private void checkKey(String key) throws PropertyTypeException {
        if (key == null || key.isEmpty()) {
            throw new PropertyTypeException("Key must not be null");
        }

        String[] keyParts = splitKey(key);
        if (keyParts == null) {
            throw new PropertyTypeException("Key must be a QName in the form prefix:localName");
        }

        if (registry.getNamespaceURI(keyParts[0]) == null) {
            throw new PropertyTypeException("Key does not use a registered Namespace prefix");
        }
    }
Checks if the given key is a valid QName with a known standard namespace prefix

@param key the key to check
@return true if the key is valid otherwise false
tika-xmp/src/main/java/org/apache/tika/xmp/XMPMetadata.java