ThreadedDemo.java

import java.io.File;
import java.util.logging.Logger;

public class ThreadedDemo {
    private static Logger logger = Logger.getLogger("demo");

    // "Inner class" represents a single thread that runs forever, looking for
    // a certain file to appear, then "processes it," for demo
    // purposes just renaming it to another name
    static class Worker implements Runnable {
        private File file;
        private long delay;

        public Worker(String sourceFile, long delay) {
            this.file = new File(sourceFile);
            this.delay = delay;
        }

        // returns the name of the current thread.
        // other logging APIs support thread name out of the box,
        // doesn't seem like java util logging does that.
        private String threadName() {
            return Thread.currentThread().getName();
        }

        // this will move file to another name
        private void processFile() {
            long timestamp = System.currentTimeMillis();
            File newFile = new File(file.getAbsolutePath() + "." + timestamp);

            logger.info(threadName() + ": moving " + file + " to " + newFile);

            if (!file.renameTo(newFile)) {
                throw new RuntimeException("cannot move file " + file + " to " + newFile);
            }
        }

        // thread sleeps for specified delay ms.
        private void sleep() {
            logger.fine(threadName() + ": sleeping " + delay + " ms.");
            try {
                Thread.sleep(delay);
            } catch (InterruptedException e) {
                logger.severe(threadName() + ": interrupted, this should really never happen");
            }
        }


        // this method is the entry point to the thread. will loop forever,
        // waking up and checking for the file
        public void run() {
            logger.info(threadName() + ": watching for file " + file);

            while(true) {
                if (file.exists()) {
                    processFile();
                }

                sleep();
            }
        }
    }

    public static void main(String[] args) {
        ThreadGroup td = new ThreadGroup("worker threads");

        // keep the running while all threads in this group are alive
        td.setDaemon(true);

        // start 5 threads
        for (int i = 1; i <= 5; i++) {
            // each thread gets its own name, logging can be configured in
            // such a way that thread name comes out in each log message
            new Thread(td, new Worker("input_" + i + ".txt", 1000), "Worker_" + i).start();
        }
    }
}