JSP/Servlet | JSTL(JSP Standard Tag Library)

Tags:

JSTL stands for Jsp Standard Tag Library. It is a set of tags that standardizes syntax used in JSP pages.

Why Is JSTL Needed?

When JSP(Java) code and HTML code are mixed together in a JSP page, the code becomes complex and difficult to maintain.

Compare the following code written only with JSP and the code written with JSTL.

Written Only with JSP

<%
    if (list.size() > 0) {
        for(int i = 0; i < list.size(); i++) {
            HashMap map = (HashMap)list.get(i);
%>
            제목 : <%=map.get("title")%>
<%
        }
    }
%>

Written with JSTL

<c:if test="${!empty list}">
    <c:forEach var="item" items="${list}">
        제목 : ${item.title}
    </c:forEach>
</c:if>

The JSTL version is clearly easier to read and understand.

JSTL Types

Core

Supports variables, flow control, and URL handling.

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

fmt

Displays locale, message formats, numbers, and date formats.

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

functions

Handles strings and collections.

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

How to Apply JSTL

Add the Library

You only need to add the jstl-1.2.jar file. If you use Maven, add the following dependency to pom.xml.

<!-- https://mvnrepository.com/artifact/javax.servlet/jstl -->
<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>jstl</artifactId>
    <version>1.2</version>
</dependency>

Declare the Tag Library

To use JSTL, you need a declaration inside the JSP file indicating that JSTL will be used.

The following directive declares the JSTL core, also known as the C tag.

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

Applied to a JSP file, it looks like this.

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<html>
<head>
    <title>JSTL</title>
</head>
<body>
Hello World!! JSTL
</body>
</html>

However, if you declare it this way, you must add the directive to every JSP file, which is inconvenient.

To include it in every JSP, set <include-prelude> in web.xml as follows.

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
    version="3.0">
 
... omitted ...

    <jsp-config>
        <jsp-property-group>
            <url-pattern>*.jsp</url-pattern>
            <el-ignored>false</el-ignored>
            <page-encoding>UTF-8</page-encoding>
            <scripting-invalid>false</scripting-invalid>
            <include-prelude>/WEB-INF/jsp/common/directive.jsp</include-prelude>
        </jsp-property-group>
    </jsp-config>

... omitted ...

</web-app>

The configured JSP file (directive.jsp) is always included in all JSP (*.jsp) files. Writing the JSTL directives there makes the setup easier.

<%@ page contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<%@ taglib prefix="c"   uri="http://java.sun.com/jsp/jstl/core"%>
<%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %>
<%@ taglib prefix="fn"  uri="http://java.sun.com/jsp/jstl/functions"%>