/**
  * Create a single instance of the db storage adapter
  * 
  * @return Tx_PtExtbase_State_Session_Storage_DBAdapter
  */
 public static function getInstance()
 {
     if (self::$instance == null) {
         self::$instance = new Tx_PtExtbase_State_Session_Storage_DBAdapter();
         self::$instance->injectStateCache(self::buildStateCache());
         self::$instance->setStateHash(self::getStateHash());
         self::$instance->init();
     }
     return self::$instance;
 }