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Apache XML Security for Java supports XML-Signature Syntax and Processing, W3C Recommendation 12 February 2002, and XML Encryption Syntax and Processing, W3C Recommendation 10 December 2002. As of version 1.4, the library supports the standard Java API JSR-105: XML Digital Signature APIs.
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/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements. See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership. The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied. See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
/*
 * Copyright 2005 Sun Microsystems, Inc. All rights reserved.
 */
package org.apache.xml.security.signature.reference;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
import java.util.NoSuchElementException;

import org.w3c.dom.NamedNodeMap;
import org.w3c.dom.Node;

/**
 * A representation of a ReferenceNodeSetData type containing a node-set.
 * This is a subtype of NodeSetData that represents a dereferenced
 * same-document URI as the root of a subdocument. The main reason is
 * for efficiency and performance, as some transforms can operate
 * directly on the subdocument and there is no need to convert it
 * first to an XPath node-set.
 */
public class ReferenceSubTreeData implements ReferenceNodeSetData {

    private boolean excludeComments;
    private Node root;

    public ReferenceSubTreeData(Node root, boolean excludeComments) {
        this.root = root;
        this.excludeComments = excludeComments;
    }

    @Override
    public Iterator iterator() {
        return new DelayedNodeIterator(root, excludeComments);
    }

    public Node getRoot() {
        return root;
    }

    public boolean excludeComments() {
        return excludeComments;
    }

    /**
     * This is an Iterator that contains a backing node-set that is
     * not populated until the caller first attempts to advance the iterator.
     */
    static class DelayedNodeIterator implements Iterator {
        private Node root;
        private List nodeSet;
        private ListIterator li;
        private boolean withComments;

        DelayedNodeIterator(Node root, boolean excludeComments) {
            this.root = root;
            this.withComments = !excludeComments;
        }

        @Override
        public boolean hasNext() {
            if (nodeSet == null) {
                nodeSet = dereferenceSameDocumentURI(root);
                li = nodeSet.listIterator();
            }
            return li.hasNext();
        }

        @Override
        public Node next() {
            if (nodeSet == null) {
                nodeSet = dereferenceSameDocumentURI(root);
                li = nodeSet.listIterator();
            }
            if (li.hasNext()) {
                return li.next();
            } else {
                throw new NoSuchElementException();
            }
        }

        @Override
        public void remove() {
            throw new UnsupportedOperationException();
        }

        /**
         * Dereferences a same-document URI fragment.
         *
         * @param node the node (document or element) referenced by the
         *	 URI fragment. If null, returns an empty set.
         * @return a set of nodes (minus any comment nodes)
         */
        private List dereferenceSameDocumentURI(Node node) {
            List nodeSet = new ArrayList<>();
            if (node != null) {
                nodeSetMinusCommentNodes(node, nodeSet, null);
            }
            return nodeSet;
        }

        /**
         * Recursively traverses the subtree, and returns an XPath-equivalent
         * node-set of all nodes traversed, excluding any comment nodes,
         * if specified.
         *
         * @param node the node to traverse
         * @param nodeSet the set of nodes traversed so far
         * @param prevSibling the previous sibling node
         */
        @SuppressWarnings("fallthrough")
        private void nodeSetMinusCommentNodes(Node node, List nodeSet,
                                              Node prevSibling)
        {
            switch (node.getNodeType()) {
                case Node.ELEMENT_NODE :
                    nodeSet.add(node);
                    NamedNodeMap attrs = node.getAttributes();
                    if (attrs != null) {
                        for (int i = 0, len = attrs.getLength(); i < len; i++) {
                            nodeSet.add(attrs.item(i));
                        }
                    }
                    Node pSibling = null;
                    for (Node child = node.getFirstChild(); child != null;
                        child = child.getNextSibling()) {
                        nodeSetMinusCommentNodes(child, nodeSet, pSibling);
                        pSibling = child;
                    }
                    break;
                case Node.DOCUMENT_NODE :
                    pSibling = null;
                    for (Node child = node.getFirstChild(); child != null;
                        child = child.getNextSibling()) {
                        nodeSetMinusCommentNodes(child, nodeSet, pSibling);
                        pSibling = child;
                    }
                    break;
                case Node.TEXT_NODE :
                case Node.CDATA_SECTION_NODE:
                    // emulate XPath which only returns the first node in
                    // contiguous text/cdata nodes
                    if (prevSibling != null &&
                        (prevSibling.getNodeType() == Node.TEXT_NODE ||
                         prevSibling.getNodeType() == Node.CDATA_SECTION_NODE)) {
                        return;
                    }
                    nodeSet.add(node);
                    break;
                case Node.PROCESSING_INSTRUCTION_NODE :
                    nodeSet.add(node);
                    break;
                case Node.COMMENT_NODE:
                    if (withComments) {
                        nodeSet.add(node);
                    }
            }
        }
    }
}