Chain of Responsibility
Proxy Design Pattern
Usually, a proxy, in its most normal 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. Most of the time we need the ability to control the access to an object. For example if we need to use only a few methods of some costly objects we'll initialize those objects when we need them entirely. Until that point we can use some light objects exposing the same interface as the heavy objects. These light objects are called proxies and they will instantiate those heavy objects when they are really need and by then we'll use some light objects instead.
This ability to control the access to an object can be required for a variety of reasons: controlling when a costly object needs to be instantiated and initialized, giving different access rights to an object, as well as providing a sophisticated means of accessing and referencing objects running in other processes, on other machines.
Consider for example an image viewer program. An image viewer program must be able to list and display high resolution photo objects that are in a folder, but how often do someone open a folder and view all the images inside. Sometimes you will be looking for a particular photo, sometimes you will only want to see an image name. The image viewer must be able to list all photo objects, but the photo objects must not be loaded into memory until they are required to be rendered.
The intent of this pattern is to provide a “Placeholder” for an object to control references to it.
ImplementationThe figure below shows a UML class diagram for the Proxy Pattern:
DescriptionA client obtains a reference to a Proxy, the client then handles the proxy in the same way it handles RealSubject and thus invoking the method doSomething(). At that point the proxy can do different things prior to invoking RealSubject’s doSomething() method. The client might create a RealSubject object at that point, perform initialization, check permissions of the client to invoke the method, and then invoke the method on the object. The client can also do additional tasks after invoking the doSomething() method, such as incrementing the number of references to the object.
Applicability & Examples
The Proxy design pattern is applicable when there is a need to control access to an Object, as well as when there is a need for a sophisticated reference to an Object. Common Situations where the proxy pattern is applicable are:
Example - Virtual Proxy Example.Consider an image viewer program that lists and displays high resolution photos. The program has to show a list of all photos however it does not need to display the actual photo until the user selects an image item from a list.
The code below shows the Proxy implementation, the image proxy is a virtual proxy that creates and loads the actual image object on demand, thus saving the cost of loading an image into memory until it needs to be rendered:
The code below displays the RealSubject Implementation, which is the concrete and heavyweight implementation of the image interface. The High resolution image, loads a high resolution image from disk, and renders it to screen when showImage() is called.
The code below illustrates a sample image viewer program; the program simply loads three images, and renders only one image, once using the proxy pattern, and another time directly. Note that when using the proxy pattern, although three images have been loaded, the High resolution image is not loaded into memory until it needs to be rendered, while in the part not using the proxy, the three images are loaded into memory although one of them is actually rendered.
Specific problems and implementation
Java Remote Method Invocation (RMI)In java RMI an object on one machine (executing in one JVM) called a client can invoke methods on an object in another machine (another JVM) the second object is called a remote object. The proxy (also called a stub) resides on the client machine and the client invokes the proxy in as if it is invoking the object itself (remember that the proxy implements the same interface that RealSubject implements). The proxy itself will handle communication to the remote object, invoke the method on that remote object, and would return the result if any to the client. The proxy in this case is a Remote proxy.