初学Mybatis

1、Mybatis概述

Mybatis基于java的持久层框架，内部封装了JDBC，开发者只需要关注SQL语句

• 特点

1、sql语句在xml里，便于统一管理和优化
2、解除sql与程序代码耦合：通过DAO层，将业务逻辑和数据访问逻辑分离
提供映射标签，

2、 第一个Mybatis项目

Mybatis3中文文档

• 2.1 搭建环境
  【1】搭建数据库
  【2】新建Maven项目，导入Mybatis依赖【pom.xml文件】

    <!-- mybatis-->
    <dependency>
      <groupId>org.mybatis</groupId>
      <artifactId>mybatis</artifactId>
      <version>3.4.5</version>
    </dependency>
    <!--    mysql驱动-->
    <dependency>
      <groupId>mysql</groupId>
      <artifactId>mysql-connector-java</artifactId>
      <version>8.0.33</version>
    </dependency>
     <!--    单元测试-->
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>3.8.1</version>
      <scope>test</scope>
    </dependency>

• 2.2 创建一个模块Module

  <modules>
    <module>mybatis_01</module>
  </modules>

• 2.3 mybatis-config.xml 核心配置文件

1、XML 配置文件中包含了对 MyBatis 系统的核心设置，包括获取数据库连接实例的数据源（DataSource）以及决定事务作用域和控制方式的事务管理器（TransactionManager）
2、在resources目录下创建mybatis-config.xml

<?xml version="1.0" encoding="utf-8" ?>
<!--Mybatis核心配置-->

<!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"></transactionManager>
           <!-- 数据库连接-->
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/springboot"/>
                <property name="username" value="root"/>
                <property name="password" value="123456"/>
            </dataSource>
        </environment>
    </environments>
    
     <!-- 加载sql语句-->
    <mappers>
        <mapper resource="com/dm/dao/EmployeeMapper.xml"/>
    </mappers>
</configuration>

• 2.4 将工程目录按照层划分
  
  pojo层是实体类

public class Employee {

    //id,name,age,habbies,address
}

dao层是Mapper接口类
【1】EmployeeDao

// mapper接口
public interface EmployeeDao {
		List<Employee> getEmployees();

【2】EmployeeMapper.xml

<?xml version="1.0" encoding="UTF8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "https://mybatis.org/dtd/mybatis-3-mapper.dtd">

       <!--namespace绑定一个对应的dao/mapper接口-->
<mapper namespace="com.dm.dao.EmployeeDao">
    <select id="getEmployees" resultType="com.dm.pojo.Employee">
        select * from employees;
    </select>
</mapper>

utils层是工具类

public class MyBatisUtils {
    private static SqlSessionFactory sqlSessionFactory;
    //使用mybatis第一步，获取sqlSessionFactory对象
    static {
        try{
            String resource = "mybatis-config.xml";
           InputStream inputStream= Resources.getResourceAsStream(resource);
           sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

        }catch (IOException e) {
            throw new RuntimeException(e);
        }
    }
    // 获取sqlSession实例
    public static SqlSession getSqlSession(){
        return sqlSessionFactory.openSession();
    }
}

记得将EmployeeMapper.xml注册到mybatis-config.xml文件中！！！

mybatis-config.xml 文件中

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

2.5 测试类

public class TestEmploy {

    // 获取sqlSession对象
    SqlSession sqlSession;
    @Test
    public void test(){
        sqlSession = MyBatisUtils.getSqlSession();

        EmployeeDao employmapper = sqlSession.getMapper(EmployeeDao.class);

        List<Employee> employList = employmapper.getEmployees();


        // employee 对象
        for (Employee employee:employList){
            System.out.println(employee.getAddress());
        }
    }
}

问题集锦

1、Cannot find class: com.mysql.cj.jdbc.Driver
2、Cause: java.io.IOException: Could not find resource com/dm/dao/EmployeeMappe
3、java不支持源选项5，请使用7或更高