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Building Wireless Applications Using Java and XML

This guide explores the development of wireless applications using Java and XML, focusing on WAP architecture for mobile devices. It examines how to implement client-server interactions through WAP gateways, the importance of security, privacy concerns, and the various wireless client types. It provides insights on server-side architecture, discussing the integration of Java technologies with XML data sources for optimal performance. Additionally, practical examples of WML are included to illustrate how to create simple yet effective wireless applications.

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Building Wireless Applications Using Java and XML

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  1. Building Wireless Applications Using Java and XML Web Applications and Services

  2. WAP Architecture Cell phone / PDA WAP Gateway HTTP Server WAP HTTP

  3. Which Gateway? • Europe: whichever • Cell phone contains modem, signal carries PPP connection to any cell ISP • U.S.: hardcoded to carrier • Same technology (?) • IP address of ISP hardcoded by vendor • Security • If I make a secure connection to my bank, maybe I don't trust my carrier's gateway not to snoop • Choice • Inhibits competition among wireless ISPs • Privacy • My carrier can monitor my behavior (traffic, email)

  4. Wireless Client Types • Physical Devices • PC, Cellphone, Palmtop, Minilaptop • HTML, WML, HDML, iMode, etc. • J2ME-compliant or not • many versions • WAP Gateways • proxy to WML browsers • Or to HDML etc. browsers • many versions

  5. On J2ME • Client-side Java • Flexible • Powerful • Not yet ubiquitous • Server-side Java • Direct access to data • Click and wait • WML/WAP much more widespread at present • Very analogous to applet vs. servlet • This talk: all about server-side

  6. Developing Wireless Applications • Get a web server • Apache • Tomcat • Add WML mime-type • text/vnd.wap.wml • Get an emulator/SDK • Phone.com • Ericsson.com • Nokia.com • Learn WML • Phone.com SDK has good docs/specs

  7. WML • XML-compliant language • Similar to HTML • Defines display, input, links for small devices • Central concept is not page but card and deck • Allows many cards (pages) to be downloaded simultaneously • WMLScript • Allows more client-side interactivity, like JavaScript • Not widely supported (phone.com browsers only)

  8. WML Example <?xml version="1.0"?> <!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.1//EN" "http://www.wapforum.org/DTD/wml_1.1.xml"> <wml> <card> <p> Hello Wireless World! </p> </card> </wml> • Thanks, Chad 

  9. WML Example 2 <wml> <card> <p> Computing Central <br/> <a href="#java_card" title="Java tips, tricks">Java News</a> <a href="#cpp_card" title="C++ news, jobs">C++ News</a> </p> </card> <card id="java_card"> <p> Welcome to the world of Java, the daily tip is ... </p> </card> <card id="cpp_card"> <p> Hot jobs in the C++ community... </p> </card> </wml>

  10. Wireless application = Web application • Since WAP gateways call regular HTTP servers • Developing wireless apps is identical to developing web apps • Except for a few small details 

  11. Server-Side Java-XML Architecture Java UI Java Business Logic XML Processors XML Data Sources WAP/HTTP Request JSP JavaBeans DOM, SAX, JDOM Filesystem XPath XML-savvy RDBMS Servlet EJB HTML/ WML XSL XML Data Feed

  12. Server-Side Architecture Notes • Note that you can skip any layer, and/or call within layers • e.g. XML->XSL->DOM->JSP, or • JSP->Servlet->DOM->XML

  13. Cache as Cache Can • Caching is essential • Whatever its advantages, XML is slow • Cache results on disk and/or in memory • See www.purpletech.com for a cache class • Also org.jdom.contrib.beans.JDOMBean

  14. Data Sources • XML files • Databases • XML database wrappers • Binary data feeds • XML data feeds

  15. Data Flow • request-driven • data-driven

  16. Request-driven flow Data Layer Java Layer WAP/HTTP Request Java Logic XML Processor XML Data Source Database or Filesystem HTML/ WML Java UI

