The fn:toUpperCase function is a JSTL function that converts the characters in a string to uppercase. The function takes a single argument, which is the input string.

The syntax of the fn:toUpperCase function is as follows:

${fn:toUpperCase(string)}

where string is the input string.

For example, the following expression converts the string "Hello World" to all uppercase characters:

${fn:toUpperCase("Hello World")}

This expression returns the string "HELLO WORLD".

You can also use variables instead of literals for the argument. For example, if you have a variable text containing a string, you can convert the string to uppercase using the following expression:

${fn:toUpperCase(text)}

This expression returns the string stored in the text variable with all characters converted to uppercase.

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" %>