code: public static void setSavePoint() {
          Connection connection = tl_conn.get();
          if (connection == null) {
              throw new RuntimeException("You do not start a Transaction so you can not set a savepoint!");
          }
          try {
              Stack<Savepoint> stack_sp = tl_sp.get();
              Savepoint sp = connection.setSavepoint();
              stack_sp.push(sp);
          } catch (SQLException e) {
              throw new RuntimeException(e);
          }
      }
docs: 用于在事务中设置一个保存点，属于线程安全的