"Content is not allowed in prolog" when parsing perfectly valid XML on GAE

[Avinash Dubey via StackOverflow]

bellow are cause above “org.xml.sax.SAXParseException: Content is not allowed in prolog” exception.

1. First check the file path of schema.xsd and file.xml.
2. The encoding in your XML and XSD (or DTD) should be same.
   XML file header: <?xml version='1.0' encoding='utf-8'?>
   XSD file header: <?xml version='1.0' encoding='utf-8'?>
3. if anything comes before the XML document type declaration.i.e: hello<?xml version='1.0' encoding='utf-16'?>

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