Proxy

Intent

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

-Design Patterns, GoF

A friend of mine -- a medical doctor, no less -- once told me that he convinced a friend to take a college exam for him. Someone who takes the place of someone else is known as a proxy. Unfortunately for my friend, his proxy drank a bit too much the night before and failed the test. – JavaWorld.com, Feb 2002

Also Known As

Surrogate

Questions

1) What are the motivations behind using the following types of proxies?

· Virtual Proxy

· Remote Proxy

· Protection Proxy

· Smart Reference

2) What is the difference between an Adapter and a Proxy?

3) If considering a virtual proxy, what would you need to sway your decision in favor of its use? In other words, what data or requirements artifacts would be necessary before the additional complexity of a Proxy was incurred?

4) When are the real objects (behind the Proxy) created?

5) Does the client ever reference the real thing? i.e. why not use just basic delegation?

6) Could you use basic inheritance and accomplish the same thing (i.e. the Proxy extends the real thing)? Why or why would you not want to do that?

7) Should you create a Proxy if there are many methods in the interface? Or if the interface is not stable?

Examples – Which are Proxies?

1) Enterprise Java Beans

2) JAX-RPC (Java API for XML-based remote procedure calls)

3) Swing’s ImageIcon and ImageIconProxy.

4) java.lang.reflect.Proxy.

-What type of proxy is this?

-Why would you want to use?

-Should it’s conceptual weight discourage use?

References

· Dynamic Proxy for Local and Remote Interface (EJB 2.0), Kjetile Helle (TheServerSide.com, August 2001)

· Take Control with the Proxy Design Pattern, David Geary (JavaWorld.com, February 2002)