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Building Wireless Applications Using Java and XML

Building Wireless Applications Using Java and XML. Alex Chaffee, alex@jguru.com http://www.purpletech.com. WAP Architecture. Cell phone / PDA. WAP Gateway. HTTP Server. WAP. HTTP. Which Gateway?. Europe: whichever

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Building Wireless Applications Using Java and XML

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  1. Building Wireless Applications Using Java and XML Alex Chaffee, alex@jguru.com http://www.purpletech.com

  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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