Taglibs : Java Glossary

One of many add-ons to JSP. A library of standard functions so you can do things in JSP without having to write Java code. They have their own syntax unlike either HTML or Java. I consider the syntax unspeakably ugly. There are many such libraries, all generically called TagLibs. The one with the most official status is called JSTL (Java Standard Tag Library).

*.tld files

Besides writing some Java code to implement your tag, you must create a *.tld file containing xml to tell the womb a few facts about your tags.

Basic Taglib tld Tags
tag	purpose
<?xml	Which version of XML is being used and which encoding.
<taglib	Miscellaneous information about where to find a DTD to describe the xml grammar of the file
<description>	Surrounds some text documenting what this jsp-tag as whole is for.
<tag>	Surrounds tags describing one jsp tag.
<tag-class	qualified class name that implements javax.servlet.jsp.tagext. Tag to implement this jsp tag.
<teiclass	optional subclass of javax.servlet.jsp.tagext.TagExtraInfo.
<body-content	Can have the values empty, JSP, scriptless or tagdependent. A one-liner jsp tag would have an empty body. A jsp tag that surrounded some text would be set to JSP to evaluate the body of the tag.
<info>	Documenation on what this jsp tag is for.
<variable>	Surrounds tags describing one EL variable exposed to the calling page.
<attribute>	Surrounds tags describing one parameter of the jsp tag.
<name	Name of the keyword parameter used in the jsp files when the tag is invoked. Your handler will be passed the value of the parameter via its setXxx method where xxx matches the name of the keyword parameter.
<required	true if the parameter must be present.
<rtexprvalue	by default false. If true, the user may calculate this parameter’s value with a scriptlet.

I don’t see any way to pass arrays or varargs as parameters.

Java Code to Implement a Custom Tag

Your Java code to implement a custom tag must implement javax.servlet.jsp.tagext.Tag, either from scratch or by extending javax.servlet.jsp.tagext.SimpleTag or javax.servlet.jsp.tagex.TagSupport.

You must write getters and setters for each parameter, and a SimpleTagSupport.doTag or TagSupport.doStartTag method to generate the HTML from the parameter values.

Navigating

In SimpleTagSupport is a method getJspContext. From JspContext you can find a few other control objects such as JspWriter with getOut where you write the expanded HTML.

Oddly, TagSupport has a completely different way of finding associated information. TagSupport has a protected field called pageContext. From there you can find the other control objects such as: JspWriter with getOut where you write the expanded HTML, Servlet with getPage, ServletRequest with getRequest, ServletResponse with getResponse, ServletConfig with getServletConfig and ServletContext with getServletContext.

Most of the navigation in convoluted, but straightforward. Getting the URL of the page you are generating is trickier:

Including

There are several ways you might include boiler plate text:

<%@ include file="filename" %> : works at translation time. Included text may contain tags that are parsed at translate time.
<jsp:include page="some.jsp" %> : works at request time (runtime). some.jsp may contain tags, which are parsed on first use of some.jsp.
<c:import url="http://any.place.com/at/all" > : works at request time too, but takes URLs which is more flexible.
Your code to implement tags has Java at its disposal to find and include text. However, unless you compose *.jsp files on the fly, you won’t be able to parse included tags at requst time.

Learning More

Sun’s Product Info on JSP Tags

Sun’s EE docs on *.tld file format

SimpleTag interface : available:

on the web at java.Sun.com
on your local Windows J: drive in the EE SDK: J:\Program Files\Sun\docs\api\javax\servlet\jsp\tagext\SimpleTag.html

SimpleTagSupport : available:

on the web at java.Sun.com
on your local Windows J: drive in the EE SDK: J:\Program Files\Sun\docs\api\javax\servlet\jsp\tagext\SimpleTagSupport.html

JspContext : available:

on the web at java.Sun.com
on your local Windows J: drive in the EE SDK: J:\Program Files\Sun\docs\api\javax\servlet\jsp\JspContext.html

Tag interface : available:

on the web at java.Sun.com
on your local Windows J: drive in the EE SDK: J:\Program Files\Sun\docs\api\javax\servlet\jsp\tagext\Tag.html

TagSupport : available:

on the web at java.Sun.com
on your local Windows J: drive in the EE SDK: J:\Program Files\Sun\docs\api\javax\servlet\jsp\tagext\TagSupport.html

TagExtraInfo : available:

on the web at java.Sun.com
on your local Windows J: drive in the EE SDK: J:\Program Files\Sun\docs\api\javax\servlet\jsp\tagext\TagExtraInfo.html

PageContext : available:

on the web at java.Sun.com
on your local Windows J: drive in the EE SDK: J:\Program Files\Sun\docs\api\javax\servlet\jsp\PageContext.html

Servlet interface : available:

on the web at java.Sun.com
on your local Windows J: drive in the EE SDK: J:\Program Files\Sun\docs\api\javax\servlet\Servlet.html

ServletRequest interface : available:

on the web at java.Sun.com
on your local Windows J: drive in the EE SDK: J:\Program Files\Sun\docs\api\javax\servlet\ServletRequest.html

ServletResponse interface : available:

on the web at java.Sun.com
on your local Windows J: drive in the EE SDK: J:\Program Files\Sun\docs\api\javax\servlet\ServletResponse.html

ServletConfig interface : available:

on the web at java.Sun.com
on your local Windows J: drive in the EE SDK: J:\Program Files\Sun\docs\api\javax\servlet\ServletConfig.html

ServletContext interface : available:

on the web at java.Sun.com
on your local Windows J: drive in the EE SDK: J:\Program Files\Sun\docs\api\javax\servlet\ServletContext.html

Sun’s Developer Online Training on Servlet Fundamentals

javax.servlet.http.HttpServlet docs : available:

on the web at java.Sun.com
on your local Windows J: drive in the EE SDK: J:\Program Files\Sun\docs\api\javax\servlet\http\HttpServlet.html

javax.servlet.http package docs : available:

on the web at java.Sun.com
on your local Windows J: drive in the EE SDK: J:\Program Files\Sun\docs\api\javax\servlet\http\package-summary.html

javax.servlet package docs : available:

on the web at java.Sun.com
on your local Windows J: drive in the EE SDK: J:\Program Files\Sun\docs\api\javax\servlet\package-summary.html

Sun’s Product Info on Dummy Servlet Classes

Download EE docs: javax
Jakarta Taglibs
JSF
JSP
JSTL
Orionserver Taglibs tutoria
STRUTS
Taglibs tutorial
Tomcat

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