editing svg in batik problem

hi,

I'm a beginner with batik library and I need to create a program that
would allow me to edit existing svg files and the save them.

The code that I have written so far doesn't seem to be working. My
program doesn't want to access my svg file, however there are no erros
when i execute it.

public void editSVG (String inputFilename) throws Exception {

        DOMImplementation impl = SVGDOMImplementation.getDOMImplementation();
        String svgNS = SVGDOMImplementation.SVG_NAMESPACE_URI;
        String svgURI = new File(inputFilename).toURL().toString();

        SVGDocument doc;
        try {
        doc = (SVGDocument)impl.createDocument(svgURI, "svg", null);

        // get the root element (the svg element)
        Element svgRoot = doc.getDocumentElement();

        // set the width and height attribute on the root svg element
        svgRoot.setAttributeNS(null, "width", "400");
        svgRoot.setAttributeNS(null, "height", "450");

        // create the rectangle
        Element rectangle = doc.createElementNS(svgNS, "rect");
        rectangle.setAttributeNS(null, "x", "200");
        rectangle.setAttributeNS(null, "y", "200");
        rectangle.setAttributeNS(null, "width", "100");
        rectangle.setAttributeNS(null, "height", "100");
        rectangle.setAttributeNS(null, "fill", "red");

        // attach the rectangle to the svg root element
        svgRoot.appendChild(rectangle);

        } catch (Exception e){
            System.out.println("Error: " + e);
        }
    }

inputFilename is a string with my svg filename (test.svg)

I would be really grateful if somebody could help.