3. Example: Automatically Reloading Pages as
Content Changes
Here is an example that lets you ask for a list of some large prime numbers.
Since this may take some time for very large numbers (e.g. 150 digits),
the servlet immediately returns the results found so far, but then keeps
calculating, using a low-priority thread so that it won't degrade Web server
performance. If the calculations are not complete, it instructs the browser to ask for
a new page in a few seconds by sending it a Refresh header.
If you want to try it out yourself, start with
the HTML front-end.
Note that, in addition to illustrating the value of HTTP response headers,
this example shows two other valuable servlet capabilities. First, it shows
that servlets can handle multiple simultaneous connections, each in their own
thread. In this case it maintains a Vector of previous requests for
prime calculations, matching the current request to previous ones by looking
at the number of primes (length of list) and number of digits (length of each
prime), and synchronizing all access to this list. Secondly, it shows how easy
it is for servlets to maintain state between requests, something that is
cumbersome to implement in traditional CGI and many CGI alternatives. This
lets the browser access the ongoing calculations when reloading the page,
plus permits the servlet to keep a list of the N most recently requested
results, returning them immediately if a new request specifies the same
parameters as a recent request. We'll discuss persistent state even further
in a later section.
3.1 PrimeNumbers.java
(Download source code)
Note: also uses ServletUtilities.java,
shown earlier, PrimeList.java for
creating a Vector of prime numbers in a background thread, and
Primes.java for generating large random
numbers of type BigInteger and checking if they are prime.
package hall;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.*;
public class PrimeNumbers extends HttpServlet {
private static Vector primeListVector = new Vector();
private static 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);
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) +
"<BODY BGCOLOR=\"#FDF5E6\">\n" +
"<H2 ALIGN=CENTER>" + title + "</H2>\n" +
"<H3>Primes found with " + numDigits +
" or more digits: " + numCurrentPrimes + ".</H3>");
if (isLastResult)
out.println("<B>Done searching.</B>");
else
out.println("<B>Still looking for " + numPrimesRemaining +
" more<BLINK>...</BLINK></B>");
out.println("<OL>");
for(int i=0; i<numCurrentPrimes; i++) {
out.println(" <LI>" + currentPrimes.elementAt(i));
}
out.println("</OL>");
out.println("</BODY></HTML>");
}
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 length of 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);
}
}
}
3.3 PrimeNumbers.html
Note: click left on the
source code link to try this out yourself. Click right to download
a copy of the HTML file.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<TITLE>Finding Large Prime Numbers</TITLE>
</HEAD>
<BODY BGCOLOR="#FDF5E6">
<H2 ALIGN="CENTER">Finding Large Prime Numbers</H2>
<BR><BR>
<CENTER>
<FORM ACTION="/servlet/hall.PrimeNumbers">
<B>Number of primes to calculate:</B>
<INPUT TYPE="TEXT" NAME="numPrimes" VALUE=25 SIZE=4><BR>
<B>Number of digits:</B>
<INPUT TYPE="TEXT" NAME="numDigits" VALUE=150 SIZE=3><BR>
<INPUT TYPE="SUBMIT" VALUE="Start Calculating">
</FORM>
</CENTER>
</BODY>
</HTML>
3.4 Front End
3.5 Intermediate Result
3.6 Final Result
This tutorial is now available as a book: Core Servlets and JavaServer Pages by Marty Hall, published by Sun Microsystems Press.
Read all about it at CoreServlets.com
Server-Side Web Applications using Java Servlets versions 2.1/2.2 and JavaServer Pages (JSP) version 1.0: A Tutorial
© 1999-2000 Marty Hall.
All source code freely available for unrestricted use.
Created for work in the Research and Technology Development Center of the Johns Hopkins University Applied Physics Lab, for courses in the Johns Hopkins Part-Time MS Program in Computer Science, and for various industry seminars and on-site Java short courses.
Please note that this is a first draft of the tutorial, so please send corrections, comments, and suggestions to me at hall@apl.jhu.edu.
Reprinted with permission from the author. Click here to visit the original version
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