	CS 635 Advanced Object-Oriented Design & Programming Spring Semester, 2001 Proxy & Adapter
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Contents of Doc 16, Proxy & Adapter

References Design Patterns: Elements of Reusable Object-Oriented Software, Gamma, Helm, Johnson, Vlissides, Addison Wesley, 1995, pp. 139-150, 207-217

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

The Design Patterns Smalltalk Companion, Alpert, Brown, Woolf, Addison Wesley Longman, 1998 pp. 105-120, 213-222

Smalltalk Scaffolding Patterns, Doble & Auer in Pattern Languages of Program Design 4, Addison Wesley, Eds. Harrison, Foote, Rohnert, 2000, pp. 199-219


Doc 16, Proxy & Adapter Slide # 2

Proxy

proxy n. pl prox-ies The agency for a person who acts as a substitute for another person, authority to act for another

Structure

The Pattern
The proxy has the same interface as the original object

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 16, Proxy & Adapter Slide # 3

Dynamics

Runtime Objects


Doc 16, Proxy & Adapter Slide # 4

Sample Proxy

public class Proxy 
   {
   Foo target;
   public float bar(int size )
      {
      preprocess here
      float answer =  target.bar( size);
      postProcess here
      return answer;
      }
   
   other methods as needed
   }

Preprocessing & post-processing depend on purpose of the proxy


Doc 16, Proxy & Adapter Slide # 5

Reasons for Object Proxies

Remote Proxy 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 16, Proxy & Adapter Slide # 6

Reasons for Object Proxies Continued
Virtual Proxy

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

Protection Proxy

Provides different objects different level of access to original object

Cache Proxy (Server Proxy)

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

Firewall Proxy

Protect local clients from outside world


Doc 16, Proxy & Adapter Slide # 7

Synchronization Proxy

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 16, Proxy & Adapter Slide # 8

Counting Proxy
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 16, Proxy & Adapter Slide # 9

Smalltalk Proxy Tricks

When an object is sent a message

The object's class and the object's classes superclasses are searched for the method

If the method is not found the object is sent the message:

doesNotUnderstand:

This method in Object raises an exception


Doc 16, Proxy & Adapter Slide # 10

Prototyping of a Proxy
One can use doesNotUnderstand: to implement a pluggable proxy

Example

Object subclass: #Proxy
   instanceVariableNames: 'target '
   classVariableNames: ''
   poolDictionaries: ''
   category: 'Whitney-Examples'

Class Method

on: anObject
   ^super new target: anObject

Instance Methods

doesNotUnderstand: aMessage 
   ^target
      perform: aMessage selector 
      withArguments: aMessage arguments

target: anObject
   target := anObject


Doc 16, Proxy & Adapter Slide # 11

Examples of Using the Proxy

| wrapper |
wrapper := Proxy on: Transcript.
wrapper open.
wrapper show: 'Hi mom'.

| wrapper |
wrapper := Proxy on: 3.
wrapper + 5.

| wrapper |
wrapper := Proxy on: 'Hi '.
wrapper , ' mom'.


Doc 16, Proxy & Adapter Slide # 12

Why just for Prototyping
doesNotUnderstand:

Can be hard to debug
Is slower than regular message send


Doc 16, Proxy & Adapter Slide # 13

Adapter

Motivating Adapter

Java CGI & Servlets
Both Java CGI and servlets are used for server-side processing of certain HTML requests, like processing HTML forms

Servlets have greater functionality and are faster, but require special Web servers or servers with special extensions

To help write Java CGI programs there is class sdsu.net.CGI

It would be useful in moving code between servers to avoid having to rewrite the code


Doc 16, Proxy & Adapter Slide # 14

One Problem
One issue is access to the CGI environment variables

There are about 20 common CGI environment variables

In servlets one has a HttpRequest class that has a getX() method for each CGI environment variable

sdsu.net.CGI class returns a hash table with one entry per CGI environment variable


Doc 16, Proxy & Adapter Slide # 15

Solution
We can write a wrapper around HttpRequest to make it act like a hash table

The Wrapper or Adapter

class CGIAdapter extends Hashtable
   {
   Hashtable CGIvariables = new Hashtable( 20);
   
   public CGIAdapter( HttpRequest CGIEnvironment )
      {
      CGIvariables.put( "AUTH_TYPE" , 
                     CGIEnvironment.getAuthType());
                     
      CGIvariables.put( "REMOTE_USER" , 
                     CGIEnvironment.getRemoteUser());
      
      etc.
      }
   
   public Object get(Object key)
      {
      return CGIvariables.get( key );
      }
      
      etc. 
   }


Doc 16, Proxy & Adapter Slide # 16

Going the other Direction
Adapting servlet code to normal CGI requires extracting the CGI environment variables out of the hash table and putting them into an object that implements the public interface of the HttpRequest class

class HTTPAdapter extends HttpRequest
   {
   Hashtable CGIvariables;
   
   public HTTPAdapter( Hashtable CGIEnvironment )
      {
      CGIvariables = CGIEnvironment;
      }
   
   public String getAuthType()
      {
      return (String) CGIvariables.get( "AUTH_TYPE" );
      }
      
   public String getRemoteUser()
      {
      return (String) CGIvariables.get( "REMOTE_USER" );
      }
      
      etc. 
   }


Doc 16, Proxy & Adapter Slide # 17

Adapter

The adapter pattern converts the interface of a class into another interface.

