只获取一次sqlSessionFactory，并且用它获取sqlSession，不然每个sql方法的开头都要用这段代码

//myBatis获取sqlSession的工具类
public class MyBatisUtils {

    private static SqlSessionFactory factory = null;

    //SqlSessionFactory只需要获取一次
    static {
        try {
            //mybatis.xml需要和项目中的文件名一致
            InputStream is = Resources.getResourceAsStream("mybatis.xml");
            SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();
            factory = builder.build(is);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    //获取sqlSession的方法
    public static SqlSession getSqlSession(){

        SqlSession sqlSession = null;
        if (factory != null){
            sqlSession = factory.openSession();//手动提交事务
            //sqlSession = factory.openSession(true);自动提交事务
        }
        return sqlSession;
    }
}