code: public synchronized SessionInfo addLock(Object bookmark, SessionInfo sessionInfo)
      {
          Utility.getLogger().info("Lock: " + bookmark + ", Session: " + sessionInfo.m_iSessionID + " success: " + !this.containsKey(bookmark));
          if (this.containsKey(bookmark))
              return (SessionInfo)this.get(bookmark);   // Error, already locked
          return (SessionInfo)this.put(bookmark, sessionInfo);    // Returns null
      }
docs: Add this lock to the table.
  @param bookmark The bookmark to lock.
  @param session The session to lock.
  @return null if successful, the session of the locking user if not successful.