Proxy pattern

In computer programming, the proxy pattern is a software design pattern. A ''proxy'', in its most general form, is a class functioning as an interface to something else. The proxy could interface to anything: a network connection, a large object in memory, a file, or some other resource that is expensive or impossible to duplicate. In short, a proxy is a wrapper or agent object that is being called by the client to access the real serving object behind the scenes. Use of the proxy can simply be forwarding to the real object, or can provide additional logic. In the proxy, extra functionality can be provided, for example caching when operations on the real object are resource intensive, or checking preconditions before operations on the real object are invoked. For the client, usage of a proxy object is similar to using the real object, because both implement the same interface.

Overview

The Proxy

design pattern is one of the twenty-three well-known
'' GoF design patterns''
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.

What problems can the Proxy design pattern solve?

* The access to an object should be controlled.
* Additional functionality should be provided when accessing an object.

When accessing sensitive objects, for example, it should be possible to check that clients have the needed access rights.

What solution does the Proxy design pattern describe?

Define a separate Proxy object that
* can be used as substitute for another object (Subject) and
* implements additional functionality to control the access to this subject.

This makes it possible to work through a Proxy object to perform additional functionality when accessing a subject. For example, to check the access rights of clients accessing a sensitive object.

To act as substitute for a subject, a proxy must implement the Subject interface.
Clients can't tell whether they work with a subject or its proxy.

See also the UML class and sequence diagram below.

Structure

UML class and sequence diagram

In the above UML class diagram,
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()).

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.

Class diagram

Possible usage scenarios

Remote proxy

In distributed object communication, 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 remote method invocation on the remote object. An example would be an ATM implementation, where the ATM might hold proxy objects for bank information that exists in the remote server.

Virtual proxy

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.

Protection proxy

A protection proxy might be used to control access to a resource based on access rights.

Example

C#

interface ICar

// Real Object
public class Car : ICar

// Proxy Object
public class ProxyCar : ICar

public class Driver

// How to use above Proxy class?
private void btnProxy_Click(object sender, EventArgs e)

Output
Sorry, the driver is too young to drive.
Car has been driven!

Notes:
* A proxy may hide information about the real object to the client.
* A proxy may perform optimization like on demand loading.
* A proxy may do additional house-keeping job like audit tasks.
* Proxy design pattern is also known as surrogate design pattern.

C++

#include
#include

class ICar ;

class Car : public ICar ;

class ProxyCar : public ICar ;

int main()

Crystal

abstract class AbstractCar
abstract def drive
end

class Car < AbstractCar
def drive
puts "Car has been driven!"
end
end

class Driver
getter age : Int32

def initialize(@age)
end
end

class ProxyCar < AbstractCar
private getter driver : Driver
private getter real_car : AbstractCar

def initialize(@driver)
@real_car = Car.new
end

def drive
if driver.age <= 16
puts "Sorry, the driver is too young to drive."
else
@real_car.drive
end
end
end

# Program
driver = Driver.new(16)
car = ProxyCar.new(driver)
car.drive

driver = Driver.new(25)
car = ProxyCar.new(driver)
car.drive

Output
Sorry, the driver is too young to drive.
Car has been driven!

Delphi / Object Pascal

// Proxy Design pattern
unit DesignPattern.Proxy;

interface

type
// Car Interface
ICar = interface
procedure DriveCar;
end;

// TCar class, implementing ICar
TCar = Class(TInterfacedObject, ICar)
class function New: ICar;
procedure DriveCar;
End;

// Driver Interface
IDriver = interface
function Age: Integer;
end;

// TDriver Class, implementing IDriver
TDriver = Class(TInterfacedObject, IDriver)
private
FAge: Integer;
public
constructor Create(Age: Integer); Overload;
class function New(Age: Integer): IDriver;
function Age: Integer;
End;

// Proxy Object
TProxyCar = Class(TInterfacedObject, ICar)
private
FDriver: IDriver;
FRealCar: ICar;
public
constructor Create(Driver: IDriver); Overload;
class function New(Driver: IDriver): ICar;
procedure DriveCar;
End;

implementation

class function TCar.New: ICar;
begin
Result := Create;
end;

procedure TCar.DriveCar;
begin
WriteLn('Car has been driven!');
end;

constructor TDriver.Create(Age: Integer);
begin
inherited Create;
FAge := Age;
end;

class function TDriver.New(Age: Integer): IDriver;
begin
Result := Create(Age);
end;

function TDriver.Age: Integer;
begin
Result := FAge;
end;

constructor TProxyCar.Create(Driver: IDriver);
begin
inherited Create;
Self.FDriver := Driver;
Self.FRealCar := TCar.Create AS ICar;
end;

class function TProxyCar.New(Driver: IDriver): ICar;
begin
Result := Create(Driver);
end;

procedure TProxyCar.DriveCar;
begin
if (FDriver.Age <= 16)
then WriteLn('Sorry, the driver is too young to drive.')
else FRealCar.DriveCar();
end;

end.

