详解Mybatis模板(已优化)适合小白

1.搭建数据库(我用的是mysql 5.5)

#创建一个mybatis数据库
create database `mybatis`;

#使用mybatis数据库
use `mybatis`;

#创建一个user表，有id、username、password三个字段
create table `user`(
  `id` int(10) not null primary key auto_increment,
  `username` varchar(30) default null,
  `password` varchar(30) default null
)engine=InnoDB default charset=utf8;

#插入数据
insert into `user`（`username`,`password`） values
('张三'，123456)，
('李四'，123456)，
('王五'，123456)；

2.导入mybatis相关的jar包（COPY，建议创建一个父子项目）

<dependencies>
    <dependency>
<!--mysql依赖包-->
      <groupId>mysql</groupId>
      <artifactId>mysql-connector-java</artifactId>
      <version>8.0.19</version>
    </dependency>
<!--mybatis依赖包-->
    <dependency>
      <groupId>org.mybatis</groupId>
      <artifactId>mybatis</artifactId>
      <version>3.4.2</version>
    </dependency>
<!--junit依赖包,为什么要用junit依赖包？@Test做单元测试用的-->
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>4.12</version>
    </dependency>
  </dependencies>

<!--maven静态资源过滤问题：配置报错，不设置会报错：ExceptionInInitializerError-->
  <build>
  <resources>
    <resource>
      <directory>src/main/resources</directory>
      <includes>
        <include>**/*.properties</include>
        <include>**/*.xml</include>
      </includes>
      <filtering>true</filtering>
    </resource>
    <resource>
      <directory>src/main/java</directory>
      <includes>
        <include>**/*.properties</include>
        <include>**/*.xml</include>
      </includes>
      <filtering>true</filtering>
    </resource>
  </resources>
</build>

3.编写MyBatisUtil工具类（COPY，低层原理还没有搞懂，直接复制的条件是要创建一个mybatis-config.xml文件  文件名要一致）

package com.kuang.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;

public class MybatisUtil {
  private static SqlSessionFactory sqlSessionFactory;
  static{
    try {
      String resource = "mybatis-config.xml";
      InputStream inputStream = Resources.getResourceAsStream(resource);
      //SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);因为重复定义导致空指针异常
      sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    } catch (IOException e) {
      e.printStackTrace();
    }
  }
  public static SqlSession getSqlSession(){
    return sqlSessionFactory.openSession();
  }

}

4.1编写db.properties配置文件（COPY，是后来优化的）

driver=com.mysql.cj.jdbc.Driver
url=jdbc:mysql://localhost:3306/mybatis?useSSL=false&useUnicode=true&characterEncoding=utf-8&serverTimezone=Asia/Shanghai
username=root
password=root

4.2编写MyBatis核心配置文件（COPY，mybatis-config.xml，已优化）

<?xml version="1.0" encoding="UTF8" ?>
<!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">
<!--第一个问题：
com.mysql.jdbc.Driver和mysql-connector-java 5一起用。
com.mysql.cj.jdbc.Driver和mysql-connector-java 6+ 一起用。
第二个问题：
serverTimezone=Asia/Shanghai 要加时区，不加时区会报错
-->
        <property name="driver" value="${driver}"/>
        <property name="url" value="${url}"/>
        <property name="username" value="${username}"/>
        <property name="password" value="${passwprd}"/>
      </dataSource>
    </environment>
  </environments>
  <mappers>
    <mapper resource="com/super/dao/UserMapper.xml"/>
  </mappers>
</configuration>

5.编写User实体类

public class User {

  private int id; //id
  private String name;  //姓名
  private String pwd;  //密码

  //构造,有参,无参
  //set/get
  //toString()

}

6.编写Mapper接口类(Mapper.java)

package com.kuang.dao;

import com.kuang.pojo.User;

import java.util.List;

public interface Mapper {
  List<User> getUserList();
}

7.编写mapper.xml配置文件

<?xml version="1.0" encoding="UTF8" ?>
<!DOCTYPE mapper
    PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
    "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.super.dao.UserDao">
  <select id="getUserList" resultType="com.kuang.pojo.User">
  select * from mybatis.user;
 </select>
</mapper>

8.编写测试类

package com.kuang.dao;

import com.kuang.pojo.User;
import com.kuang.utils.Mybatise;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import java.util.List;

public class UserDaoTest {
  @Test
  public void test(){
    SqlSession sqlSession = MybatisUtil.getSqlSession();
    UserDao mapper = sqlSession.getMapper(Mapper.class);
    List<User> userList = mapper.getUserList();
    for(User user:userList){
      System.out.println(user);
    }
    //sqlSession.commit();增删改需要添加事务
    sqlSession.close();
  }
}

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