//ExecutorType是SIMPLE,一共有三种SIMPLE(SimpleExecutor)、REUSE(ReuseExecutor)、BATCH(BatchExecutor)
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
  Transaction tx = null;
  try {
    //xml中的development节点
    final Environment environment = configuration.getEnvironment();
    //type配置的是Jbdc所以生成的是JbdcTransactionFactory工厂类
    final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
    //Jdbc生成JbdcTransactionFactory生成JbdcTransaction
    tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
    //创建CachingExecutor执行器
    final Executor executor = configuration.newExecutor(tx, execType);
    //创建DefaultSqlSession属性包括 Configuration、Executor对象
    return new DefaultSqlSession(configuration, executor, autoCommit);
  } catch (Exception e) {
    closeTransaction(tx); // may have fetched a connection so lets call
    close();
    throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
  } finally {
    ErrorContext.instance().reset();
  }
}