public static SqlSession getSqlSession(SqlSessionFactory sessionFactory, ExecutorType executorType, PersistenceExceptionTranslator exceptionTranslator) {
　　 //从从前线程的threadLocal 中获取sqlSessionHolder
    SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);
    SqlSession session = sessionHolder(executorType, holder);
　　 // 判断当前sqlSessionHolder 中是否持有sqlSession (即当前操作是否在事务当中)
    if (session != null) {
　　　　//如果持有sqlSesison 的引用，则直接获取
      return session;
    }

    if (LOGGER.isDebugEnabled()) {
      LOGGER.debug("Creating a new SqlSession");
    }
    //获取新的sqlSession 对象。这里由sessionFacory产生的defaultSqlSession
    session = sessionFactory.openSession(executorType);
    //判断判断，当前是否存在事务，将sqlSession 绑定到sqlSessionHolder 中，并放到threadLoacl 当中
    registerSessionHolder(sessionFactory, executorType, exceptionTranslator, session);
    return session;
  }

  private static void registerSessionHolder(SqlSessionFactory sessionFactory, ExecutorType executorType,
      PersistenceExceptionTranslator exceptionTranslator, SqlSession session) {
    SqlSessionHolder holder;
　　 //判断事务是否存在
    if (TransactionSynchronizationManager.isSynchronizationActive()) {
      Environment environment = sessionFactory.getConfiguration().getEnvironment();

      if (environment.getTransactionFactory() instanceof SpringManagedTransactionFactory) {
        if (LOGGER.isDebugEnabled()) {
          LOGGER.debug("Registering transaction synchronization for SqlSession [" + session + "]");
        }

        holder = new SqlSessionHolder(session, executorType, exceptionTranslator);
　　     //如果当前回话处在事务当中，则将holder 绑定到ThreadLocal 当中
        TransactionSynchronizationManager.bindResource(sessionFactory, holder);
        TransactionSynchronizationManager.registerSynchronization(new SqlSessionSynchronization(holder, sessionFactory));
        holder.setSynchronizedWithTransaction(true);
        holder.requested();
      } else {
        if (TransactionSynchronizationManager.getResource(environment.getDataSource()) == null) {
          if (LOGGER.isDebugEnabled()) {
            LOGGER.debug("SqlSession [" + session + "] was not registered for synchronization because DataSource is not transactional");
          }
        } else {
          throw new TransientDataAccessResourceException(
              "SqlSessionFactory must be using a SpringManagedTransactionFactory in order to use Spring transaction synchronization");
        }
      }
    } else {
      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("SqlSession [" + session + "] was not registered for synchronization because synchronization is not active");
      }
    }
}

  public static void closeSqlSession(SqlSession session, SqlSessionFactory sessionFactory) {
    notNull(session, NO_SQL_SESSION_SPECIFIED);
    notNull(sessionFactory, NO_SQL_SESSION_FACTORY_SPECIFIED);

    SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);
    //如果holder 中持有sqlSession 的引用，（即会话存在事务）
    if ((holder != null) && (holder.getSqlSession() == session)) {
      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Releasing transactional SqlSession [" + session + "]");
      }
　　　 //每当一个sqlSession 执行完毕，则减少holder 持有引用的次数
      holder.released();
    } else {
      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Closing non transactional SqlSession [" + session + "]");
      }
      //如果回话中，不存在事务，则直接关闭session
      session.close();
    }
  }