Daemon thread Java [duplicate]

Question

I am a new to concurrency and threads - and have been using them when developing my application at work. Essentially i have a number of threads in my RMI application (server side component) that poll changes in files (these files update every few seconds) .

When testing on a dev box I have been running the server from the command line and then closing it manually when finished and rinse and repeat throughout the day.

As it transpires - I think my threads may not be stopping when i shut the command line and still carry on processing. This is leading to some very bad side effects - although I am not 100% sure if this is possible so hopefully someone can confirm this might be the case.

If i make a thread a daemon - does this mean that when i shut the command line - these threads will automatically stop? I need some way of terminating the application nicely but because the server will eventually be run by autosys I am not sure whats the best way to make all threads finish when shut down

Thanks

Answer 1

Threads run inside a Java Virtual Machine. If you stop the JVM, the threads don't run anymore. You could see exiting the JVM as pulling the plug off your computer: nothing can run anymore.

Answer 2

The following code demonstrates creating a thread pool with a dummy running task, that task is then started and when the application is terminated, a shutdown hook runs which cancels all running tasks and cleanly closes the thread pool.

You do not have to use Callable's as I have, you can use Runnables and the threadPool.execute method and simply terminate the threadPool which is a little less elegant.

    final ExecutorService threadPool = Executors.newCachedThreadPool();
    final List<Future<Void>> runningTasks = new ArrayList<>();
    Future<Void> task = threadPool.submit(new Callable<Void>() {

        @Override
        public Void call() throws Exception {
            int count = 0;
            while(true) {
                System.out.println(++count);
                Thread.sleep(1000);
            }
        }
    });
    runningTasks.add(task);

    Runtime.getRuntime().addShutdownHook(new Thread() {
        @Override
        public void run() {
            for(Future<Void> runningTask : runningTasks) {
                runningTask.cancel(true);
            }
            threadPool.shutdownNow();
        }
    });

Shutdown hooks are for standalone applications. In a Java EE container, you could do the same with a javax.servlet.ServletContextListener