首先弄清楚流程，流程很重要

        1.1 数据库建实体表   

        1.2  idea中建立实体类 

        1.3  建立核心配置文件sqlMapConfig

        1.4  建立实体mapper接口

        1.5 建立实体mapper.xml配置文件

        1.6 建立jdbc.properties 

        1.7 完善配置文件详情

        注明 开发环境：maven+mysql+idea+jdk1.8

pom.xml <?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> <groupId>org.example</groupId> <artifactId>mavendemo</artifactId> <version>1.0-SNAPSHOT</version> <dependencies> <dependency> <groupId>org.mybatis</groupId> <artifactId>mybatis</artifactId> <version>3.4.6</version> </dependency> <dependency> <groupId>mysql</groupId> <artifactId>mysql-connector-java</artifactId> <version>8.0.15</version> </dependency> <dependency> <groupId>junit</groupId> <artifactId>junit</artifactId> <version>4.12</version> <scope>test</scope> </dependency> <dependency> <groupId>log4j</groupId> <artifactId>log4j</artifactId> <version>1.2.12</version> </dependency> <dependency> <groupId>com.github.pagehelper</groupId> <artifactId>pagehelper</artifactId> <version>3.7.5</version> </dependency> <dependency> <groupId>com.github.jsqlparser</groupId> <artifactId>jsqlparser</artifactId> <version>0.9.1</version> </dependency> </dependencies> </project>

 必须引入的坐标，mysql驱动 mybatis junit

  准备：数据库（本例为mysql）创建test数据库，创建表customer(顾客) 和表order(订单)，具体建表结构如下 CREATE TABLE `customer` ( `id` int(11) NOT NULL AUTO_INCREMENT, `cusName` varchar(10) DEFAULT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8 CREATE TABLE `order` ( `id` int(11) NOT NULL AUTO_INCREMENT, `note` varchar(15) DEFAULT NULL, `c_id` int(11) DEFAULT NULL, PRIMARY KEY (`id`), KEY `waijian` (`c_id`), CONSTRAINT `waijian` FOREIGN KEY (`c_id`) REFERENCES `customer` (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8  编写实体类Customer  Order package com.javamu.domain; import java.util.List; public class Customer { private int id; private String cusName; private List<Order> orderList; public int getId() { return id; } public void setId(int id) { this.id = id; } public String getCusName() { return cusName; } public void setCusName(String cusName) { this.cusName = cusName; } public List<Order> getOrderList() { return orderList; } public void setOrderList(List<Order> orderList) { this.orderList = orderList; } @Override public String toString() { return "Customer{" + "id=" + id + ", cusName='" + cusName + '\'' + ", orderList=" + orderList + '}'; } }

 由于customer对order为一对多关系，所以在customer类中维护一个成员 List<Order>

package com.javamu.domain; public class Order { private int id; private String note; //private Customer customer; public int getId() { return id; } public void setId(int id) { this.id = id; } public String getNote() { return note; } public void setNote(String note) { this.note = note; } public Customer getCustomer() { return customer; } public void setCustomer(Customer customer) { this.customer = customer; } @Override public String toString() { return "Order{" + "id=" + id + ", note='" + note + '\'' + ", customer=" + customer + '}'; } } 编写CustomerMapper接口 package com.javamu.mapper; import com.javamu.domain.Customer; import java.util.List; public interface CustomerMapper { List<Customer> findAll();//查询所有顾客，顾客对应多条订单 }

 注意两条信息：

CustomerMapper的接口全限定名（全路径，即com.it.mapper.CustomerMapper形式） 对应 customerMapper.xml中的<mapper >标签中的namespace

