Preserve unicode codepoints between ISO-8859-1 to UTF-8 conversions in XML in Java

Question 1

Here are three standard API methods to produce ISO-8859-1 encoded documents.

// output stream
ByteArrayOutputStream buffer = new ByteArrayOutputStream();
// transcode
StringReader xml = new StringReader("<x>pi: \u03A0</x>");
XMLEventReader reader = XMLInputFactory.newFactory().createXMLEventReader(
    xml);
XMLEventWriter writer = XMLOutputFactory.newFactory().createXMLEventWriter(
    buffer, "ISO-8859-1");
try {
  writer.add(reader);
} finally {
  writer.close();
}
// proof
String decoded = new String(buffer.toByteArray(),
    Charset.forName("ISO-8859-1"));
System.out.println(decoded);

Using the DOM API:

// output stream
ByteArrayOutputStream buffer = new ByteArrayOutputStream();
// create XML DOM
InputSource src = new InputSource(new StringReader("<x>pi: \u03A0</x>"));
Document doc = DocumentBuilderFactory.newInstance()
    .newDocumentBuilder()
    .parse(src);
// serialize
DOMImplementationLS impl = (DOMImplementationLS) doc.getImplementation();
LSOutput out = impl.createLSOutput();
out.setEncoding("ISO-8859-1");
out.setByteStream(buffer);
impl.createLSSerializer().write(doc, out);
// proof
String decoded = new String(buffer.toByteArray(),
    Charset.forName("ISO-8859-1"));
System.out.println(decoded);

Using the transform package:

// output stream
ByteArrayOutputStream buffer = new ByteArrayOutputStream();
// transformation
StreamSource src = new StreamSource(new StringReader("<x>pi: \u03A0</x>"));
StreamResult res = new StreamResult(buffer);
Transformer transformer = TransformerFactory.newInstance().newTransformer();
transformer.setOutputProperty(OutputKeys.ENCODING, "ISO-8859-1");
transformer.transform(src, res);
// proof
String decoded = new String(buffer.toByteArray(),
    Charset.forName("ISO-8859-1"));
System.out.println(decoded);

Which you would use depends on your use case; the StAX API is probably the most efficient.

All this sample code will emit documents equivalent to:

<?xml version="1.0"?><x>pi: &#x3a0;</x>

Question 2

The Unicode code points above 127 can be encoded as numeric entities like { using the following:

From Apache commons StringEscapeUtils. Read the javadoc, by default escapeXML does not convert to numeric entities.

StringEscapeUtils.ESCAPE_XML
    .with(NumericEntityEscaper.between(0x7f, Integer.MAX_VALUE));

BTW you also try sending the original XML using a header Content-Type: application/x-xml, so binary transfer.