NoClassDefFoundError是什么情况？

测试代码

@Test
public void selectAllTest() {
    SqlSession sqlSession = null;
    try {
        sqlSession = MyBatisUtils.openSession();
        List<Goods> goodsList = sqlSession.selectList("goods.selectAll");
        for (Goods g :
                goodsList) {
            System.out.println(g.getTitle());
        }
    } catch (Exception e) {
        throw new RuntimeException(e);
    } finally {
        MyBatisUtils.closeSession(sqlSession);
    }

}

Utils代码

public class MyBatisUtils {
    private static SqlSessionFactory sqlSessionFactory = null;

    static {
        try {
            Reader reader = Resources.getResourceAsReader("mybatis-config.xml");
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
        } catch (IOException e) {
            e.printStackTrace();
            // 初始化异常，抛出InInitializerError
            System.out.println("初始化异常");
            throw new ExceptionInInitializerError(e);
        }
    }

    // 获取sqlSession连接
    public static SqlSession openSession() {
        return sqlSessionFactory.openSession();
    }

    // 关闭sqlSession连接
    public static void closeSession(SqlSession sqlSession) {
        if (sqlSession != null)
            sqlSession.close();
    }
}

mybatis-config.xml文件

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <settings>
<!--        是否开启驼峰命名自动映射，即从经典数据库列名 A_COLUMN 映射到经典 Java 属性名 aColumn。-->
        <setting name="mapUnderscoreToCamelCase" value="true"/>
    </settings>
<!--    设置默认指向的数据库-->
    <environments default="dev">
        <!-- 配置环境，不同的环境不同的id名字-->
        <environment id="dev">
            <transactionManager type="JDBC"></transactionManager>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="jdbcUrl" value="jdbc:mysql://localhost:3306/babytun?useUnicode=true&amp;characterEncoding=UTF-8"/>
                <property name="username" value="root"/>
                <property name="password" value="abc123456"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="mappers.goods.xml"/>
    </mappers>
</configuration>