Java复习 - MyBatis 快速入门

简介

什么是 MyBatis

MyBatis 是一个使用 Java 编写的持久层框架。它封装了 JDBC 操作的很多细节，使开发者只需要关注 sql 语句，而无需关注注册驱动、创建连接、创建 Statement 等繁杂的过程。使用了 ORM 的思想，屏蔽的 JDBC api 底层细节，实现了对结果集的封装。

三层架构

表现层：展示数据
业务层：处理业务需求
持久层：和数据库交互

持久化

持久化就是将程序的数据在持久状态和瞬时状态转化的过程
内存：断电即失

为什么需要 MyBatis

传统 JDBC 太复杂了，简化形成框架
SQL 和代码分离，提高了可维护性

持久层技术解决方案

JDBC
Spring 和 JdbcTemplate
Apache 的 DBUtils

传统 JDBC

public static void main(String[] args) {
    Connection connection = null;
    PreparedStatement preparedStatement = null;
    ResultSet resultSet = null;
    try {
        // 1.加载数据库驱动
        Class.forName("com.mysql.jdbc.Driver");

        // 2.通过驱动管理类获取数据库链接
        connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/mybatis?characterEncoding=utf-8", "root", " root");

        // 3.定义 sql 语句 ? 表示占位符
        String sql = "SELECT * FROM user WHERE username = ?";

        // 4.获取预处理 statement
        preparedStatement = connection.prepareStatement(sql);

        // 5.设置参数，第一个参数为 sql 语句中参数的序号（从 1 开始），第二个参数为设置的参数值
        preparedStatement.setString(1, "loveccknbc");

        // 6.向数据库发出 sql 执行查询，查询出结果集
        resultSet = preparedStatement.executeQuery();

        // 7.遍历查询结果集
        while (resultSet.next()) {
            System.out.println(resultSet.getString("id") + " " + resultSet.getString("username"));
        }
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        // 8.释放资源
        if (resultSet != null) {
            try {
                resultSet.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        if (preparedStatement != null) {
            try {
                preparedStatement.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        if (connection != null) {
            try {
                connection.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
}

MaBatis 上手

步骤：搭建环境 —> 导入 MaBatis —> 编写代码 —> 测试

搭建环境

创建数据库

CREATE DATABASE mybatis;

CREATE TABLE `user`(
    id int PRIMARY KEY,
    username VARCHAR(10),
    password VARCHAR(10)
);        

INSERT INTO `user` VALUES(1,'admin','admin123');
INSERT INTO `user` VALUES(2,'root','root123');

创建 maven 项目

导入 maven 依赖（pom.xml）

<dependencies>
	<!-- JUnit -->
    <dependency>
        <groupId>junit</groupId>
        <artifactId>junit</artifactId>
        <version>4.12</version>
    </dependency>

    <!-- mybatis -->
    <dependency>
        <groupId>org.mybatis</groupId>
        <artifactId>mybatis</artifactId>
        <version>3.5.6</version>
    </dependency>
	
    <!-- 数据库驱动 -->
    <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>5.1.47</version>
    </dependency>

    <!-- lombok -->
    <dependency>
        <groupId>org.projectlombok</groupId>
        <artifactId>lombok</artifactId>
        <version>1.18.16</version>
    </dependency>

</dependencies>

开始编写代码

编写核心配置文件(resources/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.jdbc.Driver"/>
                <property name="url"
                          value="jdbc:mysql://localhost:3306/mybatis?useUnicode=true&amp;characterEncoding=utf-8&amp;serverTimezone=UTC"/>
                <property name="username" value="root"/>
                <property name="password" value="root"/>
            </dataSource>
        </environment>
    </environments>

    <!-- 每个 Mapper.xml 都要注册在 MyBatis 核心配置文件中-->
    <mappers>
        <mapper resource="mapper/UserMapper.xml"/>
    </mappers>

</configuration>

在 resources 下创建 mapper/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">

<mapper namespace="cn.imzjw.mybatis.mapper.UserMapper">

</mapper>

编写 Mabatis 工具类（cn.imzjw.mybatis.utils.MybatisUtils.java）

/**
 * sqlSessionFactory生产sqlSession
 *
 * @author https://blog.imzjw.cn
 * @date 2020/12/23 13:42
 */
public class MybatisUtils {
    /**
     * 提升作用域
     */
    private static SqlSessionFactory sqlSessionFactory;

    static {
        // 使用MaBatis第一步，获取sqlSessionFactory对象
        try {
            String resource = "mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

       /**
         * 既然有了SqlSessionFactory，就可以从中获得sqlSession实例。
         * sqlSession完全包含了面向数据库执行SQL命令所需要的所有方法
         *
         * @return
         */
    public static SqlSession getSqlSession() {
        return sqlSessionFactory.openSession();
    }
}

创建实体类（cn.imzjw.mybatis.entity.User.java）

/**
 * @author https://blog.imzjw.cn
 * @date 2020/12/23 14:09
 */
@Data
@NoArgsConstructor
@AllArgsConstructor
public class User {
    private int id;
    private String username;
    private String password;
}

mapper 接口（cn.imzjw.mybatis.mapper.UserMapper.java）

1
2
3

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

接口实现类(resources/mapper/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">

<mapper namespace="cn.imzjw.mybatis.mapper.UserMapper">

    <!--select查询语句-->
    <select id="getUserList" resultType="cn.imzjw.mybatis.entity.User">
        SELECT * FROM `user`
    </select>

</mapper>

测试

在 test 文件下编写测试类（test/java/TestMybatis.java）

/**
 * @author https://blog.imzjw.cn
 * @date 2020/12/23 13:40
 */
public class TestMybatis {

    @Test
    public void testQuery() {
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);

        userMapper.getUserList().forEach(xiaojia -> System.out.println(xiaojia));

        sqlSession.close();
    }

}

得到结果

1 2	User(id=1, username=admin, password=admin123) User(id=2, username=root, password=root123)

其它问题

如果出现以下异常

Caused by: org.apache.ibatis.builder.BuilderException: Error parsing SQL Mapper Configuration. Cause: java.io.IOException: Could not find

请在 pom.xml 最后加入资源过滤，没异常就忽略

<build>
    <resources>
        <resource>
            <directory>src/main/java</directory>
            <includes>
                <include>**/*.xml</include>
            </includes>
            <filtering>true</filtering>
        </resource>
    </resources>
</build>

我的 pom.xml 完整代码

<?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>cn.imzjw.mybatis</groupId>
    <artifactId>mybatis</artifactId>
    <version>1.0-SNAPSHOT</version>


    <dependencies>

        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.12</version>
        </dependency>

        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.5.6</version>
        </dependency>

        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.47</version>
        </dependency>

        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>1.18.16</version>
        </dependency>

    </dependencies>

    <build>
        <resources>
            <resource>
                <directory>src/main/java</directory>
                <includes>
                    <include>**/*.xml</include>
                </includes>
                <filtering>true</filtering>
            </resource>
        </resources>
    </build>

</project>