MyBatis_HelloWorld

MyBatis是目前使用最多的ORM框架之一，今天学习了MyBatis基本配置，使用MyBatis读取数据库内容并直接映射为对象

MyBatis简化了DAO层与DB层间操作

下面通过最简单的一个实例了解MyBatis

一、构建项目

1.建立JAVA项目结构如下，导入MySQL、MyBatis jar包：

2.Mybatis数据库如下：

建表SQL语句：

 1 SET FOREIGN_KEY_CHECKS=0;
 2 
 3 -- ----------------------------
 4 -- Table structure for tbl_employee
 5 -- ----------------------------
 6 CREATE TABLE `tbl_employee` (
 7   `id` int(11) NOT NULL AUTO_INCREMENT,
 8   `last_name` varchar(255) DEFAULT NULL,
 9   `gender` char(1) DEFAULT NULL,
10   `email` varchar(255) DEFAULT NULL,
11   PRIMARY KEY (`id`)
12 ) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;
13 
14 -- ----------------------------
15 -- Records of tbl_employee
16 -- ----------------------------
17 INSERT INTO `tbl_employee` VALUES ('1', 'tom', '0', '[email protected]');

3.编写与关系数据库对应的JavaBean：

 1 package com.atguigu.mybatis.bean;
 2 
 3 public class Employee {
 4 
 5     private Integer id;
 6     private String lastName;
 7     private String gender;
 8     private String email;
 9     public Integer getId() {
10         return id;
11     }
12     public void setId(Integer id) {
13         this.id = id;
14     }
15     public String getLastName() {
16         return lastName;
17     }
18     public void setLastName(String lastName) {
19         this.lastName = lastName;
20     }
21     public String getGender() {
22         return gender;
23     }
24     public void setGender(String gender) {
25         this.gender = gender;
26     }
27     public String getEmail() {
28         return email;
29     }
30     public void setEmail(String email) {
31         this.email = email;
32     }
33     
34     @Override
35     public String toString() {
36         return "Employee [id=" + id + ", lastName=" + lastName + ", gender=" + gender + ", email=" + email + "]";
37     }
38 }

二、配置MyBatis

1.在conf/下建立全局配置文件mybatis-config.xml：

 1 <?xml version="1.0" encoding="UTF-8"?>
 2 <!DOCTYPE configuration
 3  PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
 4  "http://mybatis.org/dtd/mybatis-3-config.dtd">
 5 <configuration>
 6     <environments default="development">
 7         <environment id="development">
 8             <transactionManager type="JDBC"/>
 9             <dataSource type="POOLED">
10                 <property name="driver" value="com.mysql.jdbc.Driver"/>
11                 <property name="url" value="jdbc:mysql://localhost:3306/mybatis"/>
12                 <property name="username" value="root"/>
13                 <property name="password" value="root"/>
14             </dataSource>
15         </environment>
16     </environments>
17     
18     <!-- 配置SQL语句映射文件 -->
19     <mappers>
20         <mapper resource="EmployeeMapper.xml"/>
21     </mappers>
22 </configuration>

2.建立SQL语句映射文件EmployeeMapper.xml

 1 <?xml version="1.0" encoding="UTF-8" ?>
 2 <!DOCTYPE mapper
 3  PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
 4  "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
 5 <!-- namespace可以任意填写，只是为了便于区分 -->
 6 <mapper namespace="com.atguigu.mybatis.EmployeeMapper">
 7 
 8     <!-- id是sql唯一标识，resultType相当于返回类型 -->
 9     <select id="selectEmp" resultType="com.atguigu.mybatis.bean.Employee">
10     
11         <!-- 查询结果字段与Employee属性保持一致才可以正确映射，所以这里使用别名lastName -->
12         select id,last_name lastName,gender,email from tbl_employee where id = #{id}
13     </select>
14 </mapper>

三、建立单元测试文件

 1 package com.atguigu.mybatis.test;
 2 
 3 import java.io.IOException;
 4 import java.io.InputStream;
 5 
 6 import org.apache.ibatis.io.Resources;
 7 import org.apache.ibatis.session.SqlSession;
 8 import org.apache.ibatis.session.SqlSessionFactory;
 9 import org.apache.ibatis.session.SqlSessionFactoryBuilder;
10 import org.junit.jupiter.api.Test;
11 
12 import com.atguigu.mybatis.bean.Employee;
13 
14 class MybatisTest {
15 
16     @Test
17     void test() throws IOException {
18         
19         /**
20          * 使用mybatis-config.xml获取sqlSession工厂sqlSessionFactory
21          */
22         String resource = "mybatis-config.xml";
23         InputStream inputStream = Resources.getResourceAsStream(resource);
24         SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
25     
26         /**
27          * 通过sqlSessionFactory获取SqlSession
28          */
29         SqlSession openSession = sqlSessionFactory.openSession();
30         try {
31             Employee employee = openSession.selectOne("com.atguigu.mybatis.EmployeeMapper.selectEmp", 1);
32             System.out.println(employee);
33         }finally {
34             openSession.close();    //必须关闭
35         }
36 
37     }
38 
39 }

四、执行结果

五、本次学习中遇到的问题：

1.运行测试文件时提示找不到mybatis-config.xml文件：

java.io.IOException: Could not find resource mybatis-config.xml
   at org.apache.ibatis.io.Resources.getResourceAsStream(Resources.java:114)
   at org.apache.ibatis.io.Resources.getResourceAsStream(Resources.java:100)
   at com.atguigu.mybatis.test.MybatisTest.test(MybatisTest.java:23)
   at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
   at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
   at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
　……

解决办法：

新建conf文件夹时选择Source Folder而不是Folder（参考https://ask.csdn.net/questions/252045下面xl1007的评论）

一、构建项目

二、配置MyBatis

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