Java Thread for background services in web application daemon?

I'm running a service of generating a big file (It is a report in BIRT) in Java back code and it takes a lot of time my question is what is the best way to manage it with

daemon = true or daemon = false

and the priority

new Thread(new Runnable() {
                public void run(){
                    try { 
                     task.run();
                     engine.destroy( );
                    }
                     catch ( EngineException e1 ) {
                         System.err.println( "Report " + reportFilepath + " run failed.\n" );
                         System.err.println( e1.toString( ) );
                     }
                }
        }).start();