I gonna get an SVGDocument object to fill into JSVGCanvas, but I just had an SVG string without any files, so I cannot use URI to construct.
You can read your SVG from a StringReader like this:
StringReader reader = new StringReader(svgString);
String uri = "file:make-something-up";
String parser = XMLResourceDescriptor.getXMLParserClassName();
SAXSVGDocumentFactory f = new SAXSVGDocumentFactory(parser);
SVGDocument doc = f.createSVGDocument(uri, reader);
You need to make up a valid URI but it's not important unless you make relative references to other URIs from your SVG.