java.lang.NoClassDefFoundError

这是啥情况

下面是MyBatilsUtils类和mybatis-config配置文件

package com.imooc.MyBatis;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import java.io.IOException;
import java.io.Reader;
public class MyBatilsUtils {
    private static SqlSessionFactory sqlSessionFactory=null;
    static{
        Reader reader=null;
        try{reader= Resources.getResourceAsReader("mybatis-config.xml");
            SqlSessionFactory sqlSessionFactory=new SqlSessionFactoryBuilder().build(reader);
        }catch(IOException e){e.printStackTrace();
            throw new ExceptionInInitializerError(e);}}
    public static SqlSession openSession(){
        return sqlSessionFactory.openSession();}
    public static void closeSession(SqlSession session){
        if(session!=null){session.close();}}
}

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration
        PUBLTC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <!--设置默认数据库指向-->
    <environments default="dev">
        <!--配置不同的数据库连接-->
        <environment id="dev">
            <!--采用JDBC方式对数据库事务进行commit/rollback-->
            <transactionManager type="JDBC"></transactionManager>>
        <!--采用连接池方式管理数据库连接-->
        <dataSource type="POOLED">
            <property name="driver" value="com.mysql.jdbc.Driver"/>
            <property name="url" value="jdbc:mysql://localhost:3308/babytun?useSSL=false&amp;useUnicode=true&amp;characterEncoding=UTF-8&amp;serverTimezone=Asia/Shanghai"/>
            <property name="username" value="root"/>
            <property name="password" value="www1473040752"/>
        </dataSource>
        </environment>
</environments>
    <settings>
        <setting name="mapUnderscoreToCamelCase" value="true"/>
    </settings>
    <mappers>
        <mapper resource="mappers/student.xml"></mapper>
    </mappers>
        </configuration>