wap builds java applications.htm

jsp+wml的源码,对初接触wap编程的爱好者一点帮助

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      class=atitle2>Take advantage of your existing EJBs to power a mobile 
      workforce</SPAN><BR>
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            <P>Level: Introductory</P></TD></TR></TBODY></TABLE>
      <P><A 
      href="http://www-106.ibm.com/developerworks/java/library/wi-entwap/?open&amp;l=766,t=grj,p=WAPjApps#author1"><NAME>Aashish 
      Patil</NAME></A> (<A 
      href="mailto:[e-mail]?cc=&amp;subject=WAP builds Java applications">mailto:[e-mail]?cc=&amp;subject=WAP 
      builds Java applications</A>)<BR>M.S. candidate, Computer Science 
      department, University of Southern California<BR>01 Jul 2001</P>
      <BLOCKQUOTE>If your company already relies on multitiered enterprise 
        Java applications, then you may be closer to a wireless future than you 
        think. By outlining a sample application, Aashish Patil shows you how to 
        connect your existing enterprise Java infrastructure to a wireless 
        netowrk with minimal investment in equipment and labor. Using existing 
        EJBs, modified servlets, and new WML and WMLScript pages makes the 
        process a snap.</BLOCKQUOTE>
      <P>The Wireless Application Protocol (WAP) can add value to the existing 
      Web architecture of an enterprise. If you are already using enterprise 
      Java applications, you can easily integrate them with WAP services to 
      bring useful data and functionality to a mobile workforce. In this 
      article, I will cover the basics of using J2EE with WAP and then build a 
      sample WAP/enterprise Java application that demonstrates how you can 
      connect your own EJBs to a wireless network.</P>
      <P><A name=N10038><SPAN class=atitle2>Background: J2EE and 
      WAP</SPAN></A><BR>You should have a basic understanding of the Java 2 
      Platform, Enterprise Edition (J2EE) architecture before reading this 
      article. You can find links to more information on J2EE in the <A 
      href="http://www-106.ibm.com/developerworks/java/library/wi-entwap/?open&amp;l=766,t=grj,p=WAPjApps#resources" 
      xmlns:fo="http://www.w3.org/1999/XSL/Format" 
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">Resources</A> 
      section below. As a refresher, here's a diagram of a typical J2EE 
      application designed with a desktop client in mind:</P>
      <P><A name=N10048><B>Figure 1. J2EE application structure</B></A><BR><IMG 
      height=318 alt="J2EE application structure" 
      src="WAP builds Java applications.files/wapj2ee-fig1.gif" width=556 
      xmlns:fo="http://www.w3.org/1999/XSL/Format" 
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"></P>
      <P>In Figure 1, the layer containing the JavaServer Pages (JSPs) and 
      servlets is responsible for the generation of dynamic HTML pages. In WAP 
      applications, this layer would generate dynamic Wireless Markup Language 
      (WML) pages instead. Thus, in order to alter a standard J2EE app for use 
      with mobile devices, you will have to write new JSPs and, in some cases, 
      new servlets. The Enterprise JavaBeans (EJBs) will be left untouched, 
      since they are not concerned with presentation of data.</P>
      <P>
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            <P><A name=N1005A><B>What is WML?</B></A><BR></P>Just as Web 
            browsers display data encoded in HTML, WAP-enabled devices display 
            data encoded in Wireless Markup Language (WML); and, just as Web 
            developers can use JavaScript to embed scripting functionality into 
            Web pages, device developers can use WMLScript to embed the same 
            kind of functionality into WML pages. WML is a subset of XML and 
            will look familiar to anyone versed in HTML or other markup 
            languages. One unique feature of WML to keep in mind is that of a <I 
            xmlns:fo="http://www.w3.org/1999/XSL/Format" 
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">deck</I> of <I 
            xmlns:fo="http://www.w3.org/1999/XSL/Format" 
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">cards;</I> 
            while a single HTML document displays as a single Web document, a 
            single WML document can contain multiple cards. The WAP device 
            screen will only display a single card at a time. Some WML and 
            WMLScript links are available in the <A 
            href="http://www-106.ibm.com/developerworks/java/library/wi-entwap/?open&amp;l=766,t=grj,p=WAPjApps#resources" 
            xmlns:fo="http://www.w3.org/1999/XSL/Format" 
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">Resources</A> 
            section below. </TD></TR></TBODY></TABLE></P>
      <P>Some would argue that the servlets do not need to be changed, either: a 
      simple redirection of their output to JSPs generating dynamic WML pages 
      should be sufficient. However, a servlet cannot differentiate between 
      requests coming from desktop computers and requests coming from WAP 
      devices; since WAP applications may not implement all the functionality of 
      a Web-based architecture, it is important that there be no confusion in 
      this regard. Hence, developers usually design new servlets for WAP 
      applications. However, these servlets will, in most cases, be very similar 
      to servlets that provide similar functionality over a Web-based 
      architecture. </P>
      <P>Another component that does not appear in Figure 1 but is crucial to 
      WAP applications is a <I xmlns:fo="http://www.w3.org/1999/XSL/Format" 
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">WAP gateway.</I> 
      This component is responsible for conversions from the WAP stack to the 
      Internet stack and vice versa. </P>
      <P>Figure 2 is a revised version of Figure 1, showing the structure for a 
      J2EE application with a WAP device as a client:</P>
      <P><A name=N1007A><B>Figure 2. WAP/J2EE application 
      structure</B></A><BR><IMG height=176 alt="WAP/J2EE application structure" 
      src="WAP builds Java applications.files/wapj2ee-fig2.gif" width=489 
      xmlns:fo="http://www.w3.org/1999/XSL/Format" 
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"></P>
      <P>As noted, all requests to the Web server from a WAP client must be 
      routed through a WAP gateway. Although a WAP gateway can also act as a WAP 
      server by hosting WML/WMLScript pages, it is more convenient to host these 
      on the Web server.</P>
      <P>There are many deployment scenarios for the WAP gateway. For most WAP 
      applications, the gateway is either deployed by the ISP or by the company 
      hosting the application. The latter scenario is the more secure choice, as 
      we'll see later; however, if users want general-purpose network access on 
      their WAP devices, an internal WAP gateway can be an inconvenience. Most 
      non-ISPs would not want their gateways to be used to access sites other 
      than their own; hence, to visit other sites, users will have to use an 
      ISP's gateway. But for every gateway used by the WAP client, the user has 
      to define a different connection, just as one does in Windows 98's Dialup 
      Networking -- and the number of such connections available on each device 
      is usually limited. This adds inconvenience for the user and it also 
      blocks a connection in a WAP device for access to a single site.</P>
      <P><A name=N1008D><SPAN class=atitle2>WAP application design 
      considerations</SPAN></A><BR>A developer used to building J2EE apps for 
      desktop clients will run into some new challenges when working with WAP. 
