FULL PRODUCT VERSION :
java version "1.8.0_45"
Java(TM) SE Runtime Environment (build 1.8.0_45-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.45-b02, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Linux AMDC1917 3.13.0-52-generic #86-Ubuntu SMP Mon May 4 04:32:59 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

EXTRA RELEVANT SYSTEM CONFIGURATION :
Just Jdk version

A DESCRIPTION OF THE PROBLEM :
If this is indeed an issue, a first description exists on stackoverflow:
http://stackoverflow.com/questions/27437603/setting-standalone-yes-using-xmleventwriter

When using StAX to create an XML document, the XMLEventFactory.createStartDocument does create a XMLEvent event. When adding the XMLEvent into a XMLEventWriter, it does not produce the standalone property, even if explicitly given as parameter.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1 - create a XMLOutputFactory
2 - create a XMLEventFactory
3 - create an output file (XML)
4 - create a XMLEventWriter on this file with the XMLOutputFactory
5 - create a XMLEvent as {XMLEventFactory}.createStartDocument("iso-8859-15", "1.0", true);
6 - add this event to the XMLEventWriter
7 - flush cached events if necessary

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
creation of an (empty) document with XML header as:

<?xml version="1.0" encoding="iso-8859-15" standalone="yes"?>
ACTUAL -
creation of an (empty) document with XML header as:

<?xml version="1.0" encoding="iso-8859-15"?>

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStreamWriter;

import javax.xml.stream.XMLEventFactory;
import javax.xml.stream.XMLEventWriter;
import javax.xml.stream.XMLOutputFactory;
import javax.xml.stream.XMLStreamConstants;
import javax.xml.stream.XMLStreamException;
import javax.xml.stream.events.StartDocument;
import javax.xml.stream.events.XMLEvent;



    /**
     * test for:
     * http://stackoverflow.com/questions/28629709/java-stax-standalone-property-of-startdocument
     * using XMLEventReader | XMLEventWriter
     * @param args
     */
    public testStandalone() {
        XMLOutputFactory factory_out = null;
        XMLEventFactory eventFactory = null;
        XMLEventWriter eventWriter = null;
        XMLEvent event_out = null;

        try {
            factory_out = XMLOutputFactory.newInstance();
            eventFactory = XMLEventFactory.newInstance();
            FileOutputStream out = new FileOutputStream("write_out.xml");
            OutputStreamWriter w = new OutputStreamWriter(out, "iso-8859-15");
            eventWriter = factory_out.createXMLEventWriter(w);

            event_out = eventFactory.createStartDocument("iso-8859-15", "1.0", true);
            eventWriter.add(event_out);
            eventWriter.flush();
        } catch (XMLStreamException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (Exception e) {
            //UnsupportedEncodingException or FileNotFoundException
            e.printStackTrace();
            return;
        }

    }


---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
no workaround found, maybe a missing implementation on com.sun.xml.internal.stream.writers.XMLStreamWriterImpl :

public void writeStartDocument(String encoding, String version)
    throws XMLStreamException {
    //Revisit : What about standalone ?