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JSP - What's a tag file?

Thu, 03/21/2013 - 00:20 |

admin

It's a way to include reusable code, that adds some of the flexibility of a tag handler. The code to include must be in a source with the *.tag extension.

This is an example for writing a letter, using a custom tag:

index.jsp:

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
    pageEncoding="ISO-8859-1"%>
<%@ taglib prefix="message" tagdir="/WEB-INF/tags" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
	<head>
	<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
		<title>Title here</title>
	</head>
	<body>
		<message:email dest="Arthur" author="mike">
			I'm writing to you because....
		</message:email>
	</body>
</html>

WEB-INF/tags/email.tag:

<%@ tag language="java" pageEncoding="ISO-8859-1"%>
<%@ attribute name="dest" required="true" rtexprvalue="true" %>
<%@ attribute name="author" required="true" rtexprvalue="true" %>
<%@ tag body-content="scriptless" %>

Dear ${dest}, <br/>
<jsp:doBody/>
<br/>
Bye<br/>
${author}<br/>

There are other two ways to include reusable code.

The first is by using the tag <jsp:include>. This is an example:

index.jsp:

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
    pageEncoding="ISO-8859-1"%>

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
	<head>
	<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
		<title>Title here</title>
	</head>
	<body>
		<jsp:include page="header.jsp">
			<jsp:param value="This is an header with a title" name="title"/>
		</jsp:include>
	</body>
</html>

header.jsp:

<h1>${param.title}</h1>

The second way is by using the directive <%@ include>. This is an example:

index.jsp:

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
    pageEncoding="ISO-8859-1"%>
<%@ taglib prefix="message" tagdir="/WEB-INF/tags" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
	<head>
	<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
		<title>Title here</title>
	</head>
	<body>
		<%@ include file="header.jsp" %>
	</body>
</html>

header.jsp:

<h1>This is an header</h1>

The difference between the two is that the directive is used at translation time, whereas the JSP action is executed at request time.

So if you just need a static include, then use the directive.

If you need a dynamic include and you need to pass parameters, then use the JSP action.

If you need something more sophisticated and similar to a custom tag, then use a tag file.