The JSTL (JavaServer Pages Standard Tag Library) fn tag library provides a set of functions for manipulating strings, collections, and other objects in JSP (JavaServer Pages) pages. One of the functions in the fn tag library is the fn:escapeXml function, which is used to escape special characters in a string to their corresponding XML entities.

The fn:escapeXml function takes a single argument, which is the string to be escaped. The function returns a new string with the special characters replaced by their corresponding XML entities. The syntax of the function is as follows:

${fn:escapeXml(string)}

where string is the string to be escaped.

For example, to escape the special characters in the string "<foo> & bar", you would use the following expression:

${fn:escapeXml('<foo> & bar')}

This expression returns the string "<foo> & bar", where the "<" and ">" characters have been replaced by their XML entities "<" and ">", and the "&" character has been replaced by its XML entity "&".

You can also use variables instead of literals for the string argument. For example, if you have a variable text containing a string, you can escape its special characters using the following expression:

${fn:escapeXml(text)}

This expression returns a new string with the special characters in the variable text replaced by their corresponding XML entities.

Note that the fn tag library must be imported at the beginning of the JSP page using the following tag:

<%@ taglib prefix="fn" uri="http://java.sun.com/jsp/jstl/functions" %>

This imports the fn tag library and assigns it a prefix of fn.