Running a background Java program in Tomcat [duplicate]

Use an ExecutorService.

There’s a few things you should do though, mark the thread as a daemon thread so it atleast won’t tie up tomcat in error scenarios, and you should stop the executor when your servlet context is destroyed (e.g. when you redeploy or stop your application. To do this, use a ServletContextListener:

public class ExecutorContextListener implements ServletContextListener {
    private  ExecutorService executor;

    public void contextInitialized(ServletContextEvent arg0) {
        ServletContext context = arg0.getServletContext();
        int nr_executors = 1;
        ThreadFactory daemonFactory = new DaemonThreadFactory();
        try {
            nr_executors = Integer.parseInt(context.getInitParameter("nr-executors"));
        } catch (NumberFormatException ignore ) {}

        if(nr_executors <= 1) {
        executor = Executors.newSingleThreadExecutor(daemonFactory);
        } else {
        executor = Executors.newFixedThreadPool(nr_executors,daemonFactory);
       }
          context.setAttribute("MY_EXECUTOR", executor);
      }

    public void contextDestroyed(ServletContextEvent arg0) {
        ServletContext context = arg0.getServletContext();
        executor.shutdownNow(); // or process/wait until all pending jobs are done
    }

}
import java.util.concurrent.Executors;
import java.util.concurrent.ThreadFactory;

/**
 * Hands out threads from the wrapped threadfactory with setDeamon(true), so the
 * threads won't keep the JVM alive when it should otherwise exit.
 */
public class DaemonThreadFactory implements ThreadFactory {

    private final ThreadFactory factory;

    /**
     * Construct a ThreadFactory with setDeamon(true) using
     * Executors.defaultThreadFactory()
     */
    public DaemonThreadFactory() {
        this(Executors.defaultThreadFactory());
    }

    /**
     * Construct a ThreadFactory with setDeamon(true) wrapping the given factory
     * 
     * @param thread
     *            factory to wrap
     */
    public DaemonThreadFactory(ThreadFactory factory) {
        if (factory == null)
            throw new NullPointerException("factory cannot be null");
        this.factory = factory;
    }

    public Thread newThread(Runnable r) {
        final Thread t = factory.newThread(r);
        t.setDaemon(true);
        return t;
    }
}

You’ll have to add the context listener to your web.xml, where you also can specify the number of threads you’ll want to run the background jobs:

  <listener>
    <listener-class>com.example.ExecutorContextListener</listener-class>
  </listener>

You can access the executor from your servlet and submit jobs to it:

ExecutorService executor = (ExecutorService )getServletContext().getAttribute("MY_EXECUTOR");
...
executor.submit(myJob);

If you’re using Spring, all this can probably be made even simpler