2、第一个Mybatis程序

思路：搭建环境–>导入Mybatis–>编写代码–>测试！

2.1、搭建环境

搭建数据库

CREATE DATABASE `mybatis`;
USE `mybatis`;

INSERT INTO  `user`(`id`,`name`,`pwd`) VALUES 
('5', '玲玲', '789741')

新建项目

1. 新建一个普通的Maven项目

2. 删除src目录

3. 导入maven依赖

   <?xml version="1.0" encoding="UTF-8"?>
   <project xmlns="http://maven.apache.org/POM/4.0.0"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
       <modelVersion>4.0.0</modelVersion>
   <!--父工程-->
       <groupId>org.example</groupId>
       <artifactId>Mybatis_Study</artifactId>
       <version>1.0-SNAPSHOT</version>
   
       <properties>
           <maven.compiler.source>8</maven.compiler.source>
           <maven.compiler.target>8</maven.compiler.target>
       </properties>
   
       <!--导入依赖-->
       <dependencies>
           <!--mysql驱动-->
           <dependency>
               <groupId>mysql</groupId>
               <artifactId>mysql-connector-java</artifactId>
               <version>8.0.25</version>
           </dependency>
           <!--mybatis-->
           <!-- https://mvnrepository.com/artifact/org.mybatis/mybatis -->
           <dependency>
               <groupId>org.mybatis</groupId>
               <artifactId>mybatis</artifactId>
               <version>3.5.2</version>
           </dependency>
   
           <!--junit-->
           <dependency>
               <groupId>junit</groupId>
               <artifactId>junit</artifactId>
               <version>4.12</version>
           </dependency>
       </dependencies>
   
   </project>
   

2.2、创建一个模块

• 编写mybatis的核心配置文件

  <?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核心配置文件-->
  <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://localhost:3306/mybatis?useSSL=true&amp;userUnicode=true&amp;characterEncoding=UTF-8&amp;serverTimezone=UTC"/>
                  <property name="username" value="root"/>
                  <property name="password" value="root"/>
              </dataSource>
          </environment>
      </environments>
       <!--每一个Mapper.XML都需要Mybatis核心配置文件中注册-->
      <mappers>
          <mapper resource="com/shuang/dao/UserMapper.xml"/>
      </mappers>
  </configuration>
  

• 编写mybatis的工具类

  package com.shuang.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;
  
  //sqlSessionFactory sqlSession
  public class MybatisUtils {
      private static SqlSessionFactory sqlSessionFactory;
      static {
  
          try {
              //使用Mybatis第一步：获取SqlSessionFactory对象
              String resource = "mybatis-config.xml";
              InputStream inputStream = Resources.getResourceAsStream(resource);
               sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
          } catch (IOException e) {
              e.printStackTrace();
          }
  
      }
  
      //既然有了 SqlSessionFactory，顾名思义，我们可以从中获得 SqlSession 的实例。
      // SqlSession 提供了在数据库执行 SQL 命令所需的所有方法。你可以通过 SqlSession 实例来直接执行已映射的 SQL 语句。
  
      public static SqlSession getSqlSession(){
          return sqlSessionFactory.openSession();
      }
  
  }
  
  

2.3、编写代码

• 实体类

package com.shuang.pojo;

public class User {
    private int id;
    public String name;
    public String ped;
    public User(){

    }

    public User(int id, String name, String ped) {
        this.id = id;
        this.name = name;
        this.ped = ped;
    }

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    public String getPed() {
        return ped;
    }

    public void setPed(String ped) {
        this.ped = ped;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", ped='" + ped + '\'' +
                '}';
    }
}

• Dao接口

  package com.shuang.dao;
  
  import com.shuang.pojo.User;
  
  import java.util.List;
  
  public interface UserDao {
      List<User> getUserList();
  }
  
  

• 接口实现类由原来的UserDaoimpl转换为一个Mapper配置文件

<?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">
<!--namespace绑定一个对应的Dao/Mapper接口-->
<mapper namespace="com.shuang.dao.UserDao">
    <!--查询语句-->
    <select id="getUserList" resultType="com.shuang.pojo.User">
        select * from mybatis.m_user
    </select>
</mapper>

2.4、测试

 <!--每一个Mapper.XML都需要Mybatis核心配置文件中注册-->
    <mappers>
        <mapper resource="com/shuang/dao/UserMapper.xml"/>
    </mappers>

MapperRegistry是什么？

核心配置文件中注册mappers

• junit测试

  package com.shuang.dao;
  
  import com.shuang.pojo.User;
  import com.shuang.utils.MybatisUtils;
  import org.apache.ibatis.session.SqlSession;
  import org.junit.Test;
  
  import java.util.List;
  
  public class UserDaoTest {
      @Test
      public void test(){
          //1.获得SqlSession对象
          SqlSession sqlSession= MybatisUtils.getSqlSession();
          //执行SQL
          UserDao userDao=sqlSession.getMapper(UserDao.class);
          List<User> userList=userDao.getUserList();
  
          for (User user:userList) {
              System.out.println(user);
          }
          //关闭SqlSession
          sqlSession.close();
      }
  }
  
  

  可能会遇到的问题：

  1. 配置文件没有注册
  2. 绑定接口错误
  3. 方法名不对
  4. 返回类型不对
  5. Maven导出资源问题