  17. Request-Driven Flow (Context-Driven) • client makes WAP request • gateway makes HTTP request • JSP examines context • request/device/variables/time of day/... • JSP/Servlet acquires correct XML • JSP parses XML, extracts data • JSP outputs WML

  18. Data-driven flow WAP/HTTP Request ? XSL Stylesheet XML Data Source XSLT Processor Database or Filesystem HTML/ WML XML Wrapper

  19. Data-Driven Flow • XML data source • XSL transformation into WML or HTML • How to choose… • Which XSL stylesheet to use? • Where to get the data? • How to invoke the XML wrapper?

  20. Mixed Flow • client makes request • servlet/jsp • identifies client browser • examines request • acquires XML data • acquires XSL file • invokes XSL processor • Postprocesses XML • outputs WML/HTML/HDML/...

  21. XMLC • Separates presentation template from presentation formatting code • See xmlc.enhydra.org HTML (with ID tags) Reading data XMLC Data HTML Object (automatically generated) Java Class (e.g. Servlet) Setting values HTML (dynamically-generated)

  22. XMLC + WML + HTML WML Source: <?xml version="1.0"?> <!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.1//EN" "http://www.wapforum.org/DTD/wml_1.1.xml"> <wml> <card> <p>Date: <SPAN id="date">some date</SPAN></p> <p id="motd">some message</p> </card> </wml> HTML Source: <HTML> <TITLE><HEAD>Message of the Day</HEAD></TITLE> <BODY><B>Date:</B> <SPAN id="date">some date</SPAN></p> <p id="motd">some message</p> </BODY> </HTML> Java source: obj.setElementDate( new Date().toString() ); obj.setElementMotd( "You are in a maze of twisty little passages" ); out.print(obj.toDocument());

  23. Wireless Development Problems (with solutions) • Missing HTTP features • Response size • Identifying the Client Browser • Images • Languages/Locales • Which output format (wml/html/…)? • WML Incompatibilities • Screen Scraping

  24. Missing HTTP features • Not supported by all gateways/browsers • Redirects • most do • some don't • Cookies • most gateways don't support cookies • must use URL Rewriting • Some servlet engines are bad at this • Anecdotally • 25-50% of requests don't support cookies • same model, same provider, one phone works, one doesn't

  25. Response Size • There is a hard limit on size of response • Dependent on gateway and browser/device • About 1.5 K • After response is compressed and encoded by WAP gateway • Therefore, it's impossible to know if you've reached the limit, unless you fail • "cache 22" • Solution: stay below 2K uncompressed and you should be fine

  26. Identifying the Client Browser • Hard problem, unfortunately • User-Agent header not always sufficient • Accept header not always accurate • (But those are the only headers you can use) • Also, device ¹ browser, and sometimes they provide different functions

  27. One Solution: • Allow user to set browser preference on login • Chicken-and-egg problem • Which browser do you pick first? • Make best guess: usually not unintelligible

  28. Better Solution: Capabilities • Apply rules to identify browser based on headers • Mapping from browser to capabilities • Make decisions based on capabilities, not browser • Allows migration, forward compatibility with new browsers, and making use of previously unknown features • Not tied down to what you know (or what you think you know) about a specific browser

  29. More About Capabilites • Note: be fuzzy in matching, to catch future versions of same device (hopefully) • Note: Store device in session, so you don't have to keep determining it • Yes, there are a lot of devices to keep track of • Morphis has a list, keeping it updated

  30. Example: <device id="sam-sch6100"> <name>Samsung SCH-6100</name> <imgext>bmp</imgext> <capabilities> <capability name="languages" value="English, Spanish"/> <capability name="allow_images" value="1"/> <capability name="resolution_height" value="32"/> <capability name="resolution_width" value="96"/> <capability name="max_num_lines" value="4"/> <capability name="UP_designation" value="SC03"/> <capability name="locations" value="US"/> <capability name="network" value="CDMA"/> </capabilities> <rules> <headerrule name="User-Agent" type="regexp">UP\.Browser/3.*SC03 .* </headerrule> </rules> <xsl name="wmltransform.xsl" mime="text/vnd.wap.wml"/> </device>

