Proxy Design Pattern

According to GoF definition of proxy design pattern, a proxy object provide a surrogate or placeholder for another object to control access to it. A proxy is basically a substitute for an intended object which we create due to many reasons e.g. security reasons or cost associated with creating fully initialized original object.

1. When to use proxy design pattern

A proxy object hides the original object and control access to it. We can use proxy when we may want to use a class that can perform as an interface to something else.

Proxy is heavily used to implement lazy loading related usecases where we do not want to create full object until it is actually needed.

A proxy can be used to add an additional security layer around the original object as well.

2. Real world example of proxy pattern

• In hibernate, we write the code to fetch entities from the database. Hibernate returns an object which a proxy (by dynamically constructed by Hibernate by extending the domain class) to the underlying entity class. The client code is able to read the data whatever it needs to read with the proxy.

  These proxy entity classes help in implementing lazy loading scenarios where associated entities are fetched only when they are requested explicitly. It helps in improving performance of DAO operations.

• In corporate networks, internet access is guarded behind a network proxy. All network requests goes through proxy which first check the requests for allowed websites and posted data to network. If request looks suspicious, proxy block the request – else request pass through.
• In aspect oriented programming (AOP), an object created by the AOP framework in order to implement the aspect contracts (advise method executions and so on). For example, in the Spring AOP, an AOP proxy will be a JDK dynamic proxy or a CGLIB proxy.

3. Proxy design pattern

3.1. Architecture

3.2. Design participants

• Subject – is an interface which expose the functionality available to be used by the clients.
• Real Subject – is a class implementing Subject and it is concrete implementation which needs to be hidden behind a proxy.
• Proxy – hides the real object by extending it and clients communicate to real object via this proxy object. Usually frameworks create this proxy object when client request for real object.

4. Proxy design pattern example

In given example, we have a RealObject which client need to access to do something. It will ask the framework to provide an instance of RealObject. But as the access to this object needs to be guarded, framework returns the reference to RealObjectProxy.

Any call to proxy object is used for additional requirements and the call is passed to real object.

public interface RealObject 
{
	public void doSomething();
}

public class RealObjectImpl implements RealObject {

	@Override
	public void doSomething() {
		System.out.println("Performing work in real object");
	}

}

public class RealObjectProxy extends RealObjectImpl 
{
	@Override
	public void doSomething() 
	{
		//Perform additional logic and security
		//Even we can block the operation execution
		System.out.println("Delegating work on real object");
		super.doSomething();
	}
}

public class Client 
{
	public static void main(String[] args) 
	{
		RealObject proxy = new RealObjectProxy();
		proxy.doSomething();
	}
}

Program output.

Delegating work on real object
Performing work in real object

5. FAQs

5.1. what are different types of proxies

Proxies are generally divided into four types –

1. Remote proxy – represent a remotely lactated object. To talk with remote objects, the client need to do additional work on communication over network. A proxy object does this communication on behalf of original object and client focuses on real talk to do.
2. Virtual proxy – delay the creation and initialization of expensive objects until needed, where the objects are created on demand. Hibernate created proxy entities are example of virtual proxies.
3. Protection proxy – help to implement security over original object. They may check for access rights before method invocations and allow or deny access based on the conclusion.
4. Smart Proxy – performs additional housekeeping work when an object is accessed by a client. 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.

5.2. Proxy pattern vs decorator pattern

The primary difference between both patterns are responsibilities they bear. Decorators focus on adding responsibilities, but proxies focus on controlling the access to an object.

Drop me your questions related to proxy pattern in comments.

Happy Learning !!

References:

Image Credit – Wikipedia