First thoughts on an alternative validator for EML-specific rules.

src/main/java/org/ecoinformatics/eml/EMLValidator.java

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+package org.ecoinformatics.eml;
+
+import java.io.File;
+import java.io.InputStream;
+import java.io.FileInputStream;
+import java.io.StringReader;
+import java.util.HashMap;
+import java.util.Vector;
+import javax.xml.parsers.DocumentBuilderFactory;
+import javax.xml.transform.OutputKeys;
+import javax.xml.transform.Transformer;
+import javax.xml.transform.TransformerFactory;
+import org.apache.xpath.XPathAPI;
+import org.w3c.dom.Document;
+import org.w3c.dom.Node;
+import org.w3c.dom.NodeList;
+import org.xml.sax.InputSource;
+
+public class EMLValidator {
+
+	private Document doc = null;
+
+	public EMLValidator(String filename) {
+		try {
+			FileInputStream f = new FileInputStream(new File(filename));
+			doc = parseDocument(new InputSource(f));
+		} catch (Exception e) {
+			System.err.println(e.getMessage());
+		}
+	}
+
+	public static void main(String[] args) {
+		String emlfile = args[0];
+		// System.out.println("emlfile: " + emlfile);
+		EMLValidator validator = new EMLValidator(emlfile);
+		boolean isValid = validator.validate();
+        System.err.println("isValid: " + (new Boolean(isValid)).toString());
+	}
+
+	private Document parseDocument(InputSource in) throws Exception {
+		DocumentBuilderFactory dfactory = DocumentBuilderFactory.newInstance();
+		dfactory.setNamespaceAware(false);
+		Document doc = dfactory.newDocumentBuilder().parse(in);
+		return doc;
+	}
+
+	private boolean validate() {
+		boolean isValid = true;
+
+		// TODO: All EML documents MUST have the 'eml' module as the root
+
+		// All `id` attributes within the document MUST be unique
+		Vector<String> ids = getXPathValues("//*[@id]/@id");
+		HashMap idmap = new HashMap();
+		for (String s : ids) {
+			//System.out.println(s);
+			idmap.put(s, null);
+		}
+		if (ids.size() != idmap.size()) {
+			System.err.println("Invalid: ID attributes must be unique. Duplicates exist.");
+			isValid = false;	
+		}
+
+		// If an element references another using a child `references` element or references attribute, another element with that value in its `id` attribute MUST exist in the document
+		// If an `additionalMetadata` element references another using a child `describes` element, another element with that value in its `id` attribute MUST exist in the document
+		Vector<String> refs = getXPathValues("//annotation[@references]/@references|//references|/describes");
+		for (String s : refs) {
+			System.out.println(s);
+			if (!ids.contains(s)) {
+				System.err.println("Invalid: Reference missing from IDs: " + s);
+				isValid = false;
+			}
+		}
+
+
+		// TODO: Elements which contain an `annotation` child element MUST contain an `id` attribute, unless the containing `annotation` element contains a `references` attribute
+
+		// TODO: When `references` is used, the `system` attribute MUST have the same value in both the target and source elements, or it must be absent in both.
+
+		// TODO: If an element references another using a child `references` element, it MUST not have an `id` attribute itself
+
+		return isValid;
+	}
+
+	private Vector getXPathValues(String xpath) {
+		// Use the simple XPath API to select a nodeIterator.
+		Vector<String> values = new Vector<String>();
+		try {
+			NodeList nl = XPathAPI.selectNodeList(doc, xpath);
+
+			for (int i = 0; i < nl.getLength(); i++) {
+				Node n = nl.item(i);
+				n.normalize();
+				Node t = n.getFirstChild();
+				// System.out.println(t.getNodeType() + "/" +
+				// t.getLocalName() + "/" +
+				// t.getNodeValue());
+				values.add(t.getNodeValue());
+			}
+
+		} catch (Exception e) {
+			System.out.println(e.getMessage());
+		}
+		return values;
+		//return values.toArray(new String[values.size()]);
+	}
+}
+