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:length function, which is used to get the length of a string or a collection.

The fn:length function takes one argument: the string or collection to get the length of. The function returns the length of the string or collection. The syntax of the function is as follows:

${fn:length(stringOrCollection)}

where stringOrCollection is the string or collection to get the length of.

For example, to get the length of the string "hello world", you would use the following expression:

${fn:length('hello world')}

This expression returns 11, which is the length of the string "hello world".

To get the length of a collection, you would use a similar expression, passing the collection as the argument. For example, to get the length of the collection [1, 2, 3], you would use the following expression:

${fn:length([1, 2, 3])}

This expression returns 3, which is the length of the collection.

You can also use variables instead of literals for the stringOrCollection argument. For example, if you have a variable text containing a string, and a variable list containing a collection, you can get their lengths using the following expressions:

${fn:length(text)}
${fn:length(list)}

These expressions return the lengths of the string and collection stored in the respective variables.

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.