How do I get hold of the HttpServletRequest?

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HttpServletRequest request = ServletActionContext.getRequest();
if (request != null)
{
     // do something here
}

You should always assume that ServletActionContext.getRequest() will return null. ServletActionContext is only populated if the request comes in through Struts. There are a number of circumstances in which it will not be populated, either because a web request has come in through some other path, or because there was no web request in the first place:

AJAX requests that come in via the DWR servlet
SOAP/XML-RPC requests
Scheduled tasks, including the sending of email notifications

Treat ServletActionContext as a bonus. If it's populated you can do neat things with it, but don't rely on it.