JSP tricks to make templating easier?
As skaffman suggested, JSP 2.0 Tag Files are the bee's knees.
Let's take your simple example.
Put the following in WEB-INF/tags/wrapper.tag
<%@tag description="Simple Wrapper Tag" pageEncoding="UTF-8"%>
<html><body>
<jsp:doBody/>
</body></html>
Now in your example.jsp page:
<%@page contentType="text/html" pageEncoding="UTF-8"%>
<%@taglib prefix="t" tagdir="/WEB-INF/tags" %>
<t:wrapper>
<h1>Welcome</h1>
</t:wrapper>
That does exactly what you think it does.
So, lets expand upon that to something a bit more general.
WEB-INF/tags/genericpage.tag
<%@tag description="Overall Page template" pageEncoding="UTF-8"%>
<%@attribute name="header" fragment="true" %>
<%@attribute name="footer" fragment="true" %>
<html>
<body>
<div id="pageheader">
<jsp:invoke fragment="header"/>
</div>
<div id="body">
<jsp:doBody/>
</div>
<div id="pagefooter">
<jsp:invoke fragment="footer"/>
</div>
</body>
</html>
To use this:
<%@page contentType="text/html" pageEncoding="UTF-8"%>
<%@taglib prefix="t" tagdir="/WEB-INF/tags" %>
<t:genericpage>
<jsp:attribute name="header">
<h1>Welcome</h1>
</jsp:attribute>
<jsp:attribute name="footer">
<p id="copyright">Copyright 1927, Future Bits When There Be Bits Inc.</p>
</jsp:attribute>
<jsp:body>
<p>Hi I'm the heart of the message</p>
</jsp:body>
</t:genericpage>
What does that buy you? A lot really, but it gets even better...
WEB-INF/tags/userpage.tag
<%@tag description="User Page template" pageEncoding="UTF-8"%>
<%@taglib prefix="t" tagdir="/WEB-INF/tags" %>
<%@attribute name="userName" required="true"%>
<t:genericpage>
<jsp:attribute name="header">
<h1>Welcome ${userName}</h1>
</jsp:attribute>
<jsp:attribute name="footer">
<p id="copyright">Copyright 1927, Future Bits When There Be Bits Inc.</p>
</jsp:attribute>
<jsp:body>
<jsp:doBody/>
</jsp:body>
</t:genericpage>
To use this: (assume we have a user variable in the request)
<%@page contentType="text/html" pageEncoding="UTF-8"%>
<%@taglib prefix="t" tagdir="/WEB-INF/tags" %>
<t:userpage userName="${user.fullName}">
<p>
First Name: ${user.firstName} <br/>
Last Name: ${user.lastName} <br/>
Phone: ${user.phone}<br/>
</p>
</t:userpage>
But it turns you like to use that user detail block in other places. So, we'll refactor it.
WEB-INF/tags/userdetail.tag
<%@tag description="User Page template" pageEncoding="UTF-8"%>
<%@tag import="com.example.User" %>
<%@attribute name="user" required="true" type="com.example.User"%>
First Name: ${user.firstName} <br/>
Last Name: ${user.lastName} <br/>
Phone: ${user.phone}<br/>
Now the previous example becomes:
<%@page contentType="text/html" pageEncoding="UTF-8"%>
<%@taglib prefix="t" tagdir="/WEB-INF/tags" %>
<t:userpage userName="${user.fullName}">
<p>
<t:userdetail user="${user}"/>
</p>
</t:userpage>
The beauty of JSP Tag files is that it lets you basically tag generic markup and then refactor it to your heart's content.
JSP Tag Files have pretty much usurped things like Tiles etc., at least for me. I find them much easier to use as the only structure is what you give it, nothing preconceived. Plus you can use JSP tag files for other things (like the user detail fragment above).
Here's an example that is similar to DisplayTag that I've done, but this is all done with Tag Files (and the Stripes framework, that's the s: tags..). This results in a table of rows, alternating colors, page navigation, etc:
<t:table items="${actionBean.customerList}" var="obj" css_class="display">
<t:col css_class="checkboxcol">
<s:checkbox name="customerIds" value="${obj.customerId}"
onclick="handleCheckboxRangeSelection(this, event);"/>
</t:col>
<t:col name="customerId" title="ID"/>
<t:col name="firstName" title="First Name"/>
<t:col name="lastName" title="Last Name"/>
<t:col>
<s:link href="/Customer.action" event="preEdit">
Edit
<s:param name="customer.customerId" value="${obj.customerId}"/>
<s:param name="page" value="${actionBean.page}"/>
</s:link>
</t:col>
</t:table>
Of course the tags work with the JSTL tags (like c:if, etc.). The only thing you can't do within the body of a tag file tag is add Java scriptlet code, but this isn't as much of a limitation as you might think. If I need scriptlet stuff, I just put the logic in to a tag and drop the tag in. Easy.
So, tag files can be pretty much whatever you want them to be. At the most basic level, it's simple cut and paste refactoring. Grab a chunk of layout, cut it out, do some simple parameterization, and replace it with a tag invocation.
At a higher level, you can do sophisticated things like this table tag I have here.
Based on the same basic idea as in @Will Hartung's answer, here is my magic one-tag extensible template engine. It even includes documentation and an example :-)
WEB-INF/tags/block.tag:
<%--
The block tag implements a basic but useful extensible template system.
A base template consists of a block tag without a 'template' attribute.
The template body is specified in a standard jsp:body tag, which can
contain EL, JSTL tags, nested block tags and other custom tags, but
cannot contain scriptlets (scriptlets are allowed in the template file,
but only outside of the body and attribute tags). Templates can be
full-page templates, or smaller blocks of markup included within a page.
