Gang of four Proxy Design Pattern


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Gang of four Proxy Design Pattern

  1. 1. Gang of Four – Proxy Design Pattern November 12, 20120Posted by IdioTechie on November 10, 2012Descript ion:Proxy is another Structural design pattern which works ‘on behalf of’ or ‘in place of’another object in order to access the later. ie. com echWhen t o use t his pat t ern?Proxy pattern is used when we need to create a wrapper to cover the main object’s complexity ot idifrom the client.What are t he usage scenarios? // Virtual Proxy – Imagine t p: tsituation where there is multiple database call to extract huge size hexpensive operation we can possibly use the proxy pattern which a © image. Since this is an would create multiple proxies and point to the huge size memory consuming object for further processing. The real object gets created only when a client first requests/accesses the object and after that we can just refer to the proxy to reuse the object. This avoids duplication of the object and hence saving memory. Remote Proxy – A remote proxy can be thought about the stub in the RPC call. The remote proxy provides a local representation of the object which is present in the different address location. Another example can be providing interface for remote resources such as web service or REST resources. Protective Proxy – The protective proxy acts as an authorisation layer to verify if the actual user has access to appropriate content. An example can be thought about the proxy server which provides restrictive internet access in office. Only the websites and contents which are valid will be allowed and the remaining ones will be blocked. Smart Proxy – A smart proxy provides additional layer of security by interposing specific actions when the object is accessed. An example can be to check if the real object is locked before it is accessed to ensure that no other object can change it.St ruct ure:Part icipant s:
  2. 2. Part icipant s: Subject – This object defines the common interface for RealSubject and Proxy so that a Proxy can be used anywhere a RealSubject is expected. Proxy – It maintains a reference to the RealSubject so that Proxy can access it. It also implements the same interface as the RealSubject so that Proxy can be used in place of RealSubject. Proxy also controls the access to the RealSubject and can create or delete ie. com ech this object. iot RealSubject – This refers the main object //id which the proxy represents. Proxy Design Pattern StructureExample: ht tp: article. The first one will be virtual proxy pattern and the otherWe will discuss two examples in thisone for protection proxy pattern.Virtual Proxy Example:©As mentioned earlier virtual proxy is useful to save expensive memory resources. Let’s take ascenario where the real image contains a huge size data which clients needs to access. To saveour resources and memory the implementation will be as below: Create an interface which will be accessed by the client. All its methods will be implemented by the ProxyImage class and RealImage class. RealImage runs on the different system and contains the image information is accessed from the database. The ProxyImage which is running on a different system can represent the RealImage in the new system. Using the proxy we can avoid multiple loading of the image.Class Diagram:Code Example:Image.javapublic interface Image { public void showImage();} Virtual Proxy Example
  3. 3. public class RealImage implements Image { private String fileName = null; public RealImage(String strFileName){ this.fileName = strFileName; } @Override public void showImage() { System.out.println(" Show Image:" +fileName); } com} ot ech :// idipublic class ProxyImage implements Image { ttp private RealImage img= null; h private String fileName = null; public ProxyImage(String strFileName) { © this.fileName = strFileName; } /* * (non-Javadoc) * @see com.proxy.virtualproxy.Image#showImage() */ @Override public void showImage() { if(img == null){ img = new RealImage(fileName); } img.showImage(); }}Client.javapublic class Client {public static void main(String[] args) { final Image img1 = new ProxyImage(" Image***1" ); final Image img2 = new ProxyImage(" Image***2" ); img1.showImage(); img2.showImage(); }}Protection Proxy Example: Let’s assume that company ABC starts a new policy that employees will now be prohibited internet access based on their roles. All external emails websites will be blocked. In such
  4. 4. situation we create InternetAccess interface which consists of operation grantInternetAccess(). The RealInternetAccess class which allows of internet access for all. However to restrict this access we will use ProxyInternetAccess class which will check user’s role and grant access based on their roles.Class Diagram:Code Example:InternetAccess: ie. compublic interface InternetAccess { ot ech idi public void grantInternetAccess();} :// h ttp © Protection Proxy Examplepublic class RealInternetAccess implements InternetAccess { private String employeeName = null; public RealInternetAccess(String empName) { this.employeeName = empName; } @Override public void grantInternetAccess() { System.out.println(" Internet Access granted for employee: " + employeeName); }}ProxyInternetAccess.javapublic class RealInternetAccess implements InternetAccess { private String employeeName = null; public RealInternetAccess(String empName) { this.employeeName = empName; } @Override public void grantInternetAccess() { System.out.println(" Internet Access granted for employee: " + employeeName); }}
  5. 5. Client.javapublic static void main(String[] args) { InternetAccess ia = new ProxyInternetAccess(" Idiotechie" ); ia.grantInternetAccess(); }Benef it s: om cseen in the above example is about security. ch ie. One of the advantages of Proxy pattern as you have t ewhich might be huge size and memory intensive. id ioof the application. This pattern avoids duplication of objects This in turn increases the performance //about security by installing the local code proxy (stub) in the ht tp: the server with help of the remote code. The remote proxy also ensures client machine and then accessing ©Drawbacks/Consequences:This pattern introduces another layer of abstraction which sometimes may be an issue if theRealSubject code is accessed by some of the clients directly and some of them might access theProxy classes. This might cause disparate behaviour.Int erest ing point s: There are few differences between the related patterns. Like Adapter pattern gives a different interface to its subject, while Proxy patterns provides the same interface from the original object but the decorator provides an enhanced interface. Decorator pattern adds additional behaviour at runtime. Proxy used in Java API: java.rmi.*;Please don’t forget to leave your comments. In case you like this article please share this articlesfor your friends through the social networking links.Download Sample Code:Filed in: Core Java, Design Pattern, Java, Random Tags: design pattern, download, gang of,gang of four, GoF, idiotechie, Java, protection proxy, Proxy, proxy design pattern, remote proxy,smart proxy, virtual proxy