  31. Other Capabilities Initiatives • CC/PP • W3C • UAProf • WAP Forum • Not widely supported (yet?) • Also related to Personalization

  32. Open-SourceContent Management Frameworks • Lutris Enhydra • Kargo Morphis • Cocoon • IBM

  33. One URL Fits All • Want to be able to • Email a URL to someone with a different device • Synchronize wireless/wired bookmarks • Good: • http://www.foo.com/thing.jsp • Bad: • http://www.foo.com/cellphone/nokia/9100/thing.jsp • (TM Bluestone) "Total-E-Mobile"

  34. Images • Different browsers: different… • Screen size (resolution) • 28x20, 65x25, 91x50, 96x24, 96x32, 96x44, 96x64, 101x80, 128x64, 134x60, 160x160, 160x200, 240x160, 240x320, 310x100, 480x160, 640x240, 640x480, 800x600, ... • Image types accepted • GIF, JPEG, BMP, WBMP • Color depth • B/W, grayscale, 8-bit color, ...

  35. Images: Solution • Create several base images • different sizes/colors/formats • e.g. logo.32x32.bw.wbmp, logo.96x96.col.gif • Write matching/transforming algorithm • look for closest fit • transform (scale/dither/reformat) as necessary • What if an image cannot be legibly shrunk smaller than a certain size? • May be necessary to split image across several cards

  36. One More Thing About Images • Some browsers have trouble loading more than one image per page • D'oh!

  37. Morphis Images • <wax:img srcid="foo"/> • Matches • foo.gif • foo_Gnok-7110.gif • foo_Gwml.gif • foo_Len.gif

  38. Languages/Locales/Encoding • Accept-Language header should be canonical • Once you determine language, store it in the session and/or a cookie • Should also allow override with a a "lang" parameter or persistent user preference • Language may need to be ID'd before loading data • WML supports xml:lang attribute • Usage is ambiguous • Results in large response pages (containing all languages)

  39. Java Localization Tricks • Store strings etc. in ResourceBundle object • Substitute in correct version at runtime • XMLC • Elements tagged with id attribute • Runtime substitution of element content

  40. Character encoding • Default: UTF8 • By device - some are only able to support certain encodings • iMode requires Shift-JIS • Should store as browser capability

  41. WML Incompatibilities • Really irritating • Different implementations/interpretations of WML spec • Need to tailor output to browser (not just capability) • Example: wml:select • Selectable bullet-list • UP browsers: looks nice • Nokia browser: looks ugly

  42. One solution to WML Incompatibilities • Intermediate markup language • Write your apps in this language • Provide transformations • Intermediate language • <fooml:choice>… • Transform source • phone.com (up) -> <wml:select> • nokia -> list of <wml:a> (link) • E.g. WAX (part of Morphis)

  43. Screen Scraping • Some systems accept HTML and strip/parse • Avant Go, etc. • If you depend on these, don't need to learn WML • Still need to deal with browser issues • Not a panacea • When it works, it works great • But most of the time it doesn't work 

  44. Morphis Overview • Dispatching • Translating (based on XSLT / Xalan) • Caching • WAX (an abstract markup language) • device.x ml (catalog of wireless devices)

  45. Conclusion • Wireless apps have growing pains • Some problems with the WWW were solved, many were not, plus many new ones • Be patient: • devices are getting better all the time • HTML is becoming XML • My prediction: within a few years the platforms will converge, and WML will be supplanted by HTML

  46. For More Information • www.purpletech.com • www.morphis.org • xmlc.enhydra.org

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