Do I have to manually stop threads in Java?

Question

When my application is ready to exit, either by closing a window or invoking the System.exit() method. Do I have to manually stop the threads I may have created or will Java take care of that for me?

Answer 1

In cases you use System.exit(). All the threads will stop whether or not they are daemon.

Otherwise, the JVM will automatically stop all threads that are daemon threads set by Thread.setDaemon(true). In other words, the jvm will only exit when only threads remaining are all daemon threads or no threads at all.

Consider the example below, it will continue to run even after the main method returns. but if you set it to daemon, it will terminate when the main method (the main thread) terminates.

public class Test {

    public static void main(String[] arg) throws Throwable {
       Thread t = new Thread() {
          public void run()   {
             while(true)   {
                try  {
                   Thread.sleep(300);
                   System.out.println("Woken up after 300ms");
                }catch(Exception e) {}
             }
          }
       };

       // t.setDaemon(true); // will make this thread daemon
       t.start();
       System.exit(0); // this will stop all threads whether are not they are daemon
       System.out.println("main method returning...");
    }
}

Answer 2

If you want stop threads before exit gracefully, Shutdown Hooks may be a choice.

looks like:

Runtime.getRuntime().addShutdownHook(new Thread() {
    public void run() { 
    //Stop threads }
});

See: hook-design