public enum Detail { GENERAL, DETAILED }

public class MyClass {
    Detail detail;
    /* getters & setters */
}

<app:MyClass detail="GENERAL" xmlns:app="http://example.com"/>

private XMLDescriptor createDescriptor() {
    XMLDescriptor descriptor = super.buildDescriptor(MyClass.class);
    descriptor.setDefaultRootElement("app:MyClass");
    descriptor.add(createDetailMapping());
    return descriptor;
}

private XMLTransformationMapping createDetailMapping() {
    XMLTransformationMapping mapping = new XMLTransformationMapping();
    mapping.setAttributeName("detail");
    EnumTransformer transformer = new EnumTransformer();
    mapping.setAttributeTransformer(transformer);
    mapping.addFieldTransformer("@detail", transformer);
    return mapping;
}

public class EnumTransformer implements AttributeTransformer, FieldTransformer {
  private AbstractTransformationMapping mapping;
  private XMLField detailField;

  public void initialize(AbstractTransformationMapping mapping) {
    this.mapping = mapping;
    NamespaceResolver namespaceResolver = ((XMLDescriptor)mapping.getDescriptor()).getNamespaceResolver();
    detailField = new XMLField("@detail");
    detailField.setNamespaceResolver(namespaceResolver);
  }

  public Object buildAttributeValue(Record record, Object object, Session session) {
    String detail = (String) record.get(detailField);
    return Detail.valueOf(detail);
  }
  
  public Object buildFieldValue(Object instance, String fieldName, Session session) {
    return mapping.getAttributeValueFromObject(instance);
  }
}

java.lang.NullPointerException
	at org.eclipse.persistence.oxm.record.UnmarshalRecord.startElement(UnmarshalRecord.java:551)
	at org.eclipse.persistence.internal.oxm.record.SAXUnmarshallerHandler.startElement(SAXUnmarshallerHandler.java:273)
	at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.startElement(AbstractSAXParser.java:501)
	at com.sun.org.apache.xerces.internal.impl.XMLNSDocumentScannerImpl.scanStartElement(XMLNSDocumentScannerImpl.java:400)
	at com.sun.org.apache.xerces.internal.impl.XMLNSDocumentScannerImpl$NSContentDriver.scanRootElementHook(XMLNSDocumentScannerImpl.java:626)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDriver.next(XMLDocumentFragmentScannerImpl.java:3095)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl$PrologDriver.next(XMLDocumentScannerImpl.java:921)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(XMLDocumentScannerImpl.java:648)
	at com.sun.org.apache.xerces.internal.impl.XMLNSDocumentScannerImpl.next(XMLNSDocumentScannerImpl.java:140)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(XMLDocumentFragmentScannerImpl.java:510)
	at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:807)
	at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:737)
	at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(XMLParser.java:107)
	at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(AbstractSAXParser.java:1205)
	at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(SAXParserImpl.java:522)
	at org.eclipse.persistence.internal.oxm.record.XMLReader.parse(XMLReader.java:99)
	at org.eclipse.persistence.internal.oxm.record.SAXUnmarshaller.unmarshal(SAXUnmarshaller.java:215)
	at org.eclipse.persistence.internal.oxm.record.SAXUnmarshaller.unmarshal(SAXUnmarshaller.java:192)
	at org.eclipse.persistence.oxm.XMLUnmarshaller.unmarshal(XMLUnmarshaller.java:274)
	...