CODEWITHSUNDEEP
javaxmldomxpath
Constantine
Constantine

Reputation: 2032

Java edit xml file (save attributes order)

Am working with xml file (making its validation). I need to edit some attributes before calculating CRC32 function of entire file. I using DOM parser and xPath. After I have edited my file, am converting it to byte array for crc- function:

    Node file_crc = (Node) xPath.compile("/IODevice/Stamp/@crc").evaluate(doc, XPathConstants.NODE);
    file_crc.setTextContent("");
    bos = new ByteArrayOutputStream();
    result = new StreamResult(bos);
    try {
        transformer.transform(new DOMSource(doc), result);
        crc.reset();
        crc.update(bos.toByteArray());
    } catch (TransformerException ex) {
        return false;
    }

The trouble is that DOM parser changes attributes order in xml file (sorts them alphabeticaly) - this cause invalid checksum of file. How to avoid attributes order mutation?

Upvotes: 1

Views: 1436

Answers (2)

John Jerik
John Jerik

Reputation: 39

The order of attributes is defined to be insignificant in XML: no conformant XML application should produce results that depend on the order in which attributes appear. Therefore, serializers (code that produces lexical XML as output) will usually give you no control over the order.

Now, it would sometimes be nice to have that control for cosmetic reasons, because XML is designed to be human-readable. So there's a valid reason for wanting the feature. But the fact is, I know of no serializer that offers it.

Upvotes: 3

Mathias Müller
Mathias Müller

Reputation: 22657

The order of attributes is not significant in XML. Applications are free to store attributes of an element in any order they like. So, this behaviour is to be expected from DOM and XPath.

As far as I understand, CRC32 is ill-suited in the case of XML documents because documents as

<root a="1" b="2"/>

and

<root b="2" a="1"/>

are effectively the same. As a rule, you should not write XML applications that treat attribute order as significant, because there is no way to control that. If anything, attributes and namespace declarations should be listed in "ascending lexicographic order" (xml.com).

The relevant piece of info from the XML specification says:

Note that the order of attribute specifications in a start-tag or empty-element tag is not significant.

Perhaps you'll appreciate a link to some more opinions on this?

Upvotes: 4

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