Building an eBay Rich Client using the XUI Framework Blog

Version 2



    The eBay SDK
    XUI in a Nutshell</  td>
    Integrating the Data Model
    Wrapping the SDK
    Coordinating the Interaction
    Building a Store
    Does this Constitute Rich-Client?
    Why not Ajax?

    Today eBay is a juggernaut of e-commerce, with millions of users and companies worldwide using the service on a daily basis. One of the attractions is the ease of use of the service: it's very easy to get started with eBay, whether you are buying or selling. However, while the eBay browser-based interface is perfectly adequate for many, power users may find it a bit frustrating to use on a day-to-day basis. Network latency and other delays mean that interaction with eBay, like any other web service, is not as responsive as the most demanding of users would like.

    Repetitive processes can prove frustrating, particularly when it seems that some of the information and state could be reused or handled in a smarter way, but that is the way of most browser-based applications. Rich-client applications fit nicely in this sort of niche, making use of client-side computing power and storage for more efficient use of data and fewer roundtrips to the server.

    Many users also seek additional functionality to extend and automate common tasks. Auction sniping (automated bidding), for example, is a much-used but frowned-upon add-on for many eBay power users. However, much of the potential for using eBay is not just in speeding things up, but in creating new ways of interacting with the service. In response to the demand for extensions, alternative interfaces, and integration capabilities, eBay has released a Java SDK for their service.

    Taking the eBay SDK and XUI, an open source Java and XML framework for building rich-client applications, is a good way to create an alternative to the browser based interfaces to eBay. XUI harnesses desktop computing power to improve the overall end-user experience. Building a XUI rich client for eBay makes it possible to use this latent power, add features, and improve upon a service that is already phenomenally popular.

    The eBay SDK

    The eBay SDK is delivered in several forms, including a Java SDK that wraps a set of SOAP interfaces. The Java SDK provides an easy way to interact with a large part of the eBay service, allowing you to query user and account information, add items, and create auctions.

    The SDK is a relatively thin wrapper around the SOAP services, and so the interfaces offered may not be the most natural or convenient for the desktop Java developer. For instance, the SDK is structured around the individual SOAP services and exposes some low-level details of those services. The SDK also does little to tie the various API calls together, so some work needs to be done to coordinate calls to the SDK. While a lot easier to use than a raw SOAP API, a further layer of software like XUI's eBay library can help simplify programming a little more.

    Quite apart from merely being a wrapper for the eBay SDK, the XUI framework provides a layer of caching, whereby long-lived data can be maintained within the client application. The locality of data that this provides means that many roundtrips to the server can be avoided and that network latency should be less apparent to the end user. Throw in some data binding features, and XUI ends up taking care of much of the data handling involved in a data-rich client application, leaving the developer free to concentrate on innovative ways of using the eBay service.

    XUI in a Nutshell

    As a rich-client framework, XUI takes care of some of the common tasks involved in constructing applications, and aims to promote accepted design practices like the MVC architecture and a clean separation of concerns. XUI applications run on a wide variety of platforms, from handheld computers to high-end desktops running Windows, Linux, and the Mac OS.

    XUI makes use of XML for many aspects of the framework. But once processed, the objects declared in XML are first-class Java objects and at almost any stage these objects can be customized by normal Java code. Consider the following page declaration in XUI:

    <XPage layout="Border"> <Components> <Label content="Hello world" constraint="center" align="center"/> <Button name="exitBtn" content="Goodbye" constraint="south"/> </Components> <Events> <Event target="exitBtn" type="ActionHandler" method="${mypackage.MyHelper[test].doExit()}"/> </Events> </XPage>

    This creates a simple page with the familiar "Hello world" message and a button to close the application. The sample application is shown in Figure 1:

    Screenshot of the Hello World application
    Figure 1. A simple Hello World application

    The button is declared with the Button element and an ActionHandler is then added to the button. The method declaration is an expression that refers to a method nameddoExit() in the test instance of themypackage.MyHelper class, the code for which is just some plain Java:

    package mypackage; public class MyHelper { public void doExit() { System.exit( 0 ); } }

    While this in itself is not tremendously interesting, the separation of UI and application logic is the important concept here. Unlike some XML-based formats, XUI can do a lot more than create single-page applications. In large applications, this separation of concerns can be very important. But why bother with XML at all? Well, XML, in some cases, is more suited to declaration of content: take, for example, the creation of coherent styling within an application where a XUI stylesheet can be replaced so as to rebrand an application without modifications to the application code. In fact, given the loose coupling involved, it is possible to completely modify the user interface and presentation of an application independently of the data model and the business logic, something that can be difficult to achieve without such a framework.

