Mybatis学习-初步认知与使用

Mybatis是一款优秀的持久层框架。且支持定制化SQL，存储过程以及高级映射
Mybatis几乎免除了全部的JDBC代码以及设置参数和获取结果集的工做，它使用简单的XML或注解来配置原始类型、接口和将数据库中的记录映射成POJO（Plain Old Java Objects，普通老式java对象）

为何须要Mybatis：方便；传统的JDBC代码太复杂了，简化程序员编写与数据库实现交互的代码的过程

优势：

• 简单易学
• 灵活
• SQL与代码分离，提升了可维护性
• 提供映射标签，支持对象与数据库的ORM字段关系映射
• 提供对象关系映射标签，支持对象关系组件维护
• 提供XML标签，支持编写动态SQL

第一个Mybatis程序

搭建数据库

CREATE TABLE user (
        id INT(20) NOT NULL,
	name VARCHAR(30) DEFAULT NULL,
	pwd VARCHAR(30) DEFAULT NULL,
	PRIMARY KEY (id)
)

在项目中建立一个模块Mybatis01

1. 编写Mybatis的核心配置文件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.Drive"/>
                <property name="url" value="jdbc:mysql://localhost:3306/learn?serverTimezone=GMT&amp;useSSL=false&amp;useUnicode=true&amp;characterEncoding=UTF-8"/>
                <property name="username" value="rooot "/>
                <property name="password" value="123456789"/>
            </dataSource>
        </environment>
    </environments>

    <!--每个Mapper.xml都须要在Mybatis核心配置文件中注册-->
    <mappers>
        <!--UserMapper.xml文件放在resource的mapper文件夹中-->
        <mapper resource="mapper/UerMapper.xml"/>
    </mappers>
</configuration>

2. 编写Mybatis工具类

/**
 * mybatis的sqlsessionfactory工具类，获取sqlSessionFactory对象
 * Created by 罗星
 * 2020/11/19 0019 20:16
 */
public class MybatisUtil {

    private static SqlSessionFactory sqlSessionFactory;
    
    static {
        InputStream inputStream = null;
        try {
            String resource = "mybatis-config.xml";
            inputStream = Resources.getResourceAsStream(resource);
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (IOException e) {
            e.printStackTrace();
        }

    }
    
    //获取Sqlsession实例
    public static SqlSession getSqlSession() {
        return sqlSessionFactory.openSession();
        //设置自动提交
        //return sqlSessionFactory.openSession(true);  
    }

}

3. 编写项目代码

• 实体类

public class User {

    private int id;

    private String name;

    private String pwd;

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    public String getPwd() {
        return pwd;
    }

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

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

• DAO接口

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

• 接口实现。由原来的Impl类转换为一个Mapper.xml配置文件

<?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.luoqing.dao.UserMapper">
    <!--id是须要重写方法的名字-->
    <select id="getUserList" resultType="com.luoqing.model.User">
    select * from learn.user
  </select>
</mapper>

Mapper.xml须要放在resource文件夹中才能被扫描到。若是须要放在java文件夹中则须要在主pom.xml文件中添加如下配置

<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>

• 测试

public class UserDaoTest {
    
    @Test
    public void test() {
        //获取SQLSession对象
        SqlSession sqlSession = MybatisUtil.getSqlSession();

        try {
            
            //方式一：getMapper
            UserDao mapper = sqlSession.getMapper(UserDao.class);
            List<User> userList = mapper.getUserList();

            //方式二：
            //List<User> userList = sqlSession.selectList("com.luoqing.dao.UserDao.getUserList");

            for (User user : userList) {
                System.out.println(user);
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            //关闭SQLSession
            sqlSession.close();
        }
    }
}

可能遇到的问题：

• 配置文件没有注册：因此的Mapper.xml文件都须要在mybatis-config.xml文件中的mappers标签中注册
• 绑定接口错误：Mapper.xml的namespace属性的值须要准确对应接口的路径的名字