Mybatis 之 SqlSession

电玩女神 2022-06-08 01:00 251阅读 0赞

从  [Mybatis 之 SqlSessionFactory ][Mybatis _ SqlSessionFactory] [http://blog.csdn.net/zsg88/article/details/77514475][Mybatis _ SqlSessionFactory]一文中，我们看到SqlSession在SqlSessionFactory的实现类DefaultSqlSessionFactory中被创建：

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
        Transaction tx = null;
        try {
          final Environment environment = configuration.getEnvironment();
          final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
          tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
          final Executor executor = configuration.newExecutor(tx, execType);
          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();
        }
      }
    
      private SqlSession openSessionFromConnection(ExecutorType execType, Connection connection) {
        try {
          boolean autoCommit;
          try {
            autoCommit = connection.getAutoCommit();
          } catch (SQLException e) {
            // Failover to true, as most poor drivers
            // or databases won't support transactions
            autoCommit = true;
          }      
          final Environment environment = configuration.getEnvironment();
          final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
          final Transaction tx = transactionFactory.newTransaction(connection);
          final Executor executor = configuration.newExecutor(tx, execType);
          return new DefaultSqlSession(configuration, executor, autoCommit);
        } catch (Exception e) {
          throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
        } finally {
          ErrorContext.instance().reset();
        }
      }

mybatis创建sqlsession经过了以下几个主要步骤：

1.       从核心配置文件mybatis-config.xml中获取Environment（这里面是数据源）；  
2.       从Environment中取得DataSource；  
3.       从Environment中取得TransactionFactory；  
4.       从DataSource里获取数据库连接对象Connection；  
5.       在取得的数据库连接上创建事务对象Transaction；  
6.       创建Executor对象（该对象非常重要，事实上sqlsession的所有操作都是通过它完成的）；  
7.       创建sqlsession对象。

DefaultSqlSession是SqlSession的实现类。我们来看一个具体的实现方法：

@Override
      public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
        try {
          MappedStatement ms = configuration.getMappedStatement(statement);
          return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
        } catch (Exception e) {
          throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
        } finally {
          ErrorContext.instance().reset();
        }
      }

我们看到，真正执行的代码在Executor对象里面

[Mybatis _ SqlSessionFactory]: http://blog.csdn.net/zsg88/article/details/77514475