Are your logs being filled up with errors like:

ERROR - ParametersInterceptor.setParameters(204) | ParametersInterceptor - [setParameters]: Unexpected Exception caught setting '_' on 'class com.company.web.MyAction: Error setting expression '_' with value '[Ljava.lang.String;@1491ddc'

If so, it's because Struts 2 is parsing the query string/post data and trying to "set" a value for each parameter it finds.

I'm working on an Struts 2 application utilizing the jQuery JavaScript library for the UI and the Displaytag tag library for displaying tables.

With jQuery, I'm using the "no-cache" option (cache: false) on all Ajax calls, which adds "_=timestamp" to each request. Since I don't have a property called "_" in my Action class, I get an error (mentioned above) in my logs for each request.

Same with Displaytag, except the parameters causing errors are in the form of "d-#-X", where "#" is a unique id (usually 4 or 5 digits) and "X" is either p, s or o. They are used to determine the page (p) and/or the table sort order (o for asc/desc and s for which column).

The solution: configure Struts (struts.xml) to ignore these parameters:

<interceptors>
   <interceptor-stack name="defaultStack">
      <interceptor-ref name="params">
         <!--
            Excludes the jQuery no-cache _ parameter and
            the Displaytag d-#-X parameter(s)
         -->
         <param name="excludeParams">
            _,d-\d+?-[sop]
         </param>
      </interceptor-ref>
   </interceptor-stack>
</interceptors>

If you have a different parameters that are causing problems, just put them in the excludeParams node, comma separating multiple parameters.

In jQuery 1.2.1, when using the $.ajax function with cache: false, jQuery appends a parameter with the current timestamp to the URL. This parameter makes the URL unique and therefore prevents subsequent request from being retrieved from the browser cache.

However, the code that adds this ‘no-cache’ parameter does not check to see if it already exists and so under certain circumstances you can end up with URL’s that look like:

http://mysite.com/file.html?_=1196716041523&_=1196716462963&_=1196716464245

It certainly makes the URL unique

I opened a ticket and submitted a patch, which was quickly optimized by a jQuery developer, davidserduke.

It’s nice to see a project with responsive, helpful developers.