Usage

program Project1;

uses
DesignPattern.Proxy in 'DesignPattern.Proxy.pas';
begin
TProxyCar.New(TDriver.New(16)).DriveCar;
TProxyCar.New(TDriver.New(25)).DriveCar;
end.

Output
Sorry, the driver is too young to drive.
Car has been driven!

Java

The following Java example illustrates the "virtual proxy" pattern. The ProxyImage class is used to access a remote method.

The example creates first an interface against which the pattern creates the classes. This interface contains only one method to display the image, called displayImage(), that has to be coded by all classes implementing it.

The proxy class ProxyImage is running on another system than the real image class itself and can represent the real image RealImage over there. The image information is accessed from the disk. Using the proxy pattern, the code of the ProxyImage avoids multiple loading of the image, accessing it from the other system in a memory-saving manner. The lazy loading demonstrated in this example is not part of the proxy pattern, but is merely an advantage made possible by the use of the proxy.

interface Image

// On System A
class RealImage implements Image

// On System B
class ProxyImage implements Image

class ProxyExample

Output
Loading HiRes_10MB_Photo1
Displaying HiRes_10MB_Photo1
Displaying HiRes_10MB_Photo1
Loading HiRes_10MB_Photo2
Displaying HiRes_10MB_Photo2
Displaying HiRes_10MB_Photo2
Displaying HiRes_10MB_Photo1

JavaScript

// Driver class
class Driver

// Car class
class Car

// Proxy car class
class ProxyCar

// Run program
const driver = new Driver(16)
const car = new ProxyCar(driver)
car.drive()

const driver2 = new Driver(25)
const car2 = new ProxyCar(driver2)
car2.drive()

Output
Sorry, the driver is too young to drive.
Car has been driven!

More advanced proxies involve the Proxy object, which can intercept and redefine fundamental operations such as accessing properties. The handler functions in this case are sometimes called ''traps''.

PHP

displayImage(); // Loading necessary
$image1->displayImage(); // Loading unnecessary
$image2->displayImage(); // Loading necessary
$image2->displayImage(); // Loading unnecessary
$image1->displayImage(); // Loading unnecessary

Output
Loading HiRes_10MB_Photo1
Displaying HiRes_10MB_Photo1
Displaying HiRes_10MB_Photo1
Loading HiRes_10MB_Photo2
Displaying HiRes_10MB_Photo2
Displaying HiRes_10MB_Photo2
Displaying HiRes_10MB_Photo1

Python

"""
Proxy pattern example.
"""
from abc import ABCMeta, abstractmethod

NOT_IMPLEMENTED = "You should implement this."

class AbstractCar:
__metaclass__ = ABCMeta

@abstractmethod
def drive(self):
raise NotImplementedError(NOT_IMPLEMENTED)

class Car(AbstractCar):
def drive(self) -> None:
print("Car has been driven!")

class Driver:
def __init__(self, age: int) -> None:
self.age = age

class ProxyCar(AbstractCar):
def __init__(self, driver) -> None:
self.car = Car()
self.driver = driver

def drive(self) -> None:
if self.driver.age <= 16:
print("Sorry, the driver is too young to drive.")
else:
self.car.drive()

driver = Driver(16)
car = ProxyCar(driver)
car.drive()

driver = Driver(25)
car = ProxyCar(driver)
car.drive()

Output
Sorry, the driver is too young to drive.
Car has been driven!

Rust

trait ICar

struct Car

impl ICar for Car

impl Car

struct ProxyCar<'a>

impl<'a> ICar for ProxyCar<'a>

impl<'a> ProxyCar<'a>

# fg(test)mod tests

Output
Sorry, the car is to young for you to drive.
Car has been driven!

See also

* Composite pattern
* Decorator pattern
* Lazy initialization

References

External links

*

PerfectJPattern Open Source Project
Provides componentized implementation of the Proxy Pattern in Java
*

Proxy Design Pattern
*

Proxy pattern description from the Portland Pattern Repository

{{Design Patterns Patterns

Software design patterns
Articles with example C Sharp code
Articles with example Java code