What exactly do you mean by "report"? Struts is a web framework, so its output are HTML pages (for the most part). Is that what you want to generate? The standard library for generating HTML reports (as well as PDF, and maybe a few other formats) is JasperReports.
Which version of Struts? In Struts 2 there's a plugin. In Struts 1 you'd have to create either a report file or stream and write it back to the response. Searching the web should give you several methods to do this.