private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
      // 獲取環(huán)境配置
      final Environment environment = configuration.getEnvironment();
      // 創(chuàng)建事務(wù)
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      // 創(chuàng)建執(zhí)行器
      final Executor executor = configuration.newExecutor(tx, execType);
      // 創(chuàng)建sqlsession
      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();
    }
  }

// 構(gòu)造方法
public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType,
                          PersistenceExceptionTranslator exceptionTranslator) {
    ...省略無關(guān)緊要的代碼
        this.sqlSessionProxy = (SqlSession) newProxyInstance(
        SqlSessionFactory.class.getClassLoader(),
        new Class[] { SqlSession.class },
        new SqlSessionInterceptor());
}

public class SqlSessionTemplate implements SqlSession, DisposableBean {...}

@Override
public <E> List<E> selectList(String statement, Object parameter) {
    return this.sqlSessionProxy.selectList(statement, parameter);
}

private class SqlSessionInterceptor implements InvocationHandler {
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
      // 去獲取SqlSession
      SqlSession sqlSession = getSqlSession(SqlSessionTemplate.this.sqlSessionFactory,
          SqlSessionTemplate.this.executorType, SqlSessionTemplate.this.exceptionTranslator);
      try {
        // 通過反射調(diào)用真正的處理方法
        Object result = method.invoke(sqlSession, args);
        if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
          // 提交數(shù)據(jù)
          sqlSession.commit(true);
        }
        // 返回查詢的數(shù)據(jù)
        return result;
      } catch (Throwable t) {
        ...省略無關(guān)緊要的代碼
      } finally {
        if (sqlSession != null) {
          // 關(guān)閉SqlSession的連接
          closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
        }
      }
    }
  }

public static SqlSession getSqlSession(SqlSessionFactory sessionFactory, ExecutorType executorType,
      PersistenceExceptionTranslator exceptionTranslator) {
    ...省略無關(guān)緊要的代碼
    // 從ThreadLocal變量里面獲取到Spring的事務(wù)同步管理器
    SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);
    // 調(diào)用靜態(tài)方法sessionHoler 判斷是否存在符合要求的sqlSession
    SqlSession session = sessionHolder(executorType, holder);
    if (session != null) {
      return session;
    }
    // 如果SqlSessionHolder中獲取的SqlSession為空，則新建一個SqlSession
    session = sessionFactory.openSession(executorType);
    // 判斷當(dāng)前是否存在事務(wù)，將sqlSession 綁定到sqlSessionHolder 中，并放到threadLoacl 當(dāng)中
    registerSessionHolder(sessionFactory, executorType, exceptionTranslator, session);
    return session;
  }

public static void closeSqlSession(SqlSession session, SqlSessionFactory sessionFactory) {
	...省略無關(guān)緊要的代碼
    SqlSessionHolder holder = 
          (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);
    // 查看事務(wù)同步管理器是否存在 session 
    if ((holder != null) && (holder.getSqlSession() == session)) {
      holder.released();
    } else {
      // 如果不存在就將該Session關(guān)閉掉
      session.close();
    }
  }