jdom 2.0.5 XML-Parsing error: not well formed

I have some problems by opening/ loading a generated xml-file because it's not well formed ("Invalid byte 1 of 1-byte UTF-8 sequence"). I already searched for some solutions and changed my code for getting the design as following:

System.out.println("Which Design-File shall be modified? Please enter: [file].xml");

    String file = "file.xml";

    // generate JDOM Document
    SAXBuilder builder = new SAXBuilder();
     try {
        // getting rid of the UTF8 problem when reading the modified xml file
        InputStream inputStream= new FileInputStream(file)          
        Reader reader = new InputStreamReader(inputStream,"UTF-8");

        InputSource is = new InputSource(reader);
        is.setEncoding("UTF-8");

        Document doc = builder.build(is);
     } 
     catch (JDOMException e) {
        e.printStackTrace();
     } 
     catch (IOException e) {
        e.printStackTrace();
    }

after changing the design, I write it with the XMLOutputter as followed:

    String fileNew = "file_modified.xml";
    FileWriter writer;
    try {
        Format format = Format.getPrettyFormat();
        format.setEncoding("UTF-8");
        writer = new FileWriter(fileNew);
        XMLOutputter outputter = new XMLOutputter(format);
        outputter.output(doc, writer);
    } catch (IOException e) {
        e.printStackTrace();
    }

Does somebody has a solution for my problem? I really thought that these codelines would be solve my problem, but when I load it into the firefox it still says that it's not well formed :/.