SimpleXsdSchema is not thread-safe

[nosan]

See #1043

[nosan]

I am unsure if DOM is the best way to parse XML to find the targetNamespace. I believe StAX might be a better choice here to avoid loading the entire XML document into memory.

@Override
	public void afterPropertiesSet() throws Exception {
		Assert.notNull(this.xsdResource, "'xsd' is required");
		Assert.isTrue(this.xsdResource.exists(), "xsd '" + this.xsdResource + "' does not exist");
		XmlRootElement element = XmlRootElement.of(this.xsdResource);
		Assert.isTrue(SCHEMA_NAME.getLocalPart().equals(element.localName()),
				this.xsdResource + " has invalid root element : [" + element.localName() + "] instead of [schema]");
		Assert.isTrue(SCHEMA_NAME.getNamespaceURI().equals(element.namespaceURI()),
				this.xsdResource + " has invalid root element: [" + element.namespaceURI() + "] instead of ["
						+ SCHEMA_NAME.getNamespaceURI() + "]");
		Assert.hasText(element.targetNamespace(), this.xsdResource + " has no targetNamespace");
		this.targetNamespace = element.targetNamespace();
	}


	private record XmlRootElement(String localName, String namespaceURI,
								  String targetNamespace) {
		static XmlRootElement of(Resource resource) throws Exception {
			XMLInputFactory factory = StaxUtils.createDefensiveInputFactory();
			factory.setProperty(XMLInputFactory.IS_NAMESPACE_AWARE, true);
			try (InputStream is = resource.getInputStream()) {
				XMLStreamReader reader = factory.createXMLStreamReader(SaxUtils.getSystemId(resource), is);
				try {
					while (reader.hasNext()) {
						if (reader.next() == XMLStreamConstants.START_ELEMENT) {
							return new XmlRootElement(reader.getLocalName(), reader.getNamespaceURI(),
									reader.getAttributeValue(null, "targetNamespace"));
						}
					}
				}
				finally {
					reader.close();
				}
			}
			return new XmlRootElement(null, null, null);
		}
	}

[snicoll]

Thanks @nosan. I want to acknowledge that I've seen this but will need a bit of time to review.