code: public void sweepDisk() {
          //iterate over the files in the store dir and check expiry times
          long now = System.currentTimeMillis();
          if (log.isDebugEnabled()) {
              log.debug("Sweeping " + storeDir + " for old session files");
          }
          try {
              Files.walk(storeDir.toPath(), 1, FileVisitOption.FOLLOW_LINKS)
                      .filter(p -> !Files.isDirectory(p))
                      .filter(p -> isSessionFilename(p.getFileName().toString()))
                      .forEach(p -> {
                          try {
                              sweepFile(now, p);
                          } catch (Exception e) {
                              log.warn(e.getMessage(), e);
                          }
  
                      });
          } catch (Exception e) {
              log.warn(e.getMessage(), e);
          }
      }
docs: Check all session files that do not belong to this context and
  remove any that expired long ago (ie at least 5 gracePeriods ago).