"fn:join" is a string function of JSP Standard Tag Library(JSTL). This function is used to return string with all the elements of an array seperated by a "seperator".

    String:fn:join(array,seperator)

In the above syntax the "array" is an array, "seperator" is of data type "String".

    <%@ taglib uri="http://java.sun.com/jsp/jstl/core" 
    prefix="c" %>
    <%@ taglib uri="http://java.sun.com/jsp/jstl/functions" 
    prefix="fn" %>
    <c:set var="str" value="j o i n"/>
    <p><b>The string array "join" using "|"::</b></p>
    <c:set var="jnd" value="${fn:split(str,' ')}" />
    <c:out value="${fn:join(jnd,'|')}" />

    The string array "join" using "|"::
    j|o|i|n 

In the above example we have used the string "j o i n". All the elements of the string is split and stored in an array "jnd" like {"j" "o" "i" "n"}. This array is displayed as string seperated by "|".