When running this code with the line commented, the thread is not deamon, so even if your main method has finished, you'll keep on having the console flooded until you stop it with CTRL+C. That is, the JVM will not exit.

gistfile1.txt

public class Spawner {
  public static void main(String[] args) {
    Thread t = new Thread(new Runnable() {
      public void run() {
        while (true) {
          System.out.println("I'm still alive");
        }
      }
    });
    t.start();
    // Try uncommenting/commenting this line
    // t.setDaemon(true);
    System.out.println("Main thread has finished");
  }
}