proxy

Proxy Pattern

Intent

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

Applicability

Proxy is applicable whenever there is a need for a more versatile or sophisticated reference to an object than a simple pointer. Here are several common situations in which the Proxy pattern is applicable:

1. A remote proxy provides a local representative for an object in a different address space.
2. A virtual proxy creates expensive objects on demand.
3. A protection proxy controls access to the original object. Protection proxies are useful when objects should have different access rights.
4. A smart reference is a replacement for a bare pointer that performs additional actions when an object is accessed. Typical uses include
   • counting the number of references to the real object so that it can be freed automatically when there are no more references.
   • loading a persistent object into memory when it's first referenced.
   • checking that the real object is locked before it's accessed to ensure that no other object can change it.

Structure

Participants

• Proxy
  • maintains a reference that lets the proxy access the real subject. Proxy may refer to a Subject if the RealSubject and Subject interfaces are the same.
  • provides an interface identical to Subject's so that a proxy can by substituted for the real subject.
  • controls access to the real subject and may be responsible for creating and deleting it.
  • other responsibilities depend on the kind of proxy:
    • remote proxies are responsible for encoding a request and its arguments and for sending the encoded request to the real subject in a different address space.
    • virtual proxies may cache additional information about the real subject so that they can postpone accessing it. For example, the ImageProxy from the Motivation caches the real image's extent.
    • protection proxies check that the caller has the access permissions required to perform a request.
• Subject
  • defines the common interface for RealSubject and Proxy so that a Proxy can be used anywhere a RealSubject is expected.
• RealSubject
  • defines the real object that the proxy represents.

Example

There is a computer running linux. We can SSH to the computer and feel like we are operating the real computer. SSH is the proxy.

Participants in this example:

• Computer is the Subject.
• LinuxComputer is the RealSubject.
• SecurityShell is the Proxy.

Scala Tips

None

Reference

• Design Patterns: Elements of Reusable Object-Oriented Software