Apr 28

PrimeNumbers.java Servlet that processes a request to generate n prime numbers, each with at least m digits. If these results are not complete, it sends a Refresh header instructing the browser to ask for new results a little while later. Uses the Primes #Programming Code Examples #Java/J2EE/J2ME #Servlet 

PrimeNumbers.java  Servlet that processes a request to generate n prime numbers, each with at least m digits. If these results are not complete, it sends a Refresh header instructing the browser to ask for new results a little while later. Uses the Primes, PrimeList, and ServletUtilities  classes.

package cwp;

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.
 *  

 *  Taken from Core Web Programming Java 2 Edition
 *  from Prentice Hall and Sun Microsystems Press,
 *  .
 *  May be freely used or adapted.
 */

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");
    }
    response.setContentType("text/html");
    PrintWriter out = response.getWriter();
    String title = "Some " + numDigits + "-Digit Prime Numbers";
    out.println(ServletUtilities.headWithTitle(title) +
                "n" +
                "
" + title + "
n" +
                "
Primes found with " + numDigits +
                " or more digits: " + numCurrentPrimes +
                ".
");
    if (isLastResult)
      out.println("Done searching.");
    else
      out.println("Still looking for " + numPrimesRemaining +
                  " more...");
    out.println("
    ");
    for(int i=0; i<numcurrentprimes ; i++) {
      out.println("  
  1. " + currentPrimes.elementAt(i));
    }
    out.println("
");
    out.println("");
  }

  public void doPost(HttpServletRequest request,
                     HttpServletResponse response)
      throws ServletException, IOException {
    doGet(request, response);
  }

  // See if there is an existing ongoing or completed
  // calculation with the same number of primes and number
  // of digits per prime. If so, return those results instead
  // of starting a new background thread. Keep this list
  // small so that the Web server doesn't use too much memory.
  // Synchronize access to the list since there may be
  // multiple simultaneous requests.

  private PrimeList findPrimeList(Vector primeListVector,
                                  int numPrimes,
                                  int numDigits) {
    synchronized(primeListVector) {
      for(int i=0; i<primelistvector .size(); i++) {
        PrimeList primes =
          (PrimeList)primeListVector.elementAt(i);
        if ((numPrimes == primes.numPrimes()) &&
            (numDigits == primes.numDigits()))
          return(primes);
      }
      return(null);
    }
  }
}


Primes.java

package cwp;

import java.math.BigInteger;

/** A few utilities to generate a large random BigInteger,
 *  and find the next prime number above a given BigInteger.
 *  

 *  Taken from Core Web Programming Java 2 Edition
 *  from Prentice Hall and Sun Microsystems Press,
 *  .
 *  May be freely used or adapted.
 */

public class Primes {
  // Note that BigInteger.ZERO was new in JDK 1.2, and 1.1
  // code is being used to support the most servlet engines.
  private static final BigInteger ZERO = new BigInteger("0");
  private static final BigInteger ONE = new BigInteger("1");
  private static final BigInteger TWO = new BigInteger("2");

  // Likelihood of false prime is less than 1/2^ERR_VAL
  // Assumedly BigInteger uses the Miller-Rabin test or
  // equivalent, and thus is NOT fooled by Carmichael numbers.
  // See section 33.8 of Cormen et al's Introduction to
  // Algorithms for details.
  private static final int ERR_VAL = 100;

  public static BigInteger nextPrime(BigInteger start) {
    if (isEven(start))
      start = start.add(ONE);
    else
      start = start.add(TWO);
    if (start.isProbablePrime(ERR_VAL))
      return(start);
    else
      return(nextPrime(start));
  }

  private static boolean isEven(BigInteger n) {
    return(n.mod(TWO).equals(ZERO));
  }

  private static StringBuffer[] digits =
    { new StringBuffer("0"), new StringBuffer("1"),
      new StringBuffer("2"), new StringBuffer("3"),
      new StringBuffer("4"), new StringBuffer("5"),
      new StringBuffer("6"), new StringBuffer("7"),
      new StringBuffer("8"), new StringBuffer("9") };

  private static StringBuffer randomDigit() {
    int index = (int)Math.floor(Math.random() * 10);
    return(digits[index]);
  }

  public static BigInteger random(int numDigits) {
    StringBuffer s = new StringBuffer("");
    for(int i=0; i 0)
      numDigits = Integer.parseInt(args[0]);
    else
      numDigits = 150;
    BigInteger start = random(numDigits);
    for(int i=0; i<50; i++) {
      start = nextPrime(start);
      System.out.println("Prime " + i + " = " + start);
    }
  }
}



PrimeList.java
package cwp;

import java.util.*;
import java.math.BigInteger;

/** Creates a Vector of large prime numbers, usually in
 *  a low-priority background thread. Provides a few small
 *  thread-safe access methods.
 *  

