code: public static synchronized DataUsageExtension instance() {
                  if (SINGLETON == null) {
                          SINGLETON = new DataUsageExtension();
                          XFactory factory = XFactoryRegistry.instance().currentDefault();
                          ATTR_DATA = factory.createAttributeLiteral(KEY_DATA, "", SINGLETON);
                          SINGLETON.eventAttributes.add((XAttribute) ATTR_DATA.clone());
                          // register mapping aliases
                          for (Map.Entry<String, String> mapping : EXTENSION_DESCRIPTIONS.entrySet()) {
                                  XGlobalAttributeNameMap.instance().registerMapping(mapping.getKey(), KEY_DATA, mapping.getValue());
                          }
                  }
                  return SINGLETON;
          }
docs: Provides access to the singleton instance of this extension.
  
  @return The AttributeDataUsage extension singleton.