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Subject: XOM API for Processing XML with Java

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  • From: Marcelo Schneider <marcelo.schneider AT>
  • To: xom-interest AT
  • Subject: [XOM-interest] Comparing two semantically equal XML files
  • Date: Fri, 10 Oct 2008 08:59:04 -0300

Good morning list.

In our company we currently store some graphs in XML form: the client can create a graph in a Java applet, that is sent to the server and stored as a file. Currently, all the graph <-> XML transformation is done using JDom.

What I need to do is, when a user opens an already existing graph and tries to save it, compare the new XML sent with the previously existing XML in the server, saving only if they are different (so changing dates are correct).

Googling around for a while I found out that a possible way to do that without worrying with XML structure was to use Canonicalization, and that XOM could possibly help me with that. Is this assumption correct? Here is what I'm currently doing:

| public boolean areSemanticallyEqual(String oldXML, String newXML) {
| Builder builder = new Builder();
| Document oldDoc =;
| Document newDoc =;
| // here canonicalize the old xml
| ByteArrayOutputStream oldStream = new ByteArrayOutputStream();
| Canonicalizer outputter = new Canonicalizer(oldStream , false);
| outputter.write(oldDoc);
| // here canonicalize the new xml
| ByteArrayOutputStream newStream = new ByteArrayOutputStream();
| Canonicalizer outputter = new Canonicalizer(newStream, false);
| outputter.write(newDoc);
| return Arrays.equals(oldStream.toByteArray(), newStream.toByteArray());
| }

However, the above code returns false when I compare XMLs like these (they all start with "<?xml version="1.0" encoding="UTF-8"?>"):

xml1: "<tag1><tag2 id="1" name="a">Some Text</tag2></tag1><other tag />"
xml2: "<tag1>\n\t<tag2 id="1" name="a">Some Text</tag2>\r</tag1><other tag />" (with new lines, tabs and carriage returns)
xml3: "<tag1><tag2 id="1" name="a">Some Text</tag2></tag1><other tag></other tag>" (other valid xml representation)
xml4: "<tag1><tag2 name="a" id="1">Some Text</tag2></tag1><other tag />" (attributes order reversed)
xml5: "<other tag /><tag1><tag2 id="1" name="a">Some Text</tag2></tag1>" (yet another thing that looks the same, but I'm probably wrong assuming this should count as equal)

I would expect the return to be always true, except for (maybe?) xml1 > xml5. Is this correct? Any hints are highly appreciated (:

Marcelo Frantz Schneider
SIC - TCO - Tecnologia em Engenharia do Conhecimento

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