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Chapter 14 Creating Custom JSP Tag Libraries

Listing 14.4 SimplePrimeTag.java 

package coreservlets.tags;

import javax.servlet.jsp.*;

import javax.servlet.jsp.tagext.*;

import java.io.*;

import java.math.*;

import coreservlets.*;

/** Generates a prime of approximately 50 digits.

 *  (50 is actually the length of the random number

 *  generated    the first prime above that number will

 *  be returned.)

 */

public class SimplePrimeTag extends TagSupport {

  protected int len = 50;

  public int doStartTag() {

    try {

      JspWriter out = pageContext.getOut();

      BigInteger prime = Primes.nextPrime(Primes.random(len));

      out.print(prime);

    } catch(IOException ioe) {

      System.out.println("Error generating prime: " + ioe);

    }

    return(SKIP_BODY);

  }

}

The Tag Library Descriptor File

The general format of a descriptor file is almost always the same: it should

contain an XML version identifier followed by a 

DOCTYPE

 declaration

followed by a 

taglib

 container element. To get started, just download a

sample from 

http://www.coreservlets.com/

. The important part to

understand is what goes in the 

taglib

 element: the 

tag

 element. For tags

without attributes, the 

tag

 element should contain four elements between

 and 

:

1.

name

, whose body defines the base tag name to which the prefix 

of the 

taglib

 directive will be attached. In this case, I use 

simplePrime

to assign a base tag name of 

simplePrime

.

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

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