The template is customizable by referencing named attributes within
the body (via EL). Attribute values can then be set either as attributes
of the block tag element itself (convenient for short values), or by
using nested jsp:attribute elements (better for entire blocks of markup).
Rendering a template block or extending it in a child template is then
just a matter of invoking the block tag with the 'template' attribute set
to the desired template name, and overriding template-specific attributes
as necessary to customize it.
Attribute values set when rendering a tag override those set in the template
definition, which override those set in its parent template definition, etc.
The attributes that are set in the base template are thus effectively used
as defaults. Attributes that are not set anywhere are treated as empty.
Internally, attributes are passed from child to parent via request-scope
attributes, which are removed when rendering is complete.
Here's a contrived example:
====== WEB-INF/tags/block.tag (the template engine tag)
<the file you're looking at right now>
====== WEB-INF/templates/base.jsp (base template)
<%@ page trimDirectiveWhitespaces="true" %>
<%@ taglib prefix="t" tagdir="/WEB-INF/tags" %>
<t:block>
<jsp:attribute name="title">Template Page</jsp:attribute>
<jsp:attribute name="style">
.footer { font-size: smaller; color: #aaa; }
.content { margin: 2em; color: #009; }
${moreStyle}
</jsp:attribute>
<jsp:attribute name="footer">
<div class="footer">
Powered by the block tag
</div>
</jsp:attribute>
<jsp:body>
<html>
<head>
<title>${title}</title>
<style>
${style}
</style>
</head>
<body>
<h1>${title}</h1>
<div class="content">
${content}
</div>
${footer}
</body>
</html>
</jsp:body>
</t:block>
====== WEB-INF/templates/history.jsp (child template)
<%@ page trimDirectiveWhitespaces="true" %>
<%@ taglib prefix="t" tagdir="/WEB-INF/tags" %>
<t:block template="base" title="History Lesson">
<jsp:attribute name="content" trim="false">
<p>${shooter} shot first!</p>
</jsp:attribute>
</t:block>
====== history-1977.jsp (a page using child template)
<%@ page trimDirectiveWhitespaces="true" %>
<%@ taglib prefix="t" tagdir="/WEB-INF/tags" %>
<t:block template="history" shooter="Han" />
====== history-1997.jsp (a page using child template)
<%@ page trimDirectiveWhitespaces="true" %>
<%@ taglib prefix="t" tagdir="/WEB-INF/tags" %>
<t:block template="history" title="Revised History Lesson">
<jsp:attribute name="moreStyle">.revised { font-style: italic; }</jsp:attribute>
<jsp:attribute name="shooter"><span class="revised">Greedo</span></jsp:attribute>
</t:block>
--%>
<%@ tag trimDirectiveWhitespaces="true" %>
<%@ tag import="java.util.HashSet, java.util.Map, java.util.Map.Entry" %>
<%@ tag dynamic-attributes="dynattributes" %>
<%@ attribute name="template" %>
<%
// get template name (adding default .jsp extension if it does not contain
// any '.', and /WEB-INF/templates/ prefix if it does not start with a '/')
String template = (String)jspContext.getAttribute("template");
if (template != null) {
if (!template.contains("."))
template += ".jsp";
if (!template.startsWith("/"))
template = "/WEB-INF/templates/" + template;
}
// copy dynamic attributes into request scope so they can be accessed from included template page
// (child is processed before parent template, so only set previously undefined attributes)
Map<String, String> dynattributes = (Map<String, String>)jspContext.getAttribute("dynattributes");
HashSet<String> addedAttributes = new HashSet<String>();
for (Map.Entry<String, String> e : dynattributes.entrySet()) {
if (jspContext.getAttribute(e.getKey(), PageContext.REQUEST_SCOPE) == null) {
jspContext.setAttribute(e.getKey(), e.getValue(), PageContext.REQUEST_SCOPE);
addedAttributes.add(e.getKey());
}
}
%>
<% if (template == null) { // this is the base template itself, so render it %>
<jsp:doBody/>
<% } else { // this is a page using the template, so include the template instead %>
<jsp:include page="<%= template %>" />
<% } %>
<%
// clean up the added attributes to prevent side effect outside the current tag
for (String key : addedAttributes) {
jspContext.removeAttribute(key, PageContext.REQUEST_SCOPE);
}
%>
I made quite easy, Django style JSP Template inheritance tag library. https://github.com/kwon37xi/jsp-template-inheritance
I think it make easy to manage layouts without learning curve.
example code :
base.jsp : layout
<%@page contentType="text/html; charset=UTF-8" %>
<%@ taglib uri="http://kwonnam.pe.kr/jsp/template-inheritance" prefix="layout"%>
<!DOCTYPE html>
<html lang="en">
<head>
<title>JSP Template Inheritance</title>
</head>
<h1>Head</h1>
<div>
<layout:block name="header">
header
</layout:block>
</div>
<h1>Contents</h1>
<div>
<p>
<layout:block name="contents">
<h2>Contents will be placed under this h2</h2>
</layout:block>
</p>
</div>
<div class="footer">
<hr />
<a href="https://github.com/kwon37xi/jsp-template-inheritance">jsp template inheritance example</a>
</div>
</html>
view.jsp : contents
<%@page contentType="text/html; charset=UTF-8" %>
<%@ taglib uri="http://kwonnam.pe.kr/jsp/template-inheritance" prefix="layout"%>
<layout:extends name="base.jsp">
<layout:put name="header" type="REPLACE">
<h2>This is an example about layout management with JSP Template Inheritance</h2>
</layout:put>
<layout:put name="contents">
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Proin porta,
augue ut ornare sagittis, diam libero facilisis augue, quis accumsan enim velit a mauris.
</layout:put>
</layout:extends>