基于OSGi的web开发，我碰到的一个问题

在网上看了关于OSGi部署的文章(http://www.ibm.com/developerworks/cn/web/0907_osgiweb_liuqing)，我试的过程中，碰到一个问题：
在我的web工程中的plugin.xml文件如下:
<plugin>
   <extension
         point="org.eclipse.equinox.http.registry.servlets">
      <servlet
            alias="/validate"
            class="com.sogou.servlet.UserValidate"
            load-on-startup="false">          
      </servlet>
      <servlet
            alias="/*.jsp"
            class="org.eclipse.equinox.jsp.jasper.registry.JSPFactory:/">
      </servlet>
   </extension>
</plugin>
 
系统启动时报错：
org.eclipse.core.runtime.CoreException: Plug-in ViewWeb was unable to load
class com.sogou.servlet.UserValidate.
javax.servlet.ServletException: Plug-in ViewWeb was unable to load class
com.sogou.servlet.UserValidate.
at org.eclipse.equinox.http.registry.internal.ServletManager$ServletWrapper.initializeDelegate(ServletManager.java:195)
at org.eclipse.equinox.http.registry.internal.ServletManager$ServletWrapper.service(ServletManager.java:176)
at org.eclipse.equinox.http.servlet.internal.ServletRegistration.handleRequest(ServletRegistration.java:91)
at org.eclipse.equinox.http.servlet.internal.ProxyServlet.processAlias(ProxyServlet.java:110)
at org.eclipse.equinox.http.servlet.internal.ProxyServlet.service(ProxyServlet.java:60)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:810)
at org.eclipse.equinox.servletbridge.BridgeServlet.service(BridgeServlet.java:115)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:810)
at com.caucho.server.dispatch.ServletFilterChain.doFilter(ServletFilterChain.java:103)
at com.caucho.server.webapp.WebAppFilterChain.doFilter(WebAppFilterChain.java:187)
at com.caucho.server.dispatch.ServletInvocation.service(ServletInvocation.java:266)
at com.caucho.server.http.HttpRequest.handleRequest(HttpRequest.java:270)
at com.caucho.server.port.TcpConnection.run(TcpConnection.java:678)
at com.caucho.util.ThreadPool$Item.runTasks(ThreadPool.java:721)
at com.caucho.util.ThreadPool$Item.run(ThreadPool.java:643)
at java.lang.Thread.run(Thread.java:619)请教一下，这可能是什么原因引起的？