Evaluate domtrip for XML change detection

[laeubi]

XmlComparator currently uses XmlUnit but this is rather tailored to testing. The already used domtrip library also claims to support a (semantic versus formatting) changes that we possibly can reuse here.

FYI @gnodet

[gnodet]

I looked into this and domtrip's XmlDiff API seems like a very natural fit for what XmlComparator does today. Here's a quick evaluation:

Current XmlUnit usage (narrow surface)

The comparator only uses a small slice of XmlUnit:

• DiffBuilder.compare() with Input.fromStream() to parse two XML streams
• .checkForSimilar().ignoreComments().ignoreWhitespace() for normalization
• Diff.hasDifferences() + Diff.fullDescription() for the result

domtrip equivalent

domtrip's diff API covers the same use case with a cleaner semantic/formatting separation:

Document baseline = Document.of(baselineStream);
Document reactor = Document.of(reactorStream);
DiffResult diff = XmlDiff.diff(baseline, reactor);

if (diff.hasSemanticChanges()) {
    // build delta from diff.semanticChanges()
}

Key advantages:

• Built-in semantic vs formatting distinction — DiffResult.semanticChanges() vs formattingChanges() out of the box, no need for explicit ignoreWhitespace()/ignoreComments() flags
• Granular change types — each XmlChange has a ChangeType that's classified as semantic (ELEMENT_ADDED, TEXT_CHANGED, ATTRIBUTE_CHANGED, etc.) or formatting-only (WHITESPACE_CHANGED, QUOTE_STYLE_CHANGED, ENTITY_FORM_CHANGED, EMPTY_ELEMENT_STYLE_CHANGED)
• Configurable element matching — DiffConfig.builder().matchBy("dependency", "groupId", "artifactId") for identity-based matching, which could be useful for more robust comparison of structured XML like plugin configs
• Path-scoped filtering — changesUnder(path) and changesFor(xpath, doc) for focused comparisons
• One fewer dependency — since domtrip is already in the project, this would let you drop xmlunit-core

Comment handling

One thing to note: domtrip classifies comment changes (COMMENT_ADDED/COMMENT_CHANGED/COMMENT_REMOVED) as semantic. To match the current behavior of ignoring comments, you'd filter those out from semanticChanges(), or domtrip could potentially add a DiffConfig option to exclude comment changes from the semantic category.

Summary

The migration looks straightforward — the XmlUnit surface used is small and maps 1:1 to domtrip's XmlDiff/DiffResult API. The main benefit is the native semantic vs formatting distinction, which is exactly what an artifact comparator needs (detecting real changes vs insignificant reformatting).