接口中的方法名 对应customerMapper.xml中的增删改查标签的id

编写核心配置文件 sqlMapConfig.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> <properties resource="jdbc.properties"></properties> <plugins> <plugin interceptor="com.github.pagehelper.PageHelper"> <property name="dialect" value="mysql"></property> </plugin> </plugins> <environments default="development"> <environment id="development"> <!--使用jdbc事务管理 --> <transactionManager type="JDBC"/> <!-- 数据库连接池 --> <dataSource type="POOLED"> <property name="driver" value="${jdbc.driver}"/> <property name="url" value="${jdbc.url}"/> <property name="username" value="${jdbc.username}"/> <property name="password" value="${jdbc.password}"/> </dataSource> </environment> </environments> <!-- 引入映射配置文件 --> <mappers> <mapper resource="com\javamu\mapper\UserMapper.xml"/> <mapper resource="com\javamu\mapper\OrderMapper.xml"/> <mapper resource="com\javamu\mapper\CustomerMapper.xml"/> </mappers> </configuration>

注意一点：核心配置文件的<mappers>标签必须引用接口配置文件，如CustomerMapper.xml，其余标签信息请查询互联网

编写CustomerMapper.xml <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd"> <mapper namespace="com.javamu.mapper.CustomerMapper"> <resultMap id="customerMap" type="com.javamu.domain.Customer"> <id column="c_id" property="id"/> <result column="cusName" property="cusName"/> <collection property="orderList" ofType="com.javamu.domain.Order"> <id column="o.id" property="id"/> <result column="note" property="note"/> </collection> </resultMap> <select id="findAll" resultMap="customerMap"> select * from customer c,`order` o where o.c_id=c.id; </select> </mapper>

关注<mapper>根标签的namespace的属性值，对应接口名的全限定路径；关注<select>中的id属性值，为接口的方法名

由于是一对多开发，所以查询后的值存储用resultMap属性，(请百度 resultType用法)，此属性值对应<resultMap>的id值

注意：文件和文件的关联建立往往就是靠id,全路径等能唯一建立关系的方式

关注<resultMap>标签，查询结果的装载方式,主键用<id>,成员属性用<result>；特别注意一对多查询，<collection>标签中，

property填 Customer类中的集合属性变量名，ofType填集合指定的存储类。

一个供参考的jdbc.properties文件 jdbc.driver=com.mysql.cj.jdbc.Driver jdbc.url=jdbc:mysql://localhost:3306/test?serverTimezone=GMT%2B8&useUnicode=true&characterEncoding=utf-8 jdbc.username=root jdbc.password=root

重要提示

根据本人经验，看别人的demo很容易只看只言片语就复制粘贴期望达到运行效果，强烈建议先不要直接跑程序。先用纸笔梳理开发流程，做到有条有理，不然此demo你也是看不懂的。

跑程序 @Test public void test5() throws IOException { InputStream is = Resources.getResourceAsStream("sqlMapConfig.xml"); SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(is); SqlSession sqlSession = sqlSessionFactory.openSession(); //获取代理 CustomerMapper mapper = sqlSession.getMapper(CustomerMapper.class); List<Customer> list = mapper.findAll(); System.out.println(list); }

注意看方法中的前三行为固定格式，注意第一行引入的核心配置文件；

注意第四行获取接口的代理类传的什么---接口的字节码

分享我的文件结构，很多人就是文件结构错误导致路径填写问题 结果 2020-07-04 16:20:09,974 [main] DEBUG [com.javamu.mapper.CustomerMapper.findAll] - ==> Preparing: select * from customer c,`order` o where o.c_id=c.id; 2020-07-04 16:20:10,033 [main] DEBUG [com.javamu.mapper.CustomerMapper.findAll] - ==> Parameters: 2020-07-04 16:20:10,106 [main] DEBUG [com.javamu.mapper.CustomerMapper.findAll] - <== Total: 4 [Customer{id=1, cusName='小王', orderList=[Order{id=0, note='订单1', customer=null}, Order{id=0, note='订单2', customer=null}]}, Customer{id=2, cusName='张三', orderList=[Order{id=0, note='订单3', customer=null}]}, Customer{id=3, cusName='李四', orderList=[Order{id=0, note='订单4', customer=null}]}] Process finished with exit code 0