[java] Fatal Error :1:1: Content is not allowed in prolog

I'm using Java and i'm trying to get XML document from some http link. Code I'm using is:

URL url = new URL(link);

HttpURLConnection connection = (HttpURLConnection)url.openConnection();
connection.setRequestMethod("GET");
connection.connect();
Document doc = null;

CountInputStream in = new CountInputStream(url.openStream());
doc = DocumentBuilderFactory.newInstance().newDocumentBuilder().parse(in);

Don't pay attention at CountInputStream, it's some special class acting like regular input stream.

Using the code above, I sometimes got error Fatal Error :1:1: Content is not allowed in prolog. I assume that is has something to do with bad format of xml, but I have no idea how to fix it.

Looks like you forgot adding correct headers to your get request (ask the REST API developer or you specific API description):

HttpURLConnection connection = (HttpURLConnection)url.openConnection();
connection.header("Accept", "application/xml")
connection.setRequestMethod("GET");
connection.connect();

or

connection.header("Accept", "application/xml;version=1")

Someone should mark Johannes Weiß's comment as the answer to this question. That is exactly why xml documents can't just be loaded in a DOM Document class.

http://en.wikipedia.org/wiki/Byte_order_mark

The real solution that I found for this issue was by disabling any XML Format post processors. I have added a post processor called "jp@gc - XML Format Post Processor" and started noticing the error "Fatal Error :1:1: Content is not allowed in prolog"

By disabling the post processor had stopped throwing those errors.

There are certainly some weird characters (e.g. BOM) or some whitespace before the XML preamble (<?xml ...?>)?

It could be not supported file encoding. Change it to UTF-8 for example.

I've done this using Sublime