Manifest中定义的XML Manifest元素的表示 。 
        XML Schema Definition定义为: 
           <element name="Manifest" type="ds:ManifestType"/> <complexType name="ManifestType"> <sequence> <element ref="ds:Reference" maxOccurs="unbounded"/> </sequence> <attribute name="Id" type="ID" use="optional"/> </complexType>  
 
        可以通过调用XMLSignatureFactory类的newManifest方法之一来创建Manifest实例; 
        例如: 
          XMLSignatureFactory factory = XMLSignatureFactory.getInstance("DOM");
   Reference ref = factory.newReference("#reference-1", DigestMethod.SHA1);
   List<Reference> references = Collections.singletonList(ref);
   Manifest manifest = factory.newManifest(references, "manifest-1");