Proxy pattern

<h2 id="overview">Overview</h2>
<p>The Proxy 
<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a>
design pattern is one of the twenty-three well-known 
<i><a href="/facts/Design_Patterns/sYDxSGc9">GoF design patterns</a></i> 
that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.
</p>
<h3>What problems can the Proxy design pattern solve? <a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a></h3>
<ul><li>The access to an object should be controlled.</li>
<li>Additional functionality should be provided when accessing an object.</li></ul>
<p>When accessing sensitive objects, for example, it should be possible to check that clients have the needed access rights.
</p>
<h3>What solution does the Proxy design pattern describe?</h3>
<p>Define a separate Proxy object that 
</p>
<ul><li>can be used as a substitute for another object (Subject), and</li>
<li>implements additional functionality to control the access to this subject.</li></ul>
<p>This makes it possible to work through a Proxy object to perform additional functionality when accessing a subject, like checking the access rights of clients accessing a sensitive object.
</p><p>To act as a substitute for a subject, a proxy must implement the Subject interface. Clients can't tell whether they work with a subject or its proxy.
</p><p>See also the UML class and sequence diagram below.
</p>
<h2 id="structure">Structure</h2>
<h3>UML class and sequence diagram</h3>

<p>In the above <a href="/facts/Unified_Modeling_Language/3tlNS0tH">UML</a> <a href="/facts/Class_diagram/DNnkq7xh">class diagram</a>, 
the Proxy class implements the Subject interface so that it can act as substitute for Subject objects. It maintains a reference (realSubject) 
to the substituted object (RealSubject) so that it can forward requests to it
(realSubject.operation()). 
</p><p>The sequence diagram 
shows the run-time interactions: The Client object 
works through a Proxy object that
controls the access to a RealSubject object.
In this example, the Proxy forwards the request to the RealSubject, which performs the request.
</p>
<h3>Class diagram</h3>

<h2 id="possible-usage-scenarios">Possible usage scenarios</h2>
<h3>Remote proxy</h3>
<p>In <a href="/facts/Distributed_object_communication/boz8jDHe">distributed object communication</a>, a local object represents a remote object (one that belongs to a different address space). The local object is a proxy for the remote object, and method invocation on the local object results in <a href="/facts/Remote_method_invocation/boz8jDHe">remote method invocation</a> on the remote object. An example would be an <a href="/facts/Automated_teller_machine/8y5eVFUr">ATM</a> implementation, where the ATM might hold proxy objects for bank information that exists in the remote server.
</p>
<h3>Virtual proxy</h3>
<p class="note">Further information: <a href="/facts/Lazy_loading/BpH8qyWJ">Lazy loading</a></p>
<p>In place of a complex or heavy object, a skeleton representation may be advantageous in some cases. When an underlying image is huge in size, it may be represented using a virtual proxy object, loading the real object on demand.
</p>
<h3>Protection proxy</h3>
<p>A protection proxy might be used to control access to a resource based on access rights.
</p>
<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Composite_pattern/Q2GvqrD0">Composite pattern</a></li>
<li><a href="/facts/Decorator_pattern/Pzove9TP">Decorator pattern</a></li>
<li><a href="/facts/Lazy_initialization/Xs827EU7">Lazy initialization</a></li></ul>

<h2 id="external-links">External links</h2>

The Wikibook <i>Computer Science Design Patterns</i> has a page on the topic of: <i>Proxy implementations in various languages</i>

Wikimedia Commons has media related to Proxy pattern.

<ul><li>Geary, David (February 22, 2002). <a href="https://www.infoworld.com/article/2074068/take-control-with-the-proxy-design-pattern.html">"Take control with the Proxy design pattern"</a>. <i><a href="/facts/JavaWorld/EJ0zA8FV">JavaWorld</a></i>. Retrieved 2020-07-20.</li>
<li><a href="http://perfectjpattern.sourceforge.net/dp-proxy.html">PerfectJPattern Open Source Project</a>, Provides componentized implementation of the Proxy Pattern in Java</li>
<li><a href="https://web.archive.org/web/20120311202925/http://www.netobjectives.com/PatternRepository/index.php?title=AdapterVersusProxyVersusFacadePatternComparison">Adapter vs. Proxy vs. Facade Pattern Comparison</a> at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a> (archived 2012-03-11)</li>
<li><a href="https://sourcemaking.com/design_patterns/proxy">Proxy Design Pattern</a></li>
<li><a href="https://web.archive.org/web/20141019100543/http://patterns.org.pl/proxy.html">Proxy pattern C++ implementation example</a> at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a> (archived 2014-10-19)</li>
<li><a href="https://wiki.c2.com/?ProxyPattern">Proxy pattern description from the Portland Pattern Repository</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides (1994). Design Patterns: Elements of Reusable Object-Oriented Software. Addison Wesley. pp. 207ff. ISBN 0-201-63361-2.{{cite book}}:  CS1 maint: multiple names: authors list (link) <a href="0-201-63361-2" target="_blank">0-201-63361-2</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>"The Proxy design pattern - Problem, Solution, and Applicability". w3sDesign.com. Retrieved 2017-08-12. <a href="http://w3sdesign.com/?gr=s07&ugr=proble" target="_blank">http://w3sdesign.com/?gr=s07&ugr=proble</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
</ol>

Proxy pattern open-in-new

Proxy pattern