In the *.tld,what's the use of the tag " variable "

Question

For the jsp taglib technology.
In the *.tld file,accroding to the "http://java.sun.com/dtd/web-jsptaglibrary_1_2.dtd"
there is an optional tag "<variable>".
In some document , i saw an example as below:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE taglib PUBLIC "-//Sun Microsystems, Inc.//DTD JSP Tag Library 1.2//EN" "http://java.sun.com/dtd/web-jsptaglibrary_1_2.dtd">
<taglib>
<tlib-version>1.0</tlib-version>
<jsp-version>1.2</jsp-version>
<short-name>map</short-name>
<tag>
<variable>
<name-from-attribute>id</name-from-attribute>
<variable-class>java.util.Map</variable-class>
<scope>AT_BEGIN</scope>
</variable>
</tag>
</taglib>
What use of this?
Can anybody give me an simple example to explain this?
Thank u!

Answer 1

Thank u very much?
But i want to know, if I had set the <attribute> tag
as the example i had given.Do you mean attribute, or variable in the example you gave?
Here, I will give you a full example. This tag will set a new variable (given by an attribute name) a value provided by another attribute, and puts that variable into one of the four scopes (page, request, session, application)
You would use this tag like this:
<custom:setValue var="theVar" value="What I want" scope="page"/>
Where var = the name of the variable that will be exported
value = The value to assign to the variable
scope = the scope to put the variable into.
package tags;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.http.jsp.*;
import javax.servlet.http.jsp.tagext.*;
public class SetValueTag extends SimpleTagSupport
  private String varName, scope, value;
  public void setVar(String var) { varName = var; }
  public void setScope (String s) { scope = s; }
  public void setValue(String val) { value = val; }
  public void doTag() throws JspException, IOException
    PageContext context = (PageContext)getJspContext();
    if ("application".equals(scope)) context.setAttribute(varName, value, PageContext.APPLICATION_SCOPE);
    else if ("session".equals(scope)) context.setAttribute(varName, value, PageContext.SESSION_SCOPE);
    else if ("request".equals(scope)) context.setAttribute(varName, value, PageContext.REQUEST_SCOPE);
    else context.setAttribute(varName, value, PageContext.PAGE_SCOPE);
}The tag in your tld would look like this:
  <tag>
    <description>Set the value of a scoped variable</description>
    <name>setValue</name>
    <tag-class>tags.SetValueTag</tag-class>
    <body-content>empty</body-content>
    <attribute>
      <description>The name of the exported variable</description>
      <name>var</name>
      <required>true</required>
      <rtexprvalue>false</rtexprvalue>
      <type>java.lang.String</type>
    </attribute>
    <attribute>
      <description>The value to give the exported variable</description>
      <name>value</name>
      <required>true</required>
      <rtexprvalue>true</rtexprvalue>
      <type>java.lang.String</type>
    </attribute>
    <attribute>
      <description>The the scope of the exported variable</description>
      <name>scope</name>
      <required>false</required>
      <rtexprvalue>false</rtexprvalue>
      <type>java.lang.String</type>
    </attribute>
    <variable>
      <name-from-attribute>var</name-from-attribute>
      <variable-class>java.lang.String</variable-class>
      <scope>AT_BEGIN</scope>
      <declare>true</declare>
    </variable>
  </tag>>
Then what should i do in the TagHandler programme(A
javabean handle the tag)?
And what should i do in the JSP page ?
Can u give me an simple example?