关于错误Cause: org.apache.ibatis.builder.BuilderException: Mapper’s namespace cannot be empty的解决方案

错误原因：
是Mapper.xml中的namespace没有命名，所以报错。

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper>
    <select id="selectAll" resultType="Goods">
        select * from book;
    </select>
</mapper>

    <!--其余代码略……-->

解决方案：
添加上namespace即可

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
        
<!--添加上namespace即可-->
<mapper namespace="com.ssm.mapper.BookInfoMapper">

    <select id="selectAll" resultType="Goods">
        select * from book;
    </select>
</mapper>