I want to know base using the code below what is the: 1. ISubject and Operation() 2. realSubject: RealSubject and 3. Operation() realSubject.Operation() on the UML diagram of Proxy design pattern
//Payment.java
import java.math.*; import java.rmi.*;
public interface Payment extends Remote{
public void purchase(PaymentVO payInfo, BigDecimal price)
throws PaymentException, RemoteException; }`
//PaymentProxy.java
import java.net.*;
import java.math.*;
import java.rmi.*;
public class PaymentProxy implements PaymentService{
private Payment implementation;
private String serviceMachine = "localhost";
private String serviceName = "paymentService";
public PaymentProxy() throws ServiceUnavailableException{
lookupRemoteService();
}
private void lookupRemoteService() throws ServiceUnavailableException{
try{
String url = "//" + serviceMachine + "/" + serviceName;
Object lookup = Naming.lookup(url);
if (lookup instanceof Payment){
implementation = (Payment)lookup;
}
else{
throw new ServiceUnavailableException("Cannot locate remote service");
}
}
catch (RemoteException exc){
throw new ServiceUnavailableException("Error during remote service lookup", exc);
}
catch (NotBoundException exc){
throw new ServiceUnavailableException("Remote service is not registered with naming server", exc);
}
catch (MalformedURLException exc){
throw new ServiceUnavailableException("Malformed URL for naming lookup", exc);
}
}
public void setServiceMachine(String machineName){
serviceMachine = machineName;
}
public void setServiceName(String svcName){
serviceName = svcName;
}
public void purchase(PaymentVO pay, BigDecimal price) throws PaymentException, ServiceUnavailableException{
try{
if (implementation != null){
implementation.purchase(pay, price);
}
}
catch (RemoteException exc){
try{
lookupRemoteService();
implementation.purchase(pay, price);
}
catch (RemoteException exc2){
throw new PaymentException("Cannot process payment: remote communication problems with payment service", exc2);
}
}
}
}`
//PaymentImpl.java
import java.math.*;
import java.net.*;
import java.rmi.*;
import java.rmi.server.*;
public class PaymentImpl implements Payment{
private static final String PAYMENT_SERVICE_NAME = "paymentService";
public PaymentImpl() throws RemoteException, MalformedURLException{
UnicastRemoteObject.exportObject(this);
Naming.rebind(PAYMENT_SERVICE_NAME, this);
}
public void purchase(PaymentVO payInfo, BigDecimal price)
throws PaymentException{
}
}`
The PaymentProxy
should implement Payment
instead of PaymentService
:
public class PaymentProxy implements Payment {
In this case:
Payment
is ISubject
; PaymentImpl
is RealSubject
(so implementation
in PaymentProxy
is the realSubject
field); purchase()
method corresponds to Operation()
in RealSubject
. A more robust explanation:
The Payment
interface is implemented by both classes. Additionally, each of them overrides the purchase()
method. But the PaymentProxy
is a wrapper , since it aggregates PaymentImpl
and adds null checking in the overriden method:
public void purchase(PaymentVO pay, BigDecimal price) throws PaymentException, ServiceUnavailableException, RemoteException{
if (implementation != null){
implementation.purchase(pay, price);
}
}
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