code: public static void startTransaction() {
          Connection connection = tl_conn.get();
          if (connection != null) {
              throw new RuntimeException(
                      "You have started a Transaction and can not start transaction repeatedly for the same connection!");
          }
  
          try {
              connection = JDBCUtils.getConnection();
              connection.setAutoCommit(false);
              tl_conn.set(connection);
              tl_sp.set(new Stack<Savepoint>());
          } catch (SQLException e) {
              throw new RuntimeException(e);
          }
      }
docs: 开始一个事务，该事务连接是线程安全的。