[大数据]xml映射(利用映射完成helloworld)

1. 什么是xml映射
   MyBatis提供了注解定义SQL与Mapper接口的映射关系。但是注解不能解决复杂的映射关系，例如sql拼接，所以需要使用xml映射。

2. helloWorld程序

   文件结构
   

someMapper.xml

<?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 namespace="cn.tedu.mapper.DemoMapper">
    <select id="test" resultType="java.lang.String">
        select 'hello world'
    </select>
</mapper>

• namespace对应DemoMapper地址
• id对应方法名，此处是demomapper中的test方法
• resultType对应方法的返回值，test返回值为String

DemoMapper

package cn.tedu.mapper;

import org.apache.ibatis.annotations.Select;

public interface DemoMapper {
    @Select("select empno,ename,sal,deptno from emp where empno=1;")
    String hello();

    String test();
}

MybatisConfig.java

//配置SqlSessionFactory的作用是告诉mybatis如何找到目标数据库
    @Bean
    public SqlSessionFactory sqlSessionFactory(DataSource dataSource,
           @Value("${mybatis.mapper.location}") Resource[] mapperLocation)
            throws Exception {
        SqlSessionFactoryBean bean = new SqlSessionFactoryBean();
        bean.setDataSource(dataSource);
        //将所有的Mapper映射xml文件注入给MyBatis
        bean.setMapperLocations(mapperLocation);
        return bean.getObject();
    }

• @Value("${mybatis.mapper.location}") Resource[] mapperLocation获取映射关系xml,从jdbc.properties中获取
• bean.setMapperLocations(mapperLocation);注入MyBatis

jdbc.properties

db.driver=com.mysql.cj.jdbc.Driver
db.url=jdbc:mysql://localhost:3306/newdb3?characterEncoding=utf8&useSSL=false&serverTimezone=Asia/Shanghai&rewriteBatchedStatements=true
db.username=root
db.password=
db.maxActive=10
db.initialSize=2
mybatis.mapper.location=classpath:mappers/*.xml