    In certain respects, XML can add flexibility to an application; in other respects, the mix of XML and Java adds complexity and therefore tool support is vital. XUI ships with a NetBeans plugin that manages all of the vital relationships between the various files and allows easy visual editing of the applications. Therefore, using NetBeans means that for the most part, the user is programming just another Java framework.

    The trivial example above shows the separation of the UI and the business logic, but a more realistic separation of concerns also involves separating the data management from business logic and from the presentation issues associated with that user interface. Data binding provides the key abstraction here, allowing data to be referenced and manipulated independently of the UI. Not only does this make the UI somewhat easier to manage, but it also means that your business logic is less encumbered by data management issues.

    Building an eBay application is a different proposition than a Hello World application, yet many of the same basic concepts come to play. After all, in an ideal world, we just assemble a few components and add our application-specific business logic to create such an application. Two key components in XUI come into play when considering an interface to eBay: managing all of the data and wrapping the eBay API. After that is achieved, XUI can be used like any other Swing framework, while hiding the low-level details of the eBay services.

    Integrating the Data Model

    XUI includes a managed data model that can hold a hierarchy of Java objects. The objects in the model can be from various sources, such as static property files, databases, or even data retrieved by remote calls. Nodes within the model's hierarchy can be addressed with path expressions, and for convenience, the model will also instantiate storage if you try to write to nodes that did not previously exist. In the context of user interface development, this approach is tremendously powerful, allowing you to bind UI components to an abstract model with a minimum of configuration, while also being able to reference that model from other pages or contexts without introducing dependencies between components. The data model is an essential part of an MVC architecture, and being able to bind to the model so easily can help remove a lot of glue logic from an application.

    Let's see the model in action with a modified Hello World, this time taking two pages. The first of them, shown in Figure 2, now has an edit field where some user value can be entered:

    <XPage layout="Border"> <Components> <Panel constraint="center" layout="Grid"> <Label content="Enter your name:" align="right"/> 
    <Edit name="
    userName" /> </Panel> <Button name="nextBtn" content="Next" constraint="south"/> </Components> <Events> <Event target="nextBtn" type="ActionHandler" method="${mypackage.MyHelper[test].showPage()}"/> </Events> <Data> 
    <Bind target="userName" source="myData/userName" output="myData/userName"/> </Data> </XPage>

    Screenshot of a simple form
    Figure 2. Capture some input

    The second page, shown in Figure 3, redisplays the entered text.

    <XPage layout="Border"> <Components> <Label content="Goodbye from" constraint="north" align="center"/> 
    <Label name="userName" constraint="center" align="center"/> <Button name="nextBtn" content="Exit" constraint="south"/> </Components> <Events> <Event target="nextBtn" type="ActionHandler" method="${mypackage.MyHelper[test].doExit()}"/> </Events> <Data> 
    <Bind target="userName" source="myData/userName"/> </Data> </XPage>

    Redisplay of the input form's data
    Figure 3. Redisplay of the input

    The data binding will automatically update the label on the second page with the data from the edit field on the first page once the second page is displayed, with no extra coding required. The myData/userName node is automatically created by the data binding. The first page also references a new Java method, but it too is very simple:

    public void showPage() { XProjectManager.getPageManager().showPage( "goodbye" ); }

    With this brief introduction to how XUI works, we can look at integrating the eBay SDK.

