spring-projects/spring-boot · error · IllegalStateException

Multiple '{tagName}' nodes found

Error message

Multiple '{tagName}' nodes found

What it means

IllegalStateException thrown by CustomLayersProvider.getChildElement when getElementsByTagName returns more than one match for a tag that is expected to appear at most once under a given parent. The schema may permit multiple siblings generally, but specific structural elements (e.g., a single <application> or <dependencies> root child) are required to be unique.

Source

Thrown at build-plugin/spring-boot-maven-plugin/src/main/java/org/springframework/boot/maven/CustomLayersProvider.java:167

	private List<String> getChildNodeTextContent(Element element, String tagName) {
		List<String> patterns = new ArrayList<>();
		NodeList nodes = element.getElementsByTagName(tagName);
		for (int i = 0; i < nodes.getLength(); i++) {
			Node node = nodes.item(i);
			if (node instanceof Element) {
				patterns.add(node.getTextContent());
			}
		}
		return patterns;
	}

	private @Nullable Element getChildElement(Element element, String tagName) {
		NodeList nodes = element.getElementsByTagName(tagName);
		if (nodes.getLength() == 0) {
			return null;
		}
		if (nodes.getLength() > 1) {
			throw new IllegalStateException("Multiple '" + tagName + "' nodes found");
		}
		return (Element) nodes.item(0);
	}

}

View on GitHub (pinned to 270dfe353f)

Solutions

1. Open your layers.xml and locate the tagName named in the message; remove all but one occurrence.
2. Re-run schema validation — duplicates often also trip the XSD, but this guard fires first for non-uniqueness constraints.
3. If you genuinely need multiple selections, place them as sibling <into> elements inside a single parent, not as duplicate parents.

Example fix

// before: <layers><application>...</application><application>...</application></layers>
// after:  <layers><application><into layer="a">...</into><into layer="b">...</into></application></layers>

// Reject duplicate singleton elements before invoking the goal:
Set<String> singletons = Set.of("application", "dependencies"); // adjust per schema
for (String tag : singletons) {
    if (root.getElementsByTagName(tag).getLength() > 1) {
        throw new IllegalStateException("Duplicate '" + tag + "' in layers.xml");
    }
}

try {
    // invoke goal
} catch (IllegalStateException ex) {
    if (ex.getMessage().startsWith("Multiple '") && ex.getMessage().endsWith("' nodes found")) {
        // remove the duplicate element named in the message
    }
    throw ex;
}

When it happens

Trigger: Two or more elements with the same tagName under a parent where getChildElement expects a singleton. The method explicitly checks nodes.getLength() > 1 and aborts.

Common situations: Copy-pasting a <dependencies> block twice inside <layers>; merging two layers.xml files incorrectly; an automated generator emitting duplicate sections; misunderstanding the schema and adding multiple <application> roots.

Related errors

• Failed to process custom layers configuration {source}
• Invalid layers.xml configuration
• Failed to load layers configuration with name '%s': '%s' not
• Unable to load layers XSD
• Could not build classpath

AI-assisted analysis of spring-projects/spring-boot@270dfe353f (2026-08-11). Data as JSON: /api/errors/6368dad55082b69f. Report an issue: GitHub.