Java for the WWW November 2012Slide [email protected] Tag Libraries What are they?...

Java for the WWW

November 2012 Slide [email protected]

Tag Libraries

• What are they?– Custom libraries

• Custom tags – see examples in following slides

– Standard Tag Library (JSTL) – recommend that you read the following:

• http://today.java.net/pub/a/today/2003/10/07/jstl1.html• http://today.java.net/pub/a/today/2003/11/27/jstl2.html

• Purpose and benefits for JSP web development– Lot of development and includes the AJAX technology but

this is outside the scope of this module

• For further reading on Tag libraries – here is nice tutorial to back up the following notes <link>

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March 2009 Slide [email protected]

Custom tags

• Custom tags are dynamic page elements.• This means they can provide functionality to a

web page using a syntax similar to normal HTML tags.

• The functionality is up to you…• They can provide an alternative (or an

additional) mechanism to JavaBeans for dynamic scripting.

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What do you need?

• There are two components to a tag:– A Java tag handler class that implements the tag’s

functionality.– A Tag Library Descriptor (TLD) in the form of an

XML file describing the tag.

• By including this tag on a JSP page you have access to its functionality.

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Java handler class

• Yes, you have to code the functionality in Java.

• So why bother? Why not just create a JavaBean instead?– For standard web page scripters, tags are more

familiar and provide a simpler mechanism.– For web application developers, JavaBeans

integrate more fully with the full application environment so are more powerful.

• Your choice….

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Making the handler

• You Java class needs to extend the TagSupport class.

• This class is only available in the Enterprise version of the Java Development Kit (from version 1.3.1).

• Or, the Java Web Services Developer Pack.

• However, you just need the lib/j2ee.jap file (in the case of J2EE) or the lib/servlet.jar for the JWSDP.

• You can then extract the javax.servlet.jsp.tagext class from the .jar file (using WinZip etc.) and put it into folder containing your tag classes – see screen grab on next powerpoint

Java for the WWW


Java for the WWW


Making the handler

• Once you have your custom tag Java code, you need to compile it and put the resultant .class file in your WEB-INF/classes folder.

• For the examples, this would go in:– WEB-INF/classes/mytags

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Example handler

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Tag Library Descriptor

• A TLD is a (fairly) simple XML file that describes a set of custom tags.

• You can put as many or as few custom tags in a TLD and more than one TLD can be used in a single project.

• In principle, you could create many useful tags and zip them into one .jar file for easy re-usability.

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Slide [email protected]

Example Tag Descriptor

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Using your custom tag

• To use your custom tag, you need a JSP page.

• You need to tell your JSP where to find your tag library so you use a page directive:

<%@ taglib uri=“WEB-INF/nameofdescriptor.tld” prefix=“something” %>

• Anywhere in your JSP page, you can then use your custom tag(s):

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Example with a JSP

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The use of tag libraries is all about...

• Re-usability

• Defining things once and making them available for use in more than one place

• Simplification

• Bespoke JSP work

• Or using existing tag libraries available for use - See the following

Jsptags.comJspinJakata Apache Taglibs project

Java for the WWW November 2012Slide [email protected] Tag Libraries What are they?...

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