code: public static String getStringProperty(
  			final ServiceReference<?> serviceReference, final String key) {
  		NullArgumentException.validateNotNull(serviceReference, "Service reference");
  		NullArgumentException.validateNotEmpty(key, true, "Property key");
  
  		Object value = serviceReference.getProperty(key);
  		if (value != null && !(value instanceof String)) {
  			LOG.error("Property [" + key + "] value must be a String");
  			return null;
  		}
  		return (String) value;
  	}
docs: Returns a property as String.
  
  @param serviceReference service reference; cannot be null
  @param key              property key; cannot be null
  @return property value; null if property is not set or property value is
  not a String
  @throws NullArgumentException - If service reference is null - If key is null