com_mycompany_mytaghandler.html

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<TITLE>The Quintessential Custom Tag
(Java Developers Almanac Example)
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  <h3>
    e1069.  
    The Quintessential Custom Tag</h3>

A custom tag handler must implement the <code>Tag</code> interface.  This
interface is implemented by many convenient support classes from which
a custom tag handler can extend.  As a JSP page is executed and the
custom tag is encountered, the <code>doStartTag()</code> of the tag handler is
executed. If that method does not throw an exception, the tag
handler's <code>doEndTag()</code> method is then called.

<P> The JSP container may create many instances of the tag handler
but guarantees that an instance services only one request at a time.
A JSP container also typically reuses a tag handler instance and so
instance variables should be appropriately reset if necessary.

<P> This example implements a tag handler for the simplest type of
tag - one without attributes or a body.  This custom tag simply prints
the current time and day.


<pre>
    package com.mycompany;
    
    import javax.servlet.jsp.*;
    import javax.servlet.jsp.tagext.*;
    
    // This tag simply emits the current date and time.
    public class MyTagHandler extends TagSupport {
        public int doStartTag() throws JspException {
            try {
                // Print the current date and time
                pageContext.getOut().print(<font color="#0066ff"><i>""+new java.util.Date()</i></font>);
            } catch (java.io.IOException e) {
                throw new JspTagException("MyTag: "+e.getMessage());
            }
            return SKIP_BODY;
        }
    
        public int doEndTag() {
            return EVAL_PAGE;
        }
    }
</pre>
<P> A web application wishing to use the custom tag must not only
include the custom tag's class file but also its Tag Library
Descriptor (TLD) as well.  The TLD is a file that contains information
about one or more custom tags and is needed by a JSP container to
process pages that use a custom tag.  Here's the TLD file
(<code>mytaglib.tld</code>) for the custom tag:



<pre>
    &lt;?xml version="1.0" encoding="ISO-8859-1" ?&gt;
    &lt;!DOCTYPE taglib PUBLIC "-//Sun Microsystems, Inc.//DTD JSP Tag Library 1.2//EN"
        "http://java.sun.com/dtd/web-jsptaglibrary_1_2.dtd"&gt;
    &lt;taglib&gt;
      &lt;%-- Version of this tag library; it should have the form 123 or 123.456 --%&gt;
      &lt;tlib-version&gt;0.96&lt;/tlib-version&gt;
    
      &lt;%-- The required version of JSP for this tag library --%&gt;
      &lt;jsp-version&gt;1.2&lt;/jsp-version&gt;
    
      &lt;%-- The preferred prefix for this tag library --%&gt;
      &lt;short-name&gt;my&lt;/short-name&gt;
    
      &lt;%-- This is the URI that uniquely identifies this tag library.
           The URI does not have to exist; it is merely an identifier --%&gt;
      &lt;uri&gt;http://mycompany.com/taglib/mytaglib&lt;/uri&gt;
    
      &lt;%-- This name is used by tools to represent this tag library --%&gt;
      &lt;display-name&gt;My Tag Library&lt;/display-name&gt;
    
      &lt;description&gt;This is a description of the tag library.&lt;/description&gt;
    
      &lt;tag&gt;
        &lt;%-- Name of tag in this library --%&gt;
        &lt;name&gt;simple&lt;/name&gt;
        &lt;tag-class&gt;com.mycompany.MyTagHandler&lt;/tag-class&gt;
    
        &lt;%-- Specify that the tag must not have a body --%&gt;
        &lt;body-content&gt;empty&lt;/body-content&gt;
        &lt;description&gt;
            This is a description of the tag.
        &lt;/description&gt;
      &lt;/tag&gt;
    &lt;/taglib&gt;
</pre>
Here's an example of a JSP page that uses the custom tag.  The TLD
file is assumed to be in <code>WEB-INF/tld</code> and the
<code>MyTagHandler.class</code> file in <code>WEB-INF/classes/com/mycompany</code>:


<pre>
    &lt;%@ taglib uri="/WEB-INF/tld/mytaglib.tld" prefix="my" %&gt;
    
    &lt;my:simple/&gt;
</pre>

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