How can I be sure a daemon thread is always running, in Java?

Question

In a web app my code creates some daemon threads as inner classes to check some information from incoming requests.

    private static class AlertThread extends Thread {

    private AlertThread() {
        setDaemon(true);
    }

    public void run() {
        while (true) {
            try {
               ...
            }
            catch (InterruptedException e1) {
                LogFactory.getLog(AlertThread.class)
                        .warn("InterruptedException when waiting");
                e1.printStackTrace();
            }
            catch (Exception e) {
                mainLogger.error("Error processing Alert " + e.getMessage());
            }
        }
    }
}

Code works well, and thread is running fine, but I want to be sure that this thread will be always running, and if it is killed (by any unknown reason), how should I restart a new instance?

Answer 1

The best solution is to not let it die in the first place. You can do this

public void run() {
    while(true) {
        try {
          runOne();
        } catch(Throwable t) {
          // log t
        }
    }
}

This thread will not die unless you shutdown the application.

BTW You should make the thread a daemon before you start it and you should avoid extending Thread as doing so can lead to subtle bugs.

Answer 2

As your code is written, the while loop could be terminated by any Trowable that is not an Exception . It could be an Error such as a StackOverflowError for example.