private Integer loadPropertyInt(Properties properties, String key) {
		String value = properties.getProperty(key);

		if (value == null) {
			return null;
		}
		if(!value.matches("[0-9]+")){
			throw new DBException(properties+"-value from properties file must be an integer value (0-9) but is '"+value+"'");
		}
		try {
			return Integer.parseInt(value);
		} catch (NumberFormatException e) {
			return null;
		}
	}