Java Reference
In-Depth Information
Proxy
Also known as Surrogate
Pattern Properties
Type: Structural
Level: Component
Purpose
To provide a representative of another object, for reasons such as access, speed, or security.
Introduction
Your career is taking off and your social life is humming, so your Personal Information Manager has to manage
many appointments and dates. Your address book contains all the addresses of all the people you've ever
socialized with or had professional contact with, including information on their families, hobbies, and other
potentially valuable data. The number of contacts started out small but is now in the thousands.
However, you often pull out your PIM just to change a meeting time, make a note to buy beer, or something
equally simple. Being presented with the whole address book object every time you use the PIM would be
unnecessary and annoying. Just opening the topic is a very expensive operation, unnecessarily delaying activities
that don't require its use.
As a user, you don't care about any of that—you just want the address book to be available when you need to use
it. (Ideally, the address book should be there even before you know you need to use it.) And when you use it, you
don't always need all of it. For example, you just want to know how many contacts you have in your address
book, or you want to add a new contact to your addressbook, without seeing and being able to edit the whole
thing. You just need a small part of the address book.
The solution is a placeholder object that provides the an interface to the address book, or a part of it. The
placeholder looks like the address book, but doesn't involve the overhead of running it. However, when you do
need the whole address book to perform a task like updating a colleague's address, the placeholder object creates
the real address book, to perform address book tasks assigned to it. That placeholder object is a Proxy.
Applicability
Use the Proxy pattern when you need a more elaborate reference to an object instead of just a regular one:
Remote proxy - When you need a local representative for an object in another address space (JVM).
Virtual proxy - Acts as a placeholder and delays creating expensive objects. (This is the version described in the
Introduction section.)
Protection proxy - Determines access rights to the real object.
Description
A proxy (or
stub
) is a representative for another object. To enable the proxy to represent the real object, the proxy
has to implement the exact same interface as the real object. Furthermore, the proxy keeps a reference to the real
object. The proxy needs the reference to the real object so that it can call methods on the real object if necessary.
The clients will be interacting with the proxy, but the proxy can delegate the execution to the real object. The
proxy implements the same interface as the real object, but can perform tasks that the real object does not, such as
remote communication or security.
The proxy is a sort of stand-in for the real object. You can compare the Proxy pattern to the system of filming
dangerous stunts for movies. The proxy is the body double for the real object, the movie star. During the
dangerous stunts the proxy jumps out of the plane instead of the real object. Because the proxy implements the
same interface as the real object, audiences cannot tell the difference, and think that real object jumped. But when
the camera switches to close-up (when the real object is needed for a full complement of movie star tasks), the
proxy calls the real object to perform the acting work.
