Home > Articles > Software Development & Management

Design Patterns in Java: Proxy

The intent of the Proxy pattern is to control access to an object by providing a surrogate, or placeholder, for it. Learn how to apply this pattern to your Java code.
This chapter is from the book

An ordinary object does its own work in support of the public interface that it advertises. It can happen, though, that a legitimate object cannot live up to this ordinary responsibility. This may occur when an object takes a long time to load, when the object is running on another computer, or when you need to intercept messages to the object. In these cases, a proxy object can take the responsibility that a client expects and forward requests appropriately to an underlying target object.

The intent of the Proxy pattern is to control access to an object by providing a surrogate, or placeholder, for it.

A Classic Example: Image Proxies

A proxy object usually has an interface that is nearly identical to the interface of the object it is a proxy, or substitute, for. The proxy does its work by judiciously forwarding requests to the underlying object that the proxy controls access to. A classic example of the Proxy pattern relates to avoiding the expense of loading large images into memory. Suppose that images in an application belong in pages or panels that do not initially display. To avoid loading all the images before they are needed, you might let proxies for the images act as placeholders that load the required images on demand. This section provides an example of an image proxy. But note that designs that use Proxy are sometimes brittle, because they rely on forwarding method calls to underlying objects. This forwarding may create a fragile, high-maintenance design.

Suppose that an Oozinoz engineer is working on an image proxy that will, for performance reasons, show a small, temporary image while a larger image is loading. The engineer has a prototype working (Figure 11.1). The code for this application is in the ShowProxy class in the app.proxy package. The underlying code that supports the application is in the com.oozinoz.imaging package.

Figure 11.1 Three screen shots show a mini-application before, during, and after loading a large image. (Image is in the public domain. Library of Congress, Prints and Photographs Division, Gottscho-Schleisner Collection [LC-G605-CT-00488].)

The user interface displays one of three images: one that indicates that loading has not begun, one that indicates that the real image is loading, or the real image. When the application starts, it shows Absent, a JPEG image that you have built in an image-processing tool. When the user clicks Load, the image changes almost instantly to a prebuilt _Loading... image. After a few moments, the desired image appears.

An easy way to display an image saved in, say, a JPEG file is to use an ImageIcon object as an argument to a “label” that will show the image:

ImageIcon icon = new ImageIcon("images/fest.jpg"); JLabel label = new JLabel(icon);

In the application that you are building, you want to pass into JLabel a proxy that will forward painting requests to (1) an “absent” image, (2) a “loading” image, or (3) the desired image. The message flow might look like the sequence diagram in Figure 11.2.

Figure 11.2 An ImageIconProxy object forwards paint() requests to the current ImageIcon object.

When the user clicks Load, your code will cause the ImageIconProxy object to change its current image to the Loading... image. The proxy will also begin loading the desired image. When the desired image is completely loaded, the ImageIconProxy object will change its current image to be the desired image.

To set up a proxy, you can create a subclass of ImageIcon, as Figure 11.3 shows. The code for ImageIconProxy defines two static variables that contain the Absent and Loading... images:

static final ImageIcon ABSENT = new ImageIcon(

static final ImageIcon LOADING = new ImageIcon(

Figure 11.3 An ImageIconProxy object can stand in for an ImageIcon object because an ImageIconProxy object is an ImageIcon object.

The constructor for ImageIconProxy accepts the name of an image file to eventually load. When an ImageIconProxy object’s load() method is called, it sets the image to LOADING and starts a separate thread to load the image. Using a separate thread keeps the application from waiting while the image loads. The load() method accepts a JFrame object that the run() method calls back once the desired image is loaded. The almost-complete code for ImageIconProxy.java is:

package com.oozinoz.imaging;
import java.awt.*;
import javax.swing.*;

public class ImageIconProxy 
       extends ImageIcon implements Runnable {
    static final ImageIcon ABSENT = new ImageIcon(
    static final ImageIcon LOADING = new ImageIcon(
    ImageIcon current = ABSENT;
    protected String filename;
    protected JFrame callbackFrame;
     public ImageIconProxy(String filename) {
        this.filename = filename;
     public void load(JFrame callbackFrame) {
        this.callbackFrame = callbackFrame;
        current = LOADING;
        new Thread(this).start();
     public void run() {
        current = new ImageIcon(
     public int getIconHeight() { /* Challenge! */ }
     public int getIconWidth() {  /* Challenge! */ }
     public synchronized void paintIcon(
            	Component c, Graphics g, int x, int y) {
        // Challenge!

Challenge 11.1

An ImageIconProxy object accepts three image display calls that it must pass on to the current image. Write the code for getIconHeight(), _getIconWidth(), and paintIcon() for the ImageIconProxy class.

A solution appears on page 376.

Suppose that you get the code working for this small demonstration application. Before you build the real application, which has more than just a Load button, you hold a design review, and the fragility of your design comes to light.

Challenge 11.2

The ImageIconProxy class is not a well-designed, reusable component. Point out two problems with the design.

A solution appears on page 376.

As you review someone’s design, you must concurrently form an understanding of the design and your opinion of the design. When you encounter a developer who feels that he or she is using a specific design pattern, you may disagree about whether the pattern is present. In this example, the Proxy pattern is evident but does not demonstrate that the design is good; in fact, much better designs exist. When the Proxy pattern appears in a design, its presence should be justified, because the use of forwarding can create problems that other designs may avoid. As you read the next section, you should form an opinion about whether Proxy is a desirable choice.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020