data structure at a time

Protection Proxy

Chains of Proxies