code: public void forceCheckpoint() throws PersistException {
          if (mCheckpointer != null) {
              mCheckpointer.forceCheckpoint();
          } else {
              try {
                  env_checkpoint();
              } catch (Exception e) {
                  throw toPersistException(e);
              }
          }
      }
docs: Forces a checkpoint to run now, even if checkpointer is suspended or
  disabled. If a checkpoint is in progress, then this method will block
  until it is finished, and then run another checkpoint. This method does
  not return until the requested checkpoint has finished.