CS 635: Advanced Object-Oriented Design & Programming

References

Pattern-Oriented Software: A System of Patterns, Buschman, Meunier, Rohnert, Sommerlad, Stal, 1996, pp 263-275

Advanced C++: Programming Styles and Idioms, James Coplien, 1992, pp 58-72

CS 635 Lecture notes, 1998, Doc 22 http://www.eli.sdsu.edu/courses/spring98/cs635/notes/bridge/br idge.html

Doc 24, Proxy Slide # 2

Proxy

Use common interface (or abstract class) for both the proxy and original object

Proxy contains a reference to original object, so proxy can forward requests to the original object

Doc 24, Proxy Slide # 3

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Doc 24, Proxy Slide # 4

Reasons for Object Proxies

The actual object is on a remote machine (remote address space)

Hide real details of accessing the object

Used in CORBA, Java RMI

public class HelloClient  {
     public static void main(String args[])  {
          try  {
               String server = getHelloHostAddress( args);
               Hello proxy = (Hello) Naming.lookup( server );
               String message = proxy.sayHello();
               System.out.println( message );
          } 
          catch ( Exception error) 
               {  error.printStackTrace(); }
     }
}

Doc 24, Proxy Slide # 5

Creates/accesses expensive objects on demand

You may wish to delay creating an expensive object until it is really accessed

It may be too expensive to keep entire state of the object in memory at one time

Provides different objects different level of access to original object

Multiple local clients can share results from expensive operations: remote accesses or long computations

Protect local clients from outside world

Doc 24, Proxy Slide # 6

Synchronize multiple accesses to real subject

public class Table {     
     public Object elementAt( int row, int column ){ blah     }

     public void setElementAt(Object element, int row, int column )
          { blah}
}
     
public class RowLockTable {
     Table realTable;
     Integer[] locks;
     
     public RowLockTable( Table toLock) {
          realTable = toLock;
          locks = new String[ toLock.numberOfRows() ];
          for (int row = 0; row< toLock.numberOfRows(); row++ )
               locks[row] = new Integer(row);
          }
     
     public Object elementAt( int row, int column ) {
          synchronized ( locks[row] ) {
               return realTable.elementAt( row, column);
          }
     }

     public void setElementAt(Object element, int row, int column ){
          synchronized ( locks[row] )     {
               return realTable.setElementAt(element, row, column);
          }
     }
}

Doc 24, Proxy Slide # 7

Delete original object when there are no references to it

Prevent accidental deletion of real subject

Collect usage statistics

Sample use is making C++ pointer safe

Doc 24, Proxy Slide # 8

Decorators add responsibility(ies) to an object

Proxies controls access an object

Chain can contain many objects

In the chain different objects can perform various requests

Proxy forwards (or not) requests to the original object

A facade represents a system of objects

A proxy represents a single object

A facade simplifies the interact between client and the system

A proxy controls the access to the single object

Doc 24, Proxy Slide # 9

• programming to an interface

• using indirection (composition )

These two elements are used to construct communicating objects and classes that solve a general design problem in a particular context

The structures of different patterns may be similar but the problem each pattern solves is different

Different pattern's problems may overlay at times

The goal is not to produce a disjoint taxonomy of structures but a list of common problems and a general structure for solving them

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