// Try dividing the number by all odd
// Try dividing the number by all odd numbers between 3 and its sqrt double sqrt = Math.sqrt(candidate); for (long i = 3; i <= sqrt; i += 2) { if (candidate % i == 0) return false; // found a factor } // Wasn't evenly divisible, so it's prime return true; } public void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { res.setContentType( text/plain ); PrintWriter out = res.getWriter(); if (lastprime == 0) { out.println( Still searching for first prime ); } else { out.println( The last prime discovered was + lastprime); out.println( at + lastprimeModified); } } public void destroy() { searcher.stop(); } } The searcher thread begins its search in the init()method. Its latest find is saved in lastprime, along with the time it was found in in lastprimeModified. Each time a client accesses the servlet, the doGet() method reports the largest prime found so far and the time it was found. The searcher runs independently of client accesses; even if no one accesses the servlet it continues to find primes silently. If several clients access the servlet at the same time, they all see the same current status. Notice that the destroy()method stops the searcher thread.* This is very important! If a servlet does not stop its background threads, they continue to run until the virtual machine exits. Even when a servlet is reloaded (either explicitly or because its class file changed), its threads won't be stopped. Instead, it's likely that the new servlet will create extra copies of the background threads. And, at least with the Java Web Server, even explicitly restarting the web server service doesn't stop background threads because the Java Web Server virtual machine continues its execution. * Stopping threads using the stop()method as shown here is deprecated in JDK 1.2 in favor of a safer flag-based system, where a thread must periodically examine a "flag" variable to determine when it should stop, at which point it can clean up and return from its run()method. See the JDK documentation for details. Example source code can be found in an article titled "Scott's Solutions: Programming with threads in Java 1.2", written by Scott Oaks for Java Report Online, found at http://www.sigs.com/jro/features/9711/oaks.htmlf. Last Modified Times By now, we're sure you've learned that servlets handle GETrequests with the doGet()method. And that's almost true. The full truth is that not every request really needs to invoke doGet(). For example, a web browser that repeatedly accesses PrimeSearchershould need to call doGet()only after the searcher thread has found a new prime. Until that time, any call to doGet()just generates the same page the user has already seen, a page probably stored in the browser's cache. What's really needed is a way for a servlet to report when its output has changed. That's where the getLastModified()method comes in.
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