That's an interesting limitation of the API I hadn't encountered yet. It would be great if vtd-xml-author could elaborate on technical details and why this limitation exists.
As a solution to your problem, a simple approach would be to accumulate your key-value pairs to be inserted as a String, and then to insert them in a single call after your for loop has terminated.
I've tested that this works as per your code:
private XMLModifier xm_ = new XMLModifier();
xm.bind(vn);
String insertedAttributes = "";
for (String key : attr.keySet()) {
int i = vn.getAttrVal(key);
if (i!=-1) {
xm.updateToken(i, attr.get(key));
} else {
// Store the key-values to be inserted as attributes
insertedAttributes += " " + key + "='" + attr.get(key) + "'";
}
}
if (!insertedAttributes.equals("")) {
// Insert attributes only once
xm.insertAttribute(insertedAttributes);
}
This will also work if you need to update the attributes of multiple elements, simply nest the above code in while(autoPilot.evalXPath() != -1)
and be sure to set insertedAttributes = "";
at the end of each while loop.
Hope this helps.