session创建过程

 在mybatis的常规用法当中，通过sqlSessionFactory.openSession()来创建SqlSession对象，这里的sqlSessionFactory就是在解析配置后生成的SqlSession的工厂。

public class MyBatisTest {
    public static void main(String[] args) throws Exception {
        String resource = "configuration.xml";
        Reader reader = Resources.getResourceAsReader(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder()
                .build(reader);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        User user = sqlSession.selectOne("userTest.selectUser", 1);
        System.out.println(user.getUsername());
        sqlSession.close();
    }
}

 在openSession过程中我们按照下面顺序创建SqlSession对象

• 生成mybatis的Environment对象
• 生成TransactionFactory对象
• 生成Transaction对象
• 生成Executor对象
• 生成DefaultSqlSession对象

  public SqlSession openSession() {
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
  }


  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 TransactionFactory getTransactionFactoryFromEnvironment(Environment environment) {
    if (environment == null || environment.getTransactionFactory() == null) {
      return new ManagedTransactionFactory();
    }
    return environment.getTransactionFactory();
  }

session组件依赖图

 通过下面这个图基本上把mybatis里面几个组件的创建顺序描述清楚了。

session组件类关系图

 通过下面这个图，基本上就把mybatis里面核心的几个类的继承包含关系描述清楚了。