    Wrapping the SDK

    The eBay Java SDK relies on SOAP, and each API defines many constants; for example, the payment method for a transaction can be set as follows:

    ReviseItemRequestType request = new ReviseItemRequestType(); ... BuyerPaymentMethodCodeType[] methods = new BuyerPaymentMethodCodeType[1]; methods[0] = BuyerPaymentMethodCodeType.CCAccepted; item.setPaymentMethods(methods); ...

    where the CCAccepted field is defined within the eBay SDK as:

    package com.ebay.soap.eBLBaseComponents; public class BuyerPaymentMethodCodeType implements { ... public static final org.apache.axis.types.Token _CCAccepted = new org.apache.axis.types.Token("CCAccepted"); ... public static final BuyerPaymentMethodCodeType CCAccepted = new BuyerPaymentMethodCodeType( _CCAccepted); ... }

    Unfortunately, the BuyerPaymentMethodCodeType class does not provide any support for enumeration or listing of the allowed types. To overcome the tedium of using such a limited API, we have applied a little reflection to inspect the internals of such classes and provide a more useful way of working with these constants.

    /** * Find all the instances of a class referenced * within the class * @param clazz the class of interest * @param outputModel the model where value are * stored */ public void getClassInstances( Class clazz, XModel outputModel ) { ArrayList listFields = new ArrayList(); Field[] fields = clazz.getFields(); for ( int i = 0 ; i < fields.length ; i++) { // Only choose the field instances we // requested if ( fields[ i ].getType().getClass().equals( clazz )) listFields.add( fields[ i ].getName() ); } // Add the fields to the data model int numFields = listFields.size(); for ( int i = 0; i < list.length ; i++ ) { XBaseModel listModel = new XBaseModel( outputModel, "row" + i , listFields.get( i ).toString()); } }

    The above method iterates a list of members from the specified class and adds the member instances to an output data model, theXModel parameter being the basic abstract node type used in XUI's data model. Once the data has been added to the model, it can be bound to user interface elements, as we have seen above in the Hello World example.

    Applying the same technique to other APIs goes some way towards a better integration, but we still need some work to wrap the many APIs in a suitable form.

    Coordinating the Interaction

    The parameter handling itself is a worthwhile improvement over the raw API, but it gains added importance when multiple APIs are required for a complete process, such as adding new items to an eBay store. Of course, there is nothing to stop you from doing all of the setup directly, but why bother doing the same stuff over and again when you can let a data binding do the work? Instead, you can concentrate on what is unique to the particular process or context. Data binding alleviates some of the headaches of handling and coordinating data for the APIs.

    Data binding has the added advantage of implicitly caching data, so the data needed to populate the user interface components is available locally, thereby improving performance both of the UI and of the network. In fact, rather than having to make a roundtrip to the eBay server to get configuration information, you can usually get it locally. If you don't think this is an issue, then consider that eBay has gone as far as to add a flag to many APIs indicating the amount of data to return. Something as simple as requesting the available delivery methods may return large numbers of shipping methods and all of the associated data.

    Even the process of setting a connection can be a cumbersome task, so the connection setup code is wrapped in a class to make programming of other APIs easier.

    import com.ebay.sdk.*; import com.ebay.soap.eBLBaseComponents.*; import java.util.Calendar; import; public class EbayConnection { protected ApiContext apiContext; public EbayConnection( XModel connectionModel) { apiContext = new ApiContext(); ApiCredential cred = apiContext.getApiCredential(); ac.setDeveloper( connectionModel.getValueAsString( "devID" ) ); ac.setApplication( connectionModel.getValueAsString( "appID" ) ); ac.setCertificate( connectionModel.getValueAsString( "certID" ) ); ApiAccount ac = cred.getApiAccount() ; apiContext.setApiServerUrl( connectionModel.getValueAsString( "url" ) ); apiContext.getApiCredential().seteBayToken( connectionModel.getValueAsString( "token" ) ); eBayAccount ec = cred.geteBayAccount(); ec.setUsername( connectionModel.getValueAsString( "userID" )); ApiLogging apiLogging = new ApiLogging(); apiContext.setApiLogging( apiLogging ); apiContext.setSite( SiteCodeType.fromString( connectionModel.getValueAsString( "site" ) ) ); apiContext.setWSDLVersion( connectionModel.getValueAsString( "wsdl" ) ); //Set the EPS Server URL this server is //used for uploading pictures to ebay apiContext.setEpsServerUrl( " EpsBasicApp&com.ebay.presentation. ForceXmlDownload=true" ); //For production use this EPS Server URL //context.EPSServerUrl = //" //EpsBasicApp&com.ebay.presentation. //ForceXmlDownload=true"; } public ApiContext getTheApiContext() { return apiContext; } }

    The above wrapper does all of the normal stuff needed to set up the connection, but instead of passing the connection attributes into the wrapper as method parameters, the attributes are retrieved from XUI's data model. Now, given that these parameters can be obtained from the UI without writing any code, you should begin to get an idea of how XUI's data binding can save some work. The XUI-eBay library, an open source project on the eBay developer website, goes on to wrap far more APIs in this manner. As a result, building an application becomes more a question of hooking up the appropriate data and services than one of how to use the SDK.

