Invalid subjects and properties are not variables.

Author: RubenVerborgh

src/org/linkeddatafragments/servlet/BasicLdfServlet.java

@@ -166,7 +166,7 @@ private int parseAsInteger(String value) {
 	 */
 	private Resource parseAsResource(String value) {
 		final RDFNode subject = parseAsNode(value);
-		return subject instanceof Resource ? (Resource)subject : null;
+		return subject == null || subject instanceof Resource ? (Resource)subject : INVALID_URI;
 	}
 
 	/**
@@ -178,11 +178,9 @@ private Property parseAsProperty(String value) {
 		final RDFNode predicateNode = parseAsNode(value);
 		if (predicateNode instanceof Resource) {
 			try { return ResourceFactory.createProperty(((Resource)predicateNode).getURI()); }
-			catch (InvalidPropertyURIException ex) {
-				return ResourceFactory.createProperty("urn:invalid-predicate-uri");
-			}
+			catch (InvalidPropertyURIException ex) { return INVALID_URI; }
 		}
-		return null;
+		return predicateNode == null ? null : INVALID_URI;
 	}
 
 	/**

src/org/linkeddatafragments/util/CommonResources.java

@@ -31,6 +31,8 @@ public class CommonResources {
 	public final static Property HYDRA_NEXTPAGE = createProperty(HYDRA + "nextPage");
 	public final static Property HYDRA_PREVIOUSPAGE = createProperty(HYDRA + "previousPage");
 
+	public final static Property INVALID_URI = createProperty("urn:invalid");
+	
 	private final static Property createProperty(String uri) {
 		return ResourceFactory.createProperty(uri);
 	}