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" + 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
* 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
* 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" +
"\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' 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') {
filtered.append(">");
} else if (c == '"') {
filtered.append(""");
} else if (c == '&') {
filtered.append("&");
} else {
filtered.append(c);
}
}
return(filtered.toString());
}
}
