Thursday, September 26

Proxy Design Pattern in Java, lazy loading using Proxy Design Pattern

                         Proxy Design Pattern in Java, lazy loading using Proxy Design Pattern

What is Proxy Design Pattern 

1. Proxy design patten works on the principal of exposing an Java Instance through a proxy instead of actual object. 

2. Client would never know anything about actual object and through Proxy only relevant behavior of actual object will be exposed to client.

3. Proxy Pattern can be used for applying security on actual Object. Service provider does not want actual class to be visible to any client Instead It would be shared as per Client contract agreement . Service provider may agree to share only a part of Service with it's client and for that It may expose a different contract in the form of interface in java . 





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4. This concept is very useful for lazily loading an instance . Data will be loaded only when it is actually required in an operation . Till then only proxy to Real instance will be kept as reference So as to invoke desired method later and retrieve the actual instance. 

I have tried to cover both these things directly through Java Code. 

Which class is doing what is directly written in System.out.println statement in each Class below . Executing the code will print all description on console.


Example 1.

This example covers How we can secure premium part of our Service from public cliewnts

public class ProxyClient {

    /**
     * @param args
     */
    public static void main(String[] args) {
        ServiceProxy proxy = new ServiceProxy();
        System.out.println("Clients invokes the operation on ServiceProxy instance....");
        System.out
                .println("Here Client is invoking operation on Proxy Service only. It knows nothing about actual service. Thus using proxy of actual object");
        proxy.operate();
    }

}

class RealService implements IService {

    RealService() {

        System.out
                .println("Service Provider never wish to share the instance of this class to any client publicly as it contains some premium methods as well which are just for premium clients. So it wants to restrict the access of premium methods publicly");
    }

    public void operate() {
        System.out.println("this is the real operation where actual logic is executed......");
        System.out
                .println("This is a premium method . It should not be shared publicly. It is not exposed  through IService interface");
    }

    public void execute() {
        System.out
                .println("This is a premium method . It should not be shared publicly. It is not exposed  through IService interface");

    }

    public void serve() {
        System.out
                .println("This is a premium method . It should not be shared publicly. It is not exposed  through IService interface");

    }

}

// This is the contract which can be publicly shared to all the clients to access methods available for all public
interface IService {

    public void operate();
}

class ServiceProxy implements IService {

    ServiceProxy() {

        System.out
                .println("Service provider does not want to expose actual instance of RealService publicly to everyone. It will share that only with premium clients.So an proxy is created . This Proxy implement the all methods of interface that actual service implements. Now this proxy is available to all the client to invoke public method without even knowing about the actual instance");
    }

    public void operate() {

        System.out.println("Here proxyService is delegating the call to Actual service method....");
        System.out
                .println("Here call is delegated to actual service and actual logic is executed in actual service method only . Result returned from that is received in proxy instance and is sent back to client.This Client remains totally agnostic of Actual service and Just deal with Proxy Service to invoke operations available in real service.");
        IService service = new RealService();
        service.operate();
    }

}



This example will print on console :




---------------------------
 It is Just proxy of actual object. So on instantiation is created just empty object. Data will be lazily loaded on invocation of operate() method
Clients invokes the operation on ServiceProxy instance....
Here Client is invoking operation on Proxy Service only. It knows nothing about actual service. Thus using proxy of actual object
Here proxyService is delegating the call to Actual service method....
Here call is delegated to actual service and actual logic is executed in actual service method only . Result returned from that is received in proxy instance and is sent back to client.This Client remains totally agnostic of Actual service and Just deal with Proxy Service to invoke operations available in real service.
Actual Instance is created here. It execute database calls and provide a state to the Object

--------------------------- 


Example 2

This example covers how lazy loading works and implemented using Proxy Design Pattern 


package pkg;

public class Client {
    IService proxy;

    Client() {
        System.out
                .println("Here When instance is created it only associates the proxy with it. Proxy instance is created. But Actual Service is not constructed ");
        proxy = new ServiceProxy();
    }

    /**
     * @param args
     */
    public void invoker() {

        System.out
                .println("On invocation of operate() method on proxy real instance is created. So instance of stateful object is created only when it is actually required. Till that time client only keeps the proxy to actual object. This is the concept of lazy loading which is heavily used in lot of J2ee frameworks like Spring");
        proxy.operate();
    }

    public static void main(String args[]) {
        Client client = new Client();
        client.invoker();
    }

}

class RealService implements IService {
    DBObject object;

    RealService() {
        System.out
                .println("Actual Instance is created here. It execute database calls and provide a state to the Object");
        object = new DBObject();

        // JDBC Code is executed to provide a state to object.

    }

    public void operate() {
        // TODO Auto-generated method stub
        object.decorateObject();
    }

}

// This interface has no method but is very useful in implementing Lazy loading concept
interface IService {

    void operate();

}

class ServiceProxy implements IService {

    ServiceProxy() {

        System.out
                .println("It is Just proxy of actual object. So on instantiation is created just empty object. Data will be lazily loaded on invocation of operate() method");
    }

    public void operate() {

        System.out.println("Here proxyService is delegating the call to Actual service method....");
        System.out
                .println("Here call is delegated to actual service and actual logic is executed in actual service method only . Result returned from that is received in proxy instance and is sent back to client.This Client remains totally agnostic of Actual service and Just deal with Proxy Service to invoke operations available in real service.");
        IService service = new RealService();

    }

}

class DBObject {

    public void decorateObject() {

        System.out.println("data is fetched from DB to apply other attributes to the object");
    }

}

This example will print on console :

---------------------------
Here When instance is created it only associates the proxy with it. Proxy instance is created. But Actual Service is not constructed
It is Just proxy of actual object. So on instantiation is created just empty object. Data will be lazily loaded on invocation of operate() method
On invocation of operate() method on proxy real instance is created. So instance of stateful object is created only when it is actually required. Till that time client only keeps the proxy to actual object. This is the concept of lazy loading which is heavily used in lot of J2ee frameworks like Spring
Here proxyService is delegating the call to Actual service method....
Here call is delegated to actual service and actual logic is executed in actual service method only . Result returned from that is received in proxy instance and is sent back to client.This Client remains totally agnostic of Actual service and Just deal with Proxy Service to invoke operations available in real service.
Actual Instance is created here. It execute database calls and provide a state to the Object
---------------------------






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