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Java Tag Libraries / how they work / Tomcat Deployment

A Java (JSP) tag library is a set of additional XML tags that you can include in your Java Server Page in order to create objects and set and get the attributes of those objects. Let's see an example.

I can write the following into my HTML:

<jsp:useBean id="bigmac" class="shape.Cube"/>
<jsp:setProperty name="bigmac" property="wid" value="150"/>
<jsp:setProperty name="bigmac" property="hig" value="140"/>
<jsp:setProperty name="bigmac" property="dep" value="100"/>
The Big Mac box has a volume of
<b><jsp:getProperty name="bigmac" property="vol"/> and

And that will result in an output like:

The Big Mac box has a volume of 2100 cc and

How does it work?

The extra tags in the library generate Java Byte Code (code for the Java Virtual Machine) even though there is no actual Java source code present ... but there is enough information to describe what the code needs to do - you can consider it to be another language if you like! Here's what the alternative source code would look like:

<% shape.Cube bigmac = new shape.Cube();
out.print(bigmac.getVol); %gt;

The whole of the JSP is read by the Tomcat (or other) JSP engine, and all the stuff that's NOT in the Java tags is put into a great big out.print() and the whole thing is compiled up into a Servlet, which Tomcat then runs.

Where does the calculation logic come from

The constructor for the shape.Cube object, and the accessor methods, conform to a standard called a Java Bean. This specifies that there must be a zero parameter constructor, and that there must be methods to save attributes / properties with names starting with set, and to read back properties with names starting with get. The business logic is within that class, or classes which it in turn calls.

The extra class file is stored within your web application, in teh directory WEB-INF/classes ... and in there in a subdirectory named to match the package name.

The extra class / business logic is loaded when the JSP is loaded.

Here's the start of the source code of the Java Bean to give you an idea of what it looks like:

package shape;
public class Cube {
   float x;
   float y;
   float z;
public Cube () {
   x = 0.0f;
   y = 0.0f;
   z = 0.0f;
public void setWid(String val) {
   x = Float.parseFloat(val);

Full source code of the JSP - [here]
Full source code of the Java Bean - [here]

Some Notes

Although the JSP page is rechecked by Tomcat every few seconds, the classes that it calls up are not - they're cached and assumed to be unchanging. So that means you'll need to stop and restart the application if you change the classes, but not if you change the JSP itself.

Note that the attribute names start with a lower case letter, but the method names are camel case with a capital for the attribute name. For example a property called dep will call up methods setDep and getDep.

You can learn about deploying applications that include pages that use tag libraries like this (N.B. - also other tage libraries such as Struts and Spring) on our Deploying Apache / Tomcat course. And we can teach you how to write pages like this on private Java courses - please email for a few more details, telling me a little of your background.
(written 2009-02-11)

Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
J606 - Java Roadmap - Beyond the Fundamentals
  [4305] Learning to program in Java - yes, we can help. - (2014-09-26)
  [3043] Gathering information - logging - with log4j. First steps. - (2010-11-12)
  [1910] Java - Generics - (2008-11-27)
  [1562] Java Compiler - needs all classes, compiles them all too - (2008-03-03)
  [1049] Java 6, Apache Tomcat 6. - (2007-01-21)

J711 - Java in the Web Page
  [3049] Computer Graphics is fun - even if Java Applets are Old Hat - (2010-11-13)
  [1325] Java - Client side applet applications as well as server side - (2007-08-29)

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