Threads: I only want one instance.

Question

I am starting a daemon thread from a servlet that gets loaded on startup (Tomcat 5.5). The servlet's init() method creates a thread that optimizes my Lucene indexes every 2 hours.
This is what's in the servlet's init method:
Thread optimizer = new Thread(new LuceneIndexOptimizer(conf), "Lucene Index Optimizer");
optimizer.setDaemon(true);
optimizer.setPriority(Thread.MIN_PRIORITY);
optimizer.start();     The thread looks like this:
public class LuceneIndexOptimizer implements Runnable {
     private Config conf = null;
     public LuceneIndexOptimizer(Config conf) {
          this.conf = conf;
     public void run(){
          while (true) {
               LuceneFuncs.optimize(this.conf);
               try{
                    Thread.sleep(7200000); // 2 Hours.
               catch(InterruptedException ex){
                   ex.printStackTrace();
}Now the problem is that whenever the context is reloaded, that servlet is re-initialized and a new thread gets started without the old one ever going away.
How do I make sure that only one of these threads is running at a given point in time?

Answer 1

Again, good information. Thanks.
At any rate, I ended up going with the ServletContextListener interface as I think it is cleaner.
How it works:
In my web.xml I have this entry:
<!--  Context Listener for Application Initialization -->
<listener>
     <listener-class>
          com.r10.rp.context.RPInit
     </listener-class>
</listener>The RPInit class:
public class RPInit implements ServletContextListener {
     public static final long serialVersionUID = 10;
     private LuceneIndexOptimizer indxOpt = null;
     public void contextInitialized(ServletContextEvent event) {
          // Set application specific config variables.
          System.out.println("com.r10.rp.config.RPConfig INFO: Setting Application Config Variables");          
          ServletContext ctx = event.getServletContext();
          RPConfig.createConfig(ctx);
          RPConfig conf = (RPConfig) ctx.getAttribute("RPConfig");          
          // Start Lucene Optimizer
          System.out.println("com.r10.rp.config.RPConfig INFO: Initializing Lucene Index Optimizer");
          this.indxOpt = new LuceneIndexOptimizer(conf);
          Thread optimizer = new Thread(this.indxOpt, "Lucene Index Optimizer");
          this.indxOpt.setOptimizer(optimizer);
          optimizer.setDaemon(true);
          optimizer.setPriority(Thread.MIN_PRIORITY);
          optimizer.start();                    
     public void contextDestroyed(ServletContextEvent event){
          this.indxOpt.stop();
}The IndexOptimizer thread:
public class LuceneIndexOptimizer implements Runnable {
     private RPConfig conf = null;
     private volatile Thread optimizer;
     public LuceneIndexOptimizer(RPConfig conf) {
          this.conf = conf;
     public void run() {
          Thread thisThread = Thread.currentThread();
          while (thisThread == this.optimizer) {
               LuceneFuncs.optimize(this.conf);
               try{
                    Thread.sleep(7200000); // 2 Hours.
               catch(InterruptedException ex){
                   ex.printStackTrace();
     public void stop() {
          Thread blah = this.optimizer;
        this.optimizer = null;
        blah.interrupt();
      * @param optimizer The optimizer to set.
     public void setOptimizer(Thread optimizer) {
          this.optimizer = optimizer;
}