Se guardi l'XStream documentazione, affermano chiaramente che devi aggiungere tu stesso il prolog XML:
Why does XStream not write an XML declaration?
XStream is designed to write XML snippets, so you can embed its output into
an existing stream or string. You can write the XML declaration yourself into
the Writer before using it to call XStream.toXML(writer).
Il seguente codice dovrebbe funzionare. Ho rimosso la maggior parte del tuo codice, quindi devi rimetterlo. Lo scopo è solo quello di darti un esempio di lavoro approssimativo:
private static void serializeData(Object diagData) throws Exception {
XStream xstream = new XStream();
xstream.processAnnotations(DiagData.class);
FileOutputStream fileOutputStream = null;
Writer writer = new PrintWriter(new File(your file));
CompactWriter compactWriter = new CompactWriter(writer);
try {
writer.write(your xml prolog);
writer.write(your stylesheet);
xstream.marshal(diagData, compactWriter);
} catch (FileNotFoundException e1) {
} catch (Exception e) {
} finally {
release resources
}
}