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<TITLE>The Quintessential Custom Tag (Java Developers Almanac Example)
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<B><A HREF="../javax.servlet.jsp.tagext/pkg.html">javax.servlet.jsp.tagext</A></B><font color="#666666" class="xsmall-font">
[1 examples]
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<h3>e1070. 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 <font color="#0066ff"><i>com.mycompany</i></font>;
import javax.servlet.jsp.*;
import javax.servlet.jsp.tagext.*;
// This tag simply emits the current date and time.
public class <font color="#0066ff"><i>MyTagHandler</i></font> 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> <?xml version="1.0" encoding="ISO-8859-1" ?>
<!DOCTYPE taglib PUBLIC "-//Sun Microsystems, Inc.//DTD JSP Tag Library 1.2//EN"
"http://java.sun.com/dtd/web-jsptaglibrary_1_2.dtd">
<taglib>
<%-- Version of this tag library; it should have the form 123 or 123.456 --%>
<tlib-version>0.96</tlib-version>
<%-- The required version of JSP for this tag library --%>
<jsp-version>1.2</jsp-version>
<%-- The preferred prefix for this tag library --%>
<short-name>my</short-name>
<%-- This is the URI that uniquely identifies this tag library.
The URI does not have to exist; it is merely an identifier --%>
<uri>http://com.mycompany/taglib/mytaglib</uri>
<%-- This name is used by tools to represent this tag library --%>
<display-name>My Tag Library</display-name>
<description>This is a description of the tag library.</description>
<tag>
<%-- Name of tag in this library --%>
<name>simple</name>
<tag-class>com.mycompany.MyTagHandler</tag-class>
<%-- Specify that the tag must not have a body --%>
<body-content>empty</body-content>
<description>
This is a description of the tag.
</description>
</tag>
</taglib>
</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> <%@ taglib uri="/WEB-INF/tld/mytaglib.tld" prefix="my" %>
<my:simple/>
</pre>
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