    Building a Store

    Now that some of the basic ingredients provided by XUI have been introduced, what can be done to put some richness into the user experience? To demonstrate such use of the XUI-eBay library, a sample StoreManager application (shown in Figure 4) aimed at someone who wants to set up and manage a store on eBay has been built and released by Xoetrope, as open source via the eBay Community Codebase.

    Screenshot of the my ebay page
    Figure 4. The eBay StoreManager application

    Using the browser interface, one needs multiple pages and many mouse clicks to add an individual item. Using the StoreManager application, the same task can be accomplished more quickly and easily. Even within a single session, an improvement over what is possible within the browser can be seen. However, as a XUI data model can be saved to local storage, it is possible to save an entire session and all of the associated configuration data. Saving the data in this way means that a session can be restored and the user can continue working where they left off, with no need to configure the setup again. Of course, time-sensitive information cannot be saved and reused in this way, but even so, enough information can be saved and reused so that the end user will be largely oblivious to the fact that a new session has been started.

    Perhaps more importantly, the use of Java affords many integration possibilities. The StoreManager application can store transaction histories, details of items for sale, and other information in a local database. It would not take a huge leap of imagination to then integrate with some other systems either through Java or via the local database.

    Does this Constitute Rich-Client?

    Well, like beauty, it's really in the eye of the beholder. One of the biggest drawbacks of desktop software is the perceived installation issue. In most cases, Java Web Start can neatly address that issue, but in the case of the eBay SDK, this assumption might need some qualification. While the XUI library can be as little as 194KB, the eBay library relies upon the Axis framework, which alone weighs in at a little over 9MB. Optimization might help shrink this, but there is a significant download involved. Therefore, the benefits need to outweigh the inconvenience of the download, and that largely depends on the use the application receives.

    However, once over the hump of this download, the performance improvement of a rich-client application should pay dividends for a regular user. For businesses using eBay, the added incentive of simplified integration may be reason in itself to use a Java rich client.


    One downside to the story is that the eBay API is also incomplete, lacking any bid functionality. EBay has created a PlaceOffer API, but unfortunately they will not license it for use in open source software. Therefore, we have been forced to use screen-scraping technology from the JBidWatcherproject. While perfectly functional, it will not stand up, without programmer intervention, to major changes made by eBay.

    Why not Ajax?

    Since Ajax is the framework du jour, it is worth making a brief comparison. Ajax is certainly an improvement on vanilla HTML, but it's still tied to the browser, and for the most part relies upon JavaScript for its logic. While this is perfectly adequate in many situations, it lacks the power, integration, and structural capabilities of a Java-based approach. Nor is Ajax brilliant at data management in the context of something like a data-intensive eBay application. At the same time, being browser-based largely ties an Ajax application to the browsers' concept of page lifecycle. Finally, it is doubtful if Ajax would help much with the SOAP integration.


    This is a glimpse of what can be achieved with the XUI framework. The XUI-eBay library, while workable, continues to grow and it is planned to leverage the techniques described here to make the use of eBay services even easier. Creation of the eBay StoreManager has resulted in:

    • An application that is far quicker to use than the browser-based application.
    • Extended integration capabilities.
    • A reduced learning curve for extension.
    • Technology that can just as well be applied to other eBay applications and scenarios.

    In conclusion, the techniques described here are not unique to XUI and can be applied to a wide variety of applications and APIs. By facilitating the integration of a web service API, you can tap into the power of the desktop and give your end users a far more satisfying software experience. Integrating with the desktop also gives further possibilities for working offline, in an occasionally connected way, or in concert with other systems. Working within the confines of a web browser is not for everyone, or for every situation, and this is often the case for the target audience of the StoreManager: power users. It is right here that rich-client applications can help.