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320

Chapter 14 Creating Custom JSP Tag Libraries

One of the most common things to do in the attribute handler is to simply

store the attribute in a field that will later be used by 

doStartTag

 or a similar

method. For example, following is a section of a tag implementation that adds

support for the 

message

 attribute.

private String message = "Default Message";

public void setMessage(String message) {

this.message = message;

}

If the tag handler will be accessed from other classes, it is a good idea to

provide a 

getAttributeName

 method in addition to the 

setAttributeName

method. Only 

setAttributeName

 is required, however.

Listing 14.7 shows a subclass of 

SimplePrimeTag

 that adds support for

the 

length

 attribute. When such an attribute is supplied, it results in a call to

setLength

, which converts the input 

String

 to an 

int

 and stores it in the

len

 field already used by the 

doStartTag

 method in the parent class. 

Listing 14.7 PrimeTag.java 

package coreservlets.tags;

import javax.servlet.jsp.*;

import javax.servlet.jsp.tagext.*;

import java.io.*;

import java.math.*;

import coreservlets.*;

/** Generates an N digit random prime (default N = 50).

 *  Extends SimplePrimeTag, adding a length attribute

 *  to set the size of the prime. The doStartTag

 *  method of the parent class uses the len field

 *  to determine the approximate length of the prime.

 */

public class PrimeTag extends SimplePrimeTag {

  public void setLength(String length) {

    try {

      len = Integer.parseInt(length);

    } catch(NumberFormatException nfe) {

      len = 50;

    }

  }

}

Second edition of this book: www.coreservlets.com; Sequel: www.moreservlets.com.

Servlet and JSP training courses by book's author: courses.coreservlets.com.