      The following are some questions you might find yourself asking while 
      building a WAP application.</P>
      <P><I xmlns:fo="http://www.w3.org/1999/XSL/Format" 
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">How many lines can I 
      display on a screen?</I> <BR xmlns:fo="http://www.w3.org/1999/XSL/Format" 
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">Actually, there is 
      no specific limitation on the number of lines that can be displayed as 
      long as the maximum deck size limit (which varies from device to device) 
      is not exceeded. However, in order to avoid too much scrolling, five to 
      seven lines per screen (i.e., card) is a good limit.</P>
      <P><I xmlns:fo="http://www.w3.org/1999/XSL/Format" 
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">What security issues 
      should I consider?</I> <BR xmlns:fo="http://www.w3.org/1999/XSL/Format" 
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">Some phones do not 
      support the <CODE>POST</CODE> method of sending form data. Hence, a 
      username and password must be sent by the <CODE>GET</CODE> method. At the 
      WAP gateway, if the logging function is activated and if the request is 
      logged, the administrator will be able to see the username and password. 
      This would be particularly problematic if the gateway were hosted by an 
      ISP or other third party.</P>
      <P>Even a secure connection does not completely eliminate insecurity. The 
      data sent to the WAP gateway is encrypted using WTLS (Wireless Transport 
      Layer Security), which uses the same algorithm as standard TLS. However, 
      the data sent to the WAP gateway is binary encoded (for WAP), so the 
      encrypted data must be decrypted and re-encrypted using TLS for the 
      Internet. For some time, the sensitive data is in clear text on the WAP 
      gateway. A hacker could perform a memory dump at the right time and then 
      gain access to this sensitive data.</P>
      <P>As noted, one way to get around this problem is to have the WAP gateway 
      at your own company's end instead of at the ISP's. In that case, a trusted 
      person could handle the gateway and the logging function could be turned 
      off.</P>
      <P>You could also use a custom encryption algorithm to encrypt the 
      username and the password at the client end using WMLScript. This is 
      possible only if you use simple algorithms; WMLScript is not powerful 
      enough to support algorithms of the DES category. </P>
      <P><I xmlns:fo="http://www.w3.org/1999/XSL/Format" 
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">How can I maintain a 
      session?</I> <BR xmlns:fo="http://www.w3.org/1999/XSL/Format" 
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">WAP clients do not 
      support cookies. Thus, in order to maintain information about the client 
      on the server side as he navigates through the various pages of your site, 
      a session ID must be passed as a parameter along with every request sent 
      to the server. The parameter name for this session ID is different for 
      different servlet engines.</P>
      <P>Sometimes, the default session ID length increases the length of each 
      request considerably. As a result, the client or the WAP gateway may 
      reject the request as a malformed URL. It is necessary to reduce the 
      length of the session ID. See the documentation of the servlet engine that 
      you are using for the parameter name of the session ID. If you are 
      experiencing malformed URL errors, this documentation should also offer 
      guidance on reducing the length of the session ID value.</P>
      <P><A name=N100C0><SPAN class=atitle2>Building the sample 
      application</SPAN></A><BR>XYZ Ltd. manufactures PDAs, wearable computers, 
      and other pervasive computing devices. The company's sales staff visits 
      clients to offer live demonstrations of XYZ's products; for some of these 
      demonstrations, salespeople must travel great distances to reach client 
      sites. How will they receive client lists and other vital data while on 
      the road?</P>
      <P>Using e-mail for this purpose would require bulky, expensive laptops or 
      never-ending searches for Internet cafés; using fax machines at client 
      sites would be even more impractical. Instead, XYZ's sales crew will 
      receive their data via WAP-enabled devices, such as mobile phones or PDAs. 
      Using a mobile device, the salesperson can provide immediate feedback to 
      the company on the status of the client visit. The company could then 
      arrange immediate delivery of goods to the client and maintain current 
      sales statistics.</P>