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