 *  Taken from Core Web Programming Java 2 Edition
 *  from Prentice Hall and Sun Microsystems Press,
 *  .
 *  May be freely used or adapted.
 */

public class PrimeList implements Runnable {
  private Vector primesFound;
  private int numPrimes, numDigits;

  /** Finds numPrimes prime numbers, each of which are
   *  numDigits long or longer. You can set it to only
   *  return when done, or have it return immediately,
   *  and you can later poll it to see how far it
   *  has gotten.
   */

  public PrimeList(int numPrimes, int numDigits,
                   boolean runInBackground) {
    // Using Vector instead of ArrayList
    // to support JDK 1.1 servlet engines
    primesFound = new Vector(numPrimes);
    this.numPrimes = numPrimes;
    this.numDigits = numDigits;
    if (runInBackground) {
      Thread t = new Thread(this);
      // Use low priority so you don't slow down server.
      t.setPriority(Thread.MIN_PRIORITY);
      t.start();
    } else {
      run();
    }
  }

  public void run() {
    BigInteger start = Primes.random(numDigits);
    for(int i=0; i<numprimes ; i++) {
      start = Primes.nextPrime(start);
      synchronized(this) {
        primesFound.addElement(start);
      }
    }
  }

  public synchronized boolean isDone() {
    return(primesFound.size() == numPrimes);
  }

  public synchronized Vector getPrimes() {
    if (isDone())
      return(primesFound);
    else
      return((Vector)primesFound.clone());
  }

  public int numDigits() {
    return(numDigits);
  }

  public int numPrimes() {
    return(numPrimes);
  }

  public synchronized int numCalculatedPrimes() {
    return(primesFound.size());
  }
}

ServerUtilities.java

package cwp;

import javax.servlet.*;
import javax.servlet.http.*;

/** Some simple time savers. Note that most are static methods.
 *  

 *  Taken from Core Web Programming Java 2 Edition
 *  from Prentice Hall and Sun Microsystems Press,
 *  .
 *  May be freely used or adapted.
 */

public class ServletUtilities {
  public static final String DOCTYPE =
    "";

  public static String headWithTitle(String title) {
    return(DOCTYPE + "n" +
           "n" +
           "" + title + "n");
  }

  /** Read a parameter with the specified name, convert it
   *  to an int, and return it. Return the designated default
   *  value if the parameter doesn't exist or if it is an
   *  illegal integer format.
  */

  public static int getIntParameter(HttpServletRequest request,
                                    String paramName,
                                    int defaultValue) {
    String paramString = request.getParameter(paramName);
    int paramValue;
    try {
      paramValue = Integer.parseInt(paramString);
    } catch(NumberFormatException nfe) { // null or bad format
      paramValue = defaultValue;
    }
    return(paramValue);
  }

  /** Given an array of Cookies, a name, and a default value,
   *  this method tries to find the value of the cookie with
   *  the given name. If there is no cookie matching the name
   *  in the array, then the default value is returned instead.
   */

  public static String getCookieValue(Cookie[] cookies,
                                      String cookieName,
                                      String defaultValue) {
    if (cookies != null) {
      for(int i=0; i<cookies .length; i++) {
        Cookie cookie = cookies[i];
        if (cookieName.equals(cookie.getName()))
          return(cookie.getValue());
      }
    }
    return(defaultValue);
  }

  /** Given an array of cookies and a name, this method tries
   *  to find and return the cookie from the array that has
   *  the given name. If there is no cookie matching the name
   *  in the array, null is returned.
   */

  public static Cookie getCookie(Cookie[] cookies,
                                 String cookieName) {
    if (cookies != null) {
      for(int i=0; i<cookies.length; i++) {
        Cookie cookie = cookies[i];
        if (cookieName.equals(cookie.getName()))
          return(cookie);
      }
    }
    return(null);
  }

  /** Given a string, this method replaces all occurrences of
   *  '' with
   *  '>', and (to handle cases that occur inside attribute
   *  values), all occurrences of double quotes with
   *  '"' and all occurrences of '&' with '&'.
   *  Without such filtering, an arbitrary string
   *  could not safely be inserted in a Web page.
   */

  public static String filter(String input) {
    StringBuffer filtered = new StringBuffer(input.length());
    char c;
    for(int i=0; i<input .length(); i++) {
      c = input.charAt(i);
      if (c == '') {
        filtered.append(">");
      } else if (c == '"') {
        filtered.append(""");
      } else if (c == '&') {
        filtered.append("&");
      } else {
        filtered.append(c);
      }
    }
    return(filtered.toString());
  }
}

Note: Brought from our old site: http://www.salearningschool.com/example_codes/ on Jan 2nd, 2017 From: http://sitestree.com/?p=10251
Categories:Programming Code Examples, Java/J2EE/J2ME, Servlet
Tags:Java/J2EE/J2MEServlet
Post Data:2017-01-02 16:04:28

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