A proxy is an object that controls access to another object, called a subject. The proxy and the subject have an identical interface and this allows us to transparently swap one for the other; in fact, the alternative name for this pattern is surrogate. A proxy intercepts all or some of the operations that are meant to be executed on the subject, augmenting or complementing their behavior. The following figure shows a diagrammatic representation:
The preceding figure shows us how the Proxy and the Subject have the same interface, and how this is totally transparent to the client, who can use one or the other interchangeably. The Proxy forwards each operation to the subject, enhancing its behavior with additional pre-processing or post-processing.
Note
It's important to observe that we are not talking about proxying between classes; the proxy pattern involves wrapping actual instances of the subject, thus preserving its state.
A proxy is useful in several circumstances; for example, consider...