Apex XML2JSON.cls

String xml2json(String xml) {
    XmlStreamReader reader = new XmlStreamReader(xml);
    reader.setCoalescing(true);

    //skip START_DOCUMENT
    reader.nextTag();

    JsonGenerator g = Json.createGenerator(true);
    g.writeStartObject();

    while (reader.hasNext()) {

        if (reader.isStartElement()) {

            //found open tag
            String name = reader.getLocalName();
            reader.next();

            if (reader.isWhitespace()) {
                //skip whitespace
                reader.next();
            }

            if (reader.isEndElement()) {
                //found immediately closing tag
                g.writeNullField(name);

            } else if (!reader.isCharacters()) {
                //found object
                if (name.endsWith('Set') || name.endsWith('Mapping')) {
                    g.writeFieldName(name);
                    g.writeStartArray();

                } else if (name == 'item') {
                    g.writeStartObject();

                } else {
                    g.writeFieldName(name);
                    g.writeStartObject();
                }

                continue;

            } else {
                //found text

                String text = reader.getText();

                if (text == 'true' || text == 'false') {
                    //found boolean field
                    g.writeBooleanField(name, text == 'true');

                } else {
                    //all others strings
                    g.writeStringField(name, reader.getText());
                }

                reader.nextTag(); //skip close
            }

        } else if (reader.isCharacters()) {

            //found text
            g.writeObject(reader.getText());
            reader.nextTag(); //skip close

        }  else if (reader.isEndElement()) {

            //found end tag
            String name = reader.getLocalName();
            if (name.endsWith('Response')) {
                //nothing to do
            } else if (name.endsWith('Set') || name.endsWith('Mapping')) {
                g.writeEndArray();
            } else if (name == 'item') {
                g.writeEndObject();
            } else {
                g.writeEndObject();
            }
            reader.next();
        }

        reader.next();
    }

    return g.getAsString();
}