JSP EL allows you to use functions in expressions as well. These functions must be defined in custom tag libraries. As you will see soon, there are a number of handy functions built into the JSTL.
A function has the syntax ns:func(param1, param2, ...), where ns is the namespace of the function, func is the name of the function and param1 is the first parameter value. For example, the function fn:length, which is part of the JSTL library, returns the length of a string. You can use this function to determine the length of the query string with the following expression: