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156

Chapter 7 Generating the Server Response: HTTP Response Headers

Listing 7.1  PrimeNumbers.java 

package coreservlets;

import java.io.*;

import javax.servlet.*;

import javax.servlet.http.*;

import java.util.*;

/** Servlet that processes a request to generate n

 *  prime numbers, each with at least m digits.

 *  It performs the calculations in a low priority background

 *  thread, returning only the results it has found so far.

 *  If these results are not complete, it sends a Refresh

 *  header instructing the browser to ask for new results a

 *  little while later. It also maintains a list of a

 *  small number of previously calculated prime lists

 *  to return immediately to anyone who supplies the

 *  same n and m as a recent completed computation.

 */

public class PrimeNumbers extends HttpServlet {

  private Vector primeListVector = new Vector();

  private int maxPrimeLists = 30;

  public void doGet(HttpServletRequest request,

                    HttpServletResponse response)

      throws ServletException, IOException {

    int numPrimes =

      ServletUtilities.getIntParameter(request,

                                       "numPrimes", 50);

    int numDigits =

      ServletUtilities.getIntParameter(request,

                                       "numDigits", 120);

    PrimeList primeList =

      findPrimeList(primeListVector, numPrimes, numDigits);

    if (primeList == null) {

      primeList = new PrimeList(numPrimes, numDigits, true);

      // Multiple servlet request threads share the instance

      // variables (fields) of PrimeNumbers. So

      // synchronize all access to servlet fields.

      synchronized(primeListVector) {

        if (primeListVector.size() >= maxPrimeLists)

          primeListVector.removeElementAt(0);

        primeListVector.addElement(primeList);

      }

    }

    Vector currentPrimes = primeList.getPrimes();

    int numCurrentPrimes = currentPrimes.size();

    int numPrimesRemaining = (numPrimes   numCurrentPrimes);

    boolean isLastResult = (numPrimesRemaining == 0);

    if (!isLastResult) {

      response.setHeader("Refresh", "5");

    }

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

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