初探mybatis

这里用mysql作为测试
需要的依赖包 mybatis-3.5.7.jar，mysql-connector-java-8.0.24.jar

使用xml配置

项目结构

image.png

Teacher.java是简单的java bean
Main.java代码如下

package com.ly1;

import com.ly1.entity.Teacher;
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.InputStream;

public class Main {
    public static void main(String[] args) throws IOException {
        /*final InputStream resourceAsStream =
                Main.class.getResourceAsStream("/mappings/mybatis-config.xml");
        Scanner scanner=new Scanner(resourceAsStream);
        while (scanner.hasNextLine()){
            System.out.println(scanner.nextLine());
        }*/
        String id="1";
        if(args.length==1){
            id=args[0] ;
        }
        String resource = "mappings/mybatis-config.xml";
        InputStream inputStream =  Resources.getResourceAsStream(
                resource
        );
        //或者下面形式也是可以的,注意地址
        /*resource = "/mappings/mybatis-config.xml";
        inputStream =  Main.class.getResourceAsStream(
                resource
        );*/
        SqlSessionFactory sqlSessionFactory
                = new SqlSessionFactoryBuilder().build(inputStream);
        //从factory中获取sqlSession的实例
        try(SqlSession sqlSession=sqlSessionFactory.openSession()){
            long time1=System.currentTimeMillis();
            Teacher teacher= sqlSession.selectOne(
                    "org.mybatis1.example.TeacherMapper.selectTeacher",
                    id);
            long time2=System.currentTimeMillis();
            //这里可能是由于第一次连接数据库需要耗费一些时间
            System.out.println("第一次查询使用了"+(time2-time1)+"毫秒");//700+
            time1=System.currentTimeMillis();
            Teacher teacher2= sqlSession.selectOne(
                    "org.mybatis1.example.TeacherMapper.selectTeacher",
                    3);
            time2=System.currentTimeMillis();
            System.out.println("第二次查询使用了"+(time2-time1)+"毫秒");//1ms
            System.out.println(teacher2);
        }
    }
}

注意，这里的org.mybatis1.example.TeacherMapper对应的是mapper里面的namespace
TeacherMapper.xml代码如下

<?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">
<!--这里namespace可以随便写-->
<mapper namespace="org.mybatis1.example.TeacherMapper">
    <select id="selectTeacher" resultType="com.ly1.entity.Teacher">
        select * from teacher where id = #{id}
    </select>
</mapper>

mybatis-config.xml的代码如下

<?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>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://127.0.0.1:3306/test"/>
                <property name="username" value="root"/>
                <property name="password" value="test.lwm"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <!--注意，这里的地址要写resources下的绝对地址，不能写相对地址-->
        <mapper resource="mappings\org\mybatis\example\TeacherMapper.xml" />
    </mappers>
</configuration>

稍微更改，使用Mapper注解进行映射

在mybatis-config.xml中写上要扫描的包(mappers标签中)

<mappers>
        <!--注意，这里的地址要写resources下的绝对地址，不能写相对地址-->
        <mapper resource="mappings\org\mybatis\example\TeacherMapper.xml" />
        <!--注明要扫描的包-->
        <package name="com.ly1.mappers" />
    </mappers>

新增加的代码

image.png

TeacherMapper.java

package com.ly1.mappers;
import com.ly1.entity.Teacher;
import org.apache.ibatis.annotations.Mapper;
import org.apache.ibatis.annotations.Param;
import org.apache.ibatis.annotations.Select;

@Mapper
public interface TeacherMapper {
    @Select("select * from teacher where id = #{id}")
    Teacher selectTeacher(@Param("id")String id);
}

使用Main.java文件中)

            //使用注解
            System.out.println("使用注解进行查询~");
            TeacherMapper teacherMapper=sqlSession.getMapper(TeacherMapper.class);
            final Teacher teacher2 = teacherMapper.selectTeacher("4");
            System.out.println(teacher2);
//替代了原先的使用语句,如下
/*Teacher teacher1= sqlSession.selectOne(
                    "org.mybatis1.example.TeacherMapper.selectTeacher",
                    3);*/