Use the Adapter pattern when

You want to use an existing class and its interface does not match the one you need

You want to create a reusable class that cooperates with unrelated or unforeseen classes, that is classes that don’t necessarily have compatible interfaces

You need to use several existing subclasses, but it’s impractical to adapt their interface by subclassing everyone. An object adapter can adapt the interface of its parent class

Adapter has two forms:

Class Adapter
Object Adapter


Doc 16, Proxy & Adapter Slide # 18

Class Adapter

Object Adapter


Doc 16, Proxy & Adapter Slide # 19

Class Adapter Example

class OldSquarePeg {
   public:
      void squarePegOperation()
         { do something }
}
   
class RoundPeg {
   public:
      void virtual roundPegOperation = 0;
}
   
class PegAdapter: private OldSquarePeg, 
            public RoundPeg  {
   public:
      void virtual roundPegOperation()  {
         add some corners;
         squarePegOperation();
      }
}
   
void clientMethod() {
   RoundPeg* aPeg = new PegAdapter();
   aPeg->roundPegOperation();
}


Doc 16, Proxy & Adapter Slide # 20

Object Adapter Example

class OldSquarePeg   {
   public:
      void squarePegOperation() { do something }
   }
   
class RoundPeg   {
   public:
      void virtual roundPegOperation = 0;
   }
   
class PegAdapter:  public RoundPeg   {
   private:
      OldSquarePeg* square;
      
   public:
      PegAdapter()  { square = new OldSquarePeg; }
         
      void virtual roundPegOperation()   {
         add some corners;
         square->squarePegOperation();
         }
   }


Doc 16, Proxy & Adapter Slide # 21

Consequences

A Class adapter uses inheritance so

Only adapts a class and all its parents, not all its subclasses

Lets Adapter override some of Adaptee’s behavior

Does not introduce an additional pointer indirection

An object adapter uses object composition so

Lets a single Adapter work with many Adaptees

Makes it harder to override Adaptee behavior as the Adapter may not know with Adaptee it is working with

Other issues:

How much adapting does the Adapter do?

Pluggable adapters

Using two-way adapters


Doc 16, Proxy & Adapter Slide # 22

How Much Adapting does the Adapter do?
The adapter may have to work very little or a great deal to adapt the Adaptee to the Target

The Adapter may just map one operation to another

class PegAdapter:  public RoundPeg   {
   private:
      OldSquarePeg* square;
      
   public:
      PegAdapter()   { square = new OldSquarePeg;}
         
      void roundPegOperation()   {
         square->squarePegOperation();
      }
}

The Adapter may have to work hard if the Target operation does not have a comparable operation in the Adaptee


Doc 16, Proxy & Adapter Slide # 23

Pluggable Adapters
In the CGI example we adapted a class with getX() methods to a hash table interface

It is likely that we may adapt a class with getX() methods to a hashtable in the future

It would be nice to write one class to do all such adapting

This class would be given a list of keys to getX methods and an Adaptee object

HttpRequest CGIEnvironment = getHttpRequest();
PluggableHashAdapter sample = 
   new PluggableHashAdapter( CGIEnvironment );
   
sample.adapt( "AUTH_TYPE" , getAuthType );
sample.adapt( "REMOTE_USER" , getRemoteUser );
etc.
   
sample.get( “REMOTE_USER” );

Pluggable Adapters are used in interface components, where we know in advance that we will adapt the component to other interfaces

Pluggable Adapters are common in Smalltalk, were it is easier to map strings to method calls


Doc 16, Proxy & Adapter Slide # 24

Using two-way Adapter
In the SquarePeg-RoundPeg example the SquarePeg is adapted to the RoundPeg

So a SquarePeg can be used where a RoundPeg is needed, but not the other way around.

A two-way adapter would also allow a RoundPeg be used in place of the SquarePeg

class OldSquarePeg {
   public:
      void virtual squarePegOperation() { blah }
}
   
class RoundPeg {
   public:
      void virtual roundPegOperation() { blah }
}
   
class PegAdapter: public OldSquarePeg, RoundPeg {
   public:
      void virtual roundPegOperation() {
         add some corners;
         squarePegOperation();
      }
      void virtual squarePegOperation() {
         add some corners;
         roundPegOperation();
      }
}
Doc 16, Proxy & Adapter Slide # 25

Adapter, Proxy & State

Adapter, Proxy & State forward requests to an instance object

What is the difference?

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2001 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA.
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