2.实现select

主配置文件mybatis-config.xml

<?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"/>
            <!--配置数据源：创建Connection对象-->
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url"
                          value="jdbc:mysql://localhost:3306/bjpowernode?useUnicode=true&amp;characterEncoding=UTF-8"/>
                <property name="username" value="root"/>
                <property name="password" value="admin"/>
            </dataSource>
        </environment>
    </environments>
    <!--指定其他mapper文件的位置
    找到其他mapper文件的目的是找到sql语句

    使用注意：
        resource="mapper.文件的路径，用/分割"-->

    <mappers>
        <mapper resource="Dao/StudentMapper.xml"/>
    </mappers>
</configuration>

Dao接口

package Dao;

import domain.Student;

import java.util.List;

public interface StudentDao {
    Student selectById(Integer id);

    List<Student> selectStudents();
}

mapper.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">

<!--绑定接口-->
<mapper namespace="Dao.StudentDao">

    <!--
        mapper是根标签
        mapper里面可以写<insert>,<update>,<select>等标签
            标签的id属性：正常是需要重写的方法名
            标签的resultType属性：将数据赋值给哪个类型的Java对象
        标签内部都有同名的sql语句
    -->

    <select id="selectById" resultType="domain.Student">
        select id,name,email,age from student where id=#{StudentId}
    </select>

    <select id="selectStudents" resultType="domain.Student">
        select id,name,email,age from student
    </select>
</mapper>

工具类

package utils;

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;
//工具类：先创建方法创建SqlSessionFactory
public class MyBatisUtil {
    private static SqlSessionFactory sqlSessionFactory;
    static {
        String config="mybatis-config.xml";
        try{
            InputStream inputStream = Resources.getResourceAsStream(config);
            sqlSessionFactory=new SqlSessionFactoryBuilder().build(inputStream);

        }catch(IOException e){
            e.printStackTrace();
        }
    }

    //创建方法，返回值是SqlSession对象sqlSession
    public static SqlSession getSqlSession(){
        return sqlSessionFactory.openSession();
    }
}

实体类

package domain;

public class Student {
    private Integer id;
    private String name;
    private String email;
    private Integer age;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "学生实体{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", email='" + email + '\'' +
                ", age=" + age +
                '}';
    }
}

Test类

package Dao;

import domain.Student;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
import utils.MyBatisUtil;

import java.util.List;
//selectOne方法
public class MyTest {
    @Test
    public void testSelectById(){
        //1.获取SqlSession对象
        SqlSession sqlSession= MyBatisUtil.getSqlSession();
        //2.指定sqlId
        String sqlId="Dao.StudentDao.selectById";
        //3.执行SqlSession的方法，执行SQL语句
        Student student = sqlSession.selectOne(sqlId, 001);
        System.out.println("查询的结果==="+student);
        //4.关闭sqlSession对象
        sqlSession.close();
    }
//selectList方法
    @Test
    public void testSelectStudents(){
        //1.获取SqlSession对象
        SqlSession sqlSession= MyBatisUtil.getSqlSession();
        //2.指定sqlId
        String sqlId="Dao.StudentDao.selectStudents";
        //3.执行SqlSession的方法，执行SQL语句
        List<Student> students = sqlSession.selectList(sqlId);
        for(Student student:students){
            System.out.println(student);
        }
        //4.关闭sqlSession对象
        sqlSession.close();
    }
}

测试结果

// testSelectById
查询的结果===学生实体{id=1, name='张磊', email='zl@163.com', age=23}

//testSelectStudents
学生实体{id=1, name='张磊', email='zl@163.com', age=23}
学生实体{id=2, name='陆婷婷', email='luu@163.com', age=22}
学生实体{id=3, name='陆启赛', email='kwh@qq.com', age=19}

注：本地数据库已创建student表，并且添加了数据。