JAXB always adds all namespaces that are known by the JAXBContext to the root element of the XML document for performance reasons. See this comment by Kohsuke on JAXB-103 for more information.
The only way I found to deal with this, is to traverse the document myself after it has been created with JAXB and remove all unused namespaces using the following helper class:
public class RemoveUnusedNamespaces {
private static final String XML_NAMESPACE_SCHEMA_INSTANCE = "http://www.w3.org/2001/XMLSchema-instance";
private static final String XML_NAMESPACE_NAMESPACE = "http://www.w3.org/2000/xmlns/";
private interface ElementVisitor {
void visit(Element element);
}
public void process(Document document) {
final Set<String> namespaces = new HashSet<String>();
Element element = document.getDocumentElement();
traverse(element, new ElementVisitor() {
public void visit(Element element) {
String namespace = element.getNamespaceURI();
if (namespace == null)
namespace = "";
namespaces.add(namespace);
NamedNodeMap attributes = element.getAttributes();
for (int i = 0; i < attributes.getLength(); i++) {
Node node = attributes.item(i);
if (XML_NAMESPACE_NAMESPACE.equals(node.getNamespaceURI()))
continue;
String prefix;
if (XML_NAMESPACE_SCHEMA_INSTANCE.equals(node.getNamespaceURI())) {
if ("type".equals(node.getLocalName())) {
String value = node.getNodeValue();
if (value.contains(":"))
prefix = value.substring(0, value.indexOf(":"));
else
prefix = null;
} else {
continue;
}
} else {
prefix = node.getPrefix();
}
namespace = element.lookupNamespaceURI(prefix);
if (namespace == null)
namespace = "";
namespaces.add(namespace);
}
}
});
traverse(element, new ElementVisitor() {
public void visit(Element element) {
Set<String> removeLocalNames = new HashSet<String>();
NamedNodeMap attributes = element.getAttributes();
for (int i = 0; i < attributes.getLength(); i++) {
Node node = attributes.item(i);
if (!XML_NAMESPACE_NAMESPACE.equals(node.getNamespaceURI()))
continue;
if (namespaces.contains(node.getNodeValue()))
continue;
removeLocalNames.add(node.getLocalName());
}
for (String localName : removeLocalNames)
element.removeAttributeNS(XML_NAMESPACE_NAMESPACE, localName);
}
});
}
private final void traverse(Element element, ElementVisitor visitor) {
visitor.visit(element);
NodeList children = element.getChildNodes();
for (int i = 0; i < children.getLength(); i++) {
Node node = children.item(i);
if (node.getNodeType() != Node.ELEMENT_NODE)
continue;
traverse((Element) node, visitor);
}
}
}