import com.sun.org.apache.xml.internal.security.utils.XPathFactory; //导入依赖的package包/类
/**
 * Method enginePerformTransform
 * @inheritDoc
 * @param input
 *
 * @throws TransformationException
 */
protected XMLSignatureInput enginePerformTransform(
    XMLSignatureInput input, OutputStream os, Transform transformObject
) throws TransformationException {
    try {
        /**
         * If the actual input is an octet stream, then the application MUST
         * convert the octet stream to an XPath node-set suitable for use by
         * Canonical XML with Comments. (A subsequent application of the
         * REQUIRED Canonical XML algorithm would strip away these comments.)
         *
         * ...
         *
         * The evaluation of this expression includes all of the document's nodes
         * (including comments) in the node-set representing the octet stream.
         */
        Element xpathElement =
            XMLUtils.selectDsNode(
                transformObject.getElement().getFirstChild(), Constants._TAG_XPATH, 0);

        if (xpathElement == null) {
            Object exArgs[] = { "ds:XPath", "Transform" };

            throw new TransformationException("xml.WrongContent", exArgs);
        }
        Node xpathnode = xpathElement.getChildNodes().item(0);
        String str = XMLUtils.getStrFromNode(xpathnode);
        input.setNeedsToBeExpanded(needsCircumvent(str));
        if (xpathnode == null) {
            throw new DOMException(
                DOMException.HIERARCHY_REQUEST_ERR, "Text must be in ds:Xpath"
            );
        }

        XPathFactory xpathFactory = XPathFactory.newInstance();
        XPathAPI xpathAPIInstance = xpathFactory.newXPathAPI();
        input.addNodeFilter(new XPathNodeFilter(xpathElement, xpathnode, str, xpathAPIInstance));
        input.setNodeSet(true);
        return input;
    } catch (DOMException ex) {
        throw new TransformationException("empty", ex);
    }
}