Proxy

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How to control access to an object through a representative or surrogate?

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Original author: The Gang of Four (GoF)

UML Model File:

Donload Proxy

Model Format Visual Paradigm

Advantages

It manages rhe system resources by instantiating objects on-demand, reducing memory and processing overhead.
Access control by enabling managing the permissions.
Extra layer of Security where sensitive information are involved.

Disadvantages

Complex in case of adding a lot of new classes.
The response from the service could get delayed.

Code Examples

/**
 * Normally, we adopt a “lazy instantiation” strategy for the RealSubject, 
 * creating it only when it is actually required.
 */
class RealSubject {
    … // RealSubject attribute declaration

    public RealSubject(…) {
        … // RealSubject attribute initialization
    }

    public void request() {
        … // Real implementation of the request method
    }
}

/**
 * The Proxy class contains a reference that lets the proxy access the real subject.
 * Proxy also implements the same interface as the real subject and delegates all the work to the real subject.
 */
class Proxy {
    private static RealSubject realSubject;
    … // Proxy specific attribute declaration

    public Proxy(…) {
        … // Proxy specific initialization
    }

    /**
     * The “request” method is used to access the RealSubject’s request method.
     * This could involve additional logic such as lazy instantiation, access control, or logging.
     */
    public void request() {
        if (realSubject == null) {
            realSubject = new RealSubject(…);
        }
        … // Additional logic before delegating the request
        realSubject.request();
        … // Additional logic after delegating the request
    }
}

/**
 * If we are concerned with thread safety when using multi-threading, we need to ensure 
 * that our Proxy class handles simultaneous calls properly, especially during instantiation of the RealSubject.
 */
class Proxy {
    private static volatile RealSubject realSubject;
    … // Proxy specific attribute declaration

    public Proxy(…) {
        … // Proxy specific initialization
    }

    /**
     * We use synchronized blocks inside the request method to ensure that RealSubject is
     * instantiated only once, even with concurrent access.
     */
    public static void request() {
        if (realSubject == null) {
            synchronized (Proxy.class) {
                if (realSubject == null) {
                    realSubject = new RealSubject(…);
                }
            }
        }
        realSubject.request();
    }
}

/**
 * Alternatively, we may use an “early instantiation” strategy for RealSubject, creating it
 * at the time of Proxy class loading. This simplifies the request method as it no longer needs 
 * to check if RealSubject exists.
 */
class Proxy {
    private static final RealSubject realSubject = new RealSubject(…);
    … // Proxy specific attribute declaration

    public Proxy(…) {
        … // Proxy specific initialization
    }

    /**
     * The “request” method simply delegates the request to the RealSubject.
     */
    public void request() {
        realSubject.request();
    }
}

/**

* Normally, we adopt a “lazy instantiation” strategy for the RealSubject,

* creating it only when it is actually required.

class RealSubject {

... // RealSubject attribute declaration

public RealSubject(...) {

... // RealSubject attribute initialization

}

public void request() {

... // Real implementation of the request method

}

/**

* The Proxy class contains a reference that lets the proxy access the real subject.

* Proxy also implements the same interface as the real subject and delegates all the work to the real subject.

class Proxy {

private static RealSubject realSubject;

... // Proxy specific attribute declaration

public Proxy(...) {

... // Proxy specific initialization

}

/**

* The “request” method is used to access the RealSubject’s request method.

* This could involve additional logic such as lazy instantiation, access control, or logging.

public void request() {

if (realSubject == null) {

realSubject = new RealSubject(...);

}

... // Additional logic before delegating the request

realSubject.request();

... // Additional logic after delegating the request

}

/**

* If we are concerned with thread safety when using multi-threading, we need to ensure

* that our Proxy class handles simultaneous calls properly, especially during instantiation of the RealSubject.

class Proxy {

private static volatile RealSubject realSubject;

... // Proxy specific attribute declaration

public Proxy(...) {

... // Proxy specific initialization

}

/**

* We use synchronized blocks inside the request method to ensure that RealSubject is

* instantiated only once, even with concurrent access.

public static void request() {

if (realSubject == null) {

synchronized (Proxy.class) {

if (realSubject == null) {

realSubject = new RealSubject(...);

}

realSubject.request();

}

/**

* Alternatively, we may use an “early instantiation” strategy for RealSubject, creating it

* at the time of Proxy class loading. This simplifies the request method as it no longer needs

* to check if RealSubject exists.

class Proxy {

private static final RealSubject realSubject = new RealSubject(...);

... // Proxy specific attribute declaration

public Proxy(...) {

... // Proxy specific initialization

}

/**

* The “request” method simply delegates the request to the RealSubject.

public void request() {

realSubject.request();

}