Mybatis程序

思路：搭建环境-->导入Mybatis-->编写代码-->测试

搭建环境

搭建数据库

CREATE TABLE user(
id int(20) not null PRIMARY KEY,
name VARCHAR(30) DEFAULT NULL,
pwd VARCHAR(30) DEFAULT NULL
)ENGINE=INNODB DEFAULT CHARSET=utf8;

INSERT INTO user(id,name,pwd) values 
(1,"迪迦",123)

新建项目
1，新建普通的maven项目
2，删除src目录
3，导入maven依赖
<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>

    <!--父工程-->
    <groupId>org.god</groupId>
    <artifactId>Mybatis-Study</artifactId>
    <version>1.0-SNAPSHOT</version>

    <!--导入依赖-->
    <dependencies>

        <!-- Source: https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.33</version>
            <scope>compile</scope>
        </dependency>

        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.5.17</version>
        </dependency>

        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>3.8.1</version>
        </dependency>

    </dependencies>

    <properties>
        <maven.compiler.source>25</maven.compiler.source>
        <maven.compiler.target>25</maven.compiler.target>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>

</project>

创建模块

编写mybatis的核心配置文件
核心配置文件 <Mybatis-config.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">

<!--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;useUnicode=true&amp;characterEncoding=UTF-8"/>
                <property name="username" value="root"/>
                <property name="password" value="Dominus0825."/>
            </dataSource>
        </environment>
    </environments>

</configuration>

编写mybatis工具类
工具类

package com.god.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.InputStream;

//sqlSessionFactory -->sqlSession
public class MybatisUtils {
    private static SqlSessionFactory sqlSessionFactory;

    static{
        try {
            //使用Mybatis,第一步获取sqlSessionFactory对象
            // 1. 加载配置文件
            String resource = "mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
            // 2. 构建SqlSessionFactory
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (Exception 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。例如：
    // 3. 获取SqlSession
    public static SqlSession getSqlSession(){
        return sqlSessionFactory.openSession();

    }

}

编写代码
实体类
实体类<User.java>

package com.god.pojo;

public class User {
    private int id;
    private String name;
    private String pwd;

    public User() {
    }

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

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public String getPwd() {
        return pwd;
    }

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

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

    public void setPwd(String pwd) {
        this.pwd = pwd;
    }

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

Dao 接口和它的 XML 实现文件必须 “一一对应”

1. Dao 接口和 XML 实现文件是绑定关系，改一个必须改另一个；
2. 接口负责 “定义功能”（要做什么），XML 负责 “实现功能”（怎么做）；
3. 核心匹配规则：XML 标签的 id = 接口方法名。

Dao接口 <UserDao.java>

public interface UserDao {
// 查询所有用户
    List<User> getUserList();
}

接口实现类<UserMapper.xml>

<?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.god.dao.UserDao">
    <!--select 查询语句-->
    <select id="getUserList" resultType="com.god.pojo.User">
        select * from mybatis.user;
    </select>
</mapper>

测试

注意点：
org.apache.ibatis.binding.BindingException: Type interface com.god.dao.UserDao is not known to the MapperRegistry.
MapperRegistry是什么？
核心配置文件中注册mappers
用 JUnit 写测试类

package com.god.dao;

import com.god.pojo.User;
import com.god.utils.MybatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import java.util.List;

public class UserDaoTest {
    @Test
    public void test(){

        //第一步：获得SqlSession对象
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        //方式一：getMapper
        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