private static List<FileObject> getRequiredXMLFiles(Project p, FileAcceptor a) {
        Collection<? extends FileObject> roots = QuerySupport.findRoots(p, null, Collections.<String>emptyList(), Collections.<String>emptyList());
        try {
            String regexp = ".*\\.xml"; // all *.xml files
            QuerySupport q = QuerySupport.forRoots(
                    "org-netbeans-modules-jumpto-file-FileIndexer", // org.netbeans.modules.jumpto.file.FileIndexer.ID
                    1,                                              // org.netbeans.modules.jumpto.file.FileIndexer.VERSION
                    roots.toArray(new FileObject [roots.size()]));
            Collection<? extends IndexResult> results = q.query(
                    "ci-file-name",                                 // org.netbeans.modules.jumpto.file.FileIndexer.FIELD_CASE_INSENSITIVE_NAME
                    regexp,
                    QuerySupport.Kind.CASE_INSENSITIVE_REGEXP);
            ArrayList<FileObject> files = new ArrayList<FileObject>();
            for(IndexResult r : results) {
                FileObject file = r.getFile();
                if (file == null || !file.isValid()) {
                    // the file has been deleted in the meantime
                    continue;
                }
                if (a.accept(file)) {
                    files.add(file);
                }
            }
            ....
        } catch (PatternSyntaxException pse) {
            return Collections.<FileObject>emptyList();
        } catch (IOException ioe) {
            logger.log(Level.WARNING, null, ioe);
            return Collections.<FileObject>emptyList();
        }
    }

    /** Used to narrow the search results by MIME type */
    private static interface FileAcceptor {
        public boolean accept(FileObject fo);
    }