code: protected boolean isDaemon(Object command) {
  		if (command == null) {
  			return false;
  		}
  
  		if (command instanceof Command) {
  			Command command2 = (Command) command;
  			return command2.isDaemon();
  		}
  		return false;
  	}
docs: 是否执行System.exit(0)，如果有线程在后台运行则不能执行System.exit();
  
  @return