filter not executing on static content request

I'm trying to implement making gwt apps crawlable, but the CrawlFilter isn't being executed, ever. What am I doing wrong? Is static content not subject to filters?

<web-app>
  <!-- Crawling servlet filter -->
  <filter>
    <filter-name>AjaxCrawlFilter</filter-name>
    <filter-class>com.bitdual.server.CrawlServlet<开发者_JAVA百科/filter-class>
  </filter>

  <filter-mapping>
    <filter-name>AjaxCrawlFilter</filter-name>
    <url-pattern>/*</url-pattern>
  </filter-mapping>

  <!-- Default page to serve -->
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
  </welcome-file-list>

</web-app>

Static content is served from an entirely different infrastructure (like a CDN) so I wouldn't be surprised if filters are not executed on content hosted statically.