Java Servlet Programming

Background Processing

Servlets can do more than simply persist between accesses. They can also execute between accesses. Any thread started by a servlet can continue executing even after the response has been sent. This ability proves most useful for long-running tasks whose incremental results should be made available to multiple clients. A background thread started in init() performs continuous work while request-handling threads display the current status with doGet(). It’s a similar technique to that used in animation applets, where one thread changes the picture and another paints the display.

Example 3.6 shows a servlet that searches for prime numbers above one quadrillion. It starts with such a large number to make the calculation slow enough to adequately demonstrate caching effects—something we need for the next section. The algorithm it uses couldn’t be simpler: it selects odd-numbered candidates and attempts to divide them by every odd integer between 3 and their square root. If none of the integers evenly divides the candidate, it is declared prime.

Example 3-6. On the hunt for primes

import java.io.*; import java.util.*; import javax.servlet.*; import javax.servlet.http.*; public class PrimeSearcher extends HttpServlet implements Runnable { long lastprime = 0; // last prime found Date lastprimeModified = new Date(); // when it was found Thread searcher; // background search thread public void init(ServletConfig config) throws ServletException { super.init(config); // ...