Problems with JAXB and UTF-16 encoding

Hi I have a small APP that reads content from an xml file and put it into a corresponding Java object.

Here is the XML:

<?xml version="1.0" encoding="UTF-16"?>
<Marker>
<TimePosition>2700</TimePosition>
<SamplePosition>119070</SamplePosition>
</Marker>

here is the corresponding Java code:

JAXBContext jaxbContext = JAXBContext.newInstance(MarkerDto.class);
Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();
InputStream inputStream = new FileInputStream("D:/marker.xml");
Reader reader = new InputStreamReader(inputStream, StandardCharsets.UTF_16.toString());
MarkerDto markerDto = (MarkerDto) jaxbUnmarshaller.unmarshal(reader);

If I run this code I get an "Content is not allowed in prolog." exception. If I run the same with UTF-8 everything works fine. Does anyone have a clue what might be the problem?

There's several things wrong here (ranging from slightly suboptimal, to potentially very wrong). In increasing order of likelihood of causing the problem:

1. When constructing an InputStreamReader, there's no need to call toString() on the Charset, because that class has a constructor that takes a Charset, so simply remove the .toString():

   Reader reader = new InputStreamReader(inputStream, StandardCharsets.UTF_16);
   

   This is a tiny nitpick and has no effect on functionality.

2. Don't construct a Reader at all! XML is a format that's self-describing when it comes to encoding: Valid XML files can be parsed without knowing the encoding up-front. So instead of creating a Reader, simply pass the InputStream directly into your XML-handling code. Delete the line that creates the Reader and change the next one to this:

    MarkerDto markerDto = (MarkerDto) jaxbUnmarshaller.unmarshal(inputStream);
   

   This may or may not fix your problem, depending on whether the input is well-formed.

3. Your XML file might have encoding="UTF-16" in the header and not actually be UTF-16 encoded. If that's the case, then it is malformed and a conforming parser will decline to parse it. Verify this by opening the file with the advanced text editor of your choice (I suggest Notepad++ on Windows, Linux users probably know what their preference is) and check if it shows "UTF-16" as encoding (and the content is readable).

   > If I run the same with UTF-8 everything works fine.

   This line suggests that that's what's actually happening here: the XML file is mis-labeling itself. This needs to be fixed at the point where the XML file is created.

   Notably, this demo code provides exactly the same Content is not allowed in prolog. exception message that is reported in the question:

   String xml = "<?xml version=\"1.0\" encoding=\"UTF-16\"?>\n<foo />";
   JAXBContext jaxbContext = JAXBContext.newInstance();
   Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();
   InputStream inputStream = new ByteArrayInputStream(xml.getBytes(StandardCharsets.UTF_8));
   jaxbUnmarshaller.unmarshal(inputStream);
   

   Note that the XML encoding attribute claims UTF-16, but the actual data handed to the XML parser is UTF-8 encoded.