Mybatis學習-初步認知與使用

Mybatis是一款優秀的持久層框架。且支持定製化SQL，存儲過程以及高級映射
Mybatis幾乎免除了全部的JDBC代碼以及設置參數和獲取結果集的工做，它使用簡單的XML或註解來配置原始類型、接口和將數據庫中的記錄映射成POJO（Plain Old Java Objects，普通老式java對象）

爲何須要Mybatis：方便；傳統的JDBC代碼太複雜了，簡化程序員編寫與數據庫實現交互的代碼的過程

優勢：

• 簡單易學
• 靈活
• SQL與代碼分離，提升了可維護性
• 提供映射標籤，支持對象與數據庫的ORM字段關係映射
• 提供對象關係映射標籤，支持對象關係組件維護
• 提供XML標籤，支持編寫動態SQL

第一個Mybatis程序

搭建數據庫

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

在項目中建立一個模塊Mybatis01

1. 編寫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>

    <!--每個Mapper.xml都須要在Mybatis核心配置文件中註冊-->
    <mappers>
        <!--UserMapper.xml文件放在resource的mapper文件夾中-->
        <mapper resource="mapper/UerMapper.xml"/>
    </mappers>
</configuration>

2. 編寫Mybatis工具類

/**
 * mybatis的sqlsessionfactory工具類，獲取sqlSessionFactory對象
 * Created by 羅星
 * 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();
        }

    }
    
    //獲取Sqlsession實例
    public static SqlSession getSqlSession() {
        return sqlSessionFactory.openSession();
        //設置自動提交
        //return sqlSessionFactory.openSession(true);  
    }

}

3. 編寫項目代碼

• 實體類

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();
}

• 接口實現。由原來的Impl類轉換爲一個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綁定一個對應的Dao/Mapper接口-->
<mapper namespace="com.luoqing.dao.UserMapper">
    <!--id是須要重寫方法的名字-->
    <select id="getUserList" resultType="com.luoqing.model.User">
    select * from learn.user
  </select>
</mapper>

Mapper.xml須要放在resource文件夾中才能被掃描到。若是須要放在java文件夾中則須要在主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>

• 測試

public class UserDaoTest {
    
    @Test
    public void test() {
        //獲取SQLSession對象
        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 {
            //關閉SQLSession
            sqlSession.close();
        }
    }
}

可能遇到的問題：

• 配置文件沒有註冊：因此的Mapper.xml文件都須要在mybatis-config.xml文件中的mappers標籤中註冊
• 綁定接口錯誤：Mapper.xml的namespace屬性的值須要準確對應接口的路徑的名字