Mybatis初体验之为什么获取SqlSession报空指针异常

报错如图

2019-05-17 12:39:37,988 0      [           main] DEBUG ache.ibatis.logging.LogFactory  - Logging initialized using 'class org.apache.ibatis.logging.log4j.Log4jImpl' adapter.
Exception in thread "main" org.apache.ibatis.exceptions.PersistenceException: 
### Error opening session.  Cause: java.lang.NullPointerException
### Cause: java.lang.NullPointerException
	at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:30)
	at org.apache.ibatis.session.defaults.DefaultSqlSessionFactory.openSessionFromDataSource(DefaultSqlSessionFactory.java:100)
	at org.apache.ibatis.session.defaults.DefaultSqlSessionFactory.openSession(DefaultSqlSessionFactory.java:47)
	at com.itheima.test.MybatisTest.main(MybatisTest.java:26)
Caused by: java.lang.NullPointerException
	at org.apache.ibatis.session.defaults.DefaultSqlSessionFactory.openSessionFromDataSource(DefaultSqlSessionFactory.java:95)
	... 2 more

Process finished with exit code 1

主测试代码如下

public class MybatisTest {
    public static void main(String[] args) throws IOException {
        InputStream in = Resources.getResourceAsStream("SqlMapConfig.xml");
        SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();
        SqlSessionFactory factory  = builder.build(in);
        SqlSession session = factory.openSession();
        //就是上面这行报错！！！
        IUserDao userDao = session.getMapper(IUserDao.class);
          List users = userDao.findAll();
        for (User u:users) {
            System.out.println(u);

        }
        session.close();
        in.close();
    }
}

其中SqlSession session= factory.openSession();这行报错。查了CSDN上的同样问题，他们都是Mybatis主配置文件的问题！但是我不是，我的代码如下

特来求教！！