1、SqlSession session = sqlSessionFactory.openSession(); // 获取SqlSession 对象

1）configuration.getDefaultExecutorType()获取默认的执行器类型，一共有三种，默认是SIMPLE

public enum ExecutorType {
   SIMPLE, // 简单类型
   REUSE, // 重用类型 
   BATCH  // 批量处理类型
}

2）openSessionFromDataSource方法

SqlSession session = sqlSessionFactory.openSession();
  // configuration.getDefaultExecutorType()获取默认的执行器类型
  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);
    // 创建事务
    tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
    // 创建默认执行器SIMPLE
    final Executor executor = configuration.newExecutor(tx, execType);
    // 创建DefaultSqlSession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();
  }
}

2）获取mapper接口代理对象

获取之前SqlSessionFactory初始化时保存的Mapper代理工厂，创建出mapper代理对象，调用栈

UserMapper mapper = session.getMapper(UserMapper.class);
  configuration.getMapper(type, this);
    mapperRegistry.getMapper(type, sqlSession);
      // 获取mapper代理工厂
	  final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
        // mapper代理工厂创建mapper代理对象  
	    mapperProxyFactory.newInstance(sqlSession);
	      newInstance(mapperProxy);
		    (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);

3) 根据mapper代理对象调用mapper接口方法时，就会执行invocationHandler中的invoke方法，执行sql并解析结果返回。