gistfile1.java

@JRubyMethod
public IRubyObject native_write_to(ThreadContext context, IRubyObject io, IRubyObject encoding, IRubyObject indentString, IRubyObject options) {
StringWriter sw = new StringWriter();
try {
    Transformer t = TransformerFactory.newInstance().newTransformer();
    t.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "yes");
    t.transform(new DOMSource(this.node), new StreamResult(sw));
} catch (TransformerException te) {
    throw context.getRuntime().newRuntimeError("couldn't transform the node back to string");
}

RuntimeHelpers.invoke(context, io, "write", context.getRuntime().newString(sw.toString()));

return io;
}

gistfile2.h

/*
 * call-seq:
 *  native_write_to(io, encoding, options)
 *
 * Write this Node to +io+ with +encoding+ and +options+
 */
static VALUE native_write_to(
    VALUE self,
    VALUE io,
    VALUE encoding,
    VALUE indent_string,
    VALUE options
) {
  xmlNodePtr node;

  Data_Get_Struct(self, xmlNode, node);

  xmlIndentTreeOutput = 1;

  xmlTreeIndentString = StringValuePtr(indent_string);

  xmlSaveCtxtPtr savectx = xmlSaveToIO(
      (xmlOutputWriteCallback)io_write_callback,
      (xmlOutputCloseCallback)io_close_callback,
      (void *)io,
      RTEST(encoding) ? StringValuePtr(encoding) : NULL,
      NUM2INT(options)
  );

  xmlSaveTree(savectx, node);
  xmlSaveClose(savectx);
  return io;
}