MyBatis核心之实践初体验

MyBatis核心之实践初体验

一、MyBatis搭建开发环境

首先我们需要在Maven项目中引入依赖坐标，包括MyBatis核心依赖、Junit测试依赖、MySQL数据库依赖以及日志log4j依赖：

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<dependencies>
    <dependency>
        <groupId>org.mybatis</groupId>
        <artifactId>mybatis</artifactId>
        <version>3.5.7</version>
    </dependency>
    <dependency>
        <groupId>junit</groupId>
        <artifactId>junit</artifactId>
        <version>4.13.1</version>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>8.0.28</version>
    </dependency>
    <dependency>
        <groupId>log4j</groupId>
        <artifactId>log4j</artifactId>
        <version>1.2.17</version>
    </dependency>
</dependencies>

然后，我们需要引入日志配置文件log4j.xml，可以方便我们观察MyBatis的运行时信息，帮助我们理解MyBatis的运行原理，

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<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd">
<log4j:configuration xmlns:log4j="http://jakarta.apache.org/log4j/">
    <appender name="STDOUT" class="org.apache.log4j.ConsoleAppender">
        <param name="Encoding" value="UTF-8" />
        <layout class="org.apache.log4j.PatternLayout">
            <param name="ConversionPattern" value="%-5p %d{MM-dd HH:mm:ss,SSS} %m (%F:%L) \n" />
        </layout>
    </appender>
    <logger name="java.sql">
        <level value="debug" />
    </logger>
    <logger name="org.apache.ibatis">
        <level value="info" />
    </logger>
    <root>
        <level value="debug" />
        <appender-ref ref="STDOUT" />
    </root>
</log4j:configuration>

其次，我们需要编写MyBatis的核心配置文件，这是MyBatis实践中最重要的一环，其大致内容如下：

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<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "https://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <!--数据源连接的配置信息-->
    <properties resource="jdbc.properties"/>
    <typeAliases>
        <!--实体类所在的包-->
        <package name="com.huling.pojo"/>
    </typeAliases>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driver}"/>
                <property name="url" value="${jdbc.url}"/>
                <property name="username" value="${jdbc.username}"/>
                <property name="password" value="${jdbc.password}"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <!--需要跟Mapper接口所在的包保持一致-->
        <package name="com.huling.mapper"/>
    </mappers>
</configuration>

上述配置文件中引入了数据源连接的配置文件jdbc.properties，其内容如下：

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jdbc.driver=com.mysql.cj.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/mybatis?serverTimezone=UTC&characterEncoding=utf8&useUnicode=true&useSSL=false&allowPublicKeyRetrieval=true
jdbc.username=root
jdbc.password=root

接着，我们还需要建立项目的源代码目录结构如实体类包com.huling.pojo、数据持久层包com.huling.mapper等，整体项目的结构如下图：

image-20231124152126118

其中，实体类User关联数据库中的t_user表，对象属性与数据表字段一一对应：

image-20231124152306799

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package com.huling.pojo;

public class User {
    private Integer id;
    private String username;
    private String password;
    private Integer age;
    private String sex;
    private String email;

    public User() {
    }

    public User(Integer id, String username, String password, Integer age, String sex, String email) {
        this.id = id;
        this.username = username;
        this.password = password;
        this.age = age;
        this.sex = sex;
        this.email = email;
    }

    public Integer getId() {
        return id;
    }

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

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public String getSex() {
        return sex;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", username='" + username + '\'' +
                ", password='" + password + '\'' +
                ", age=" + age +
                ", sex='" + sex + '\'' +
                ", email='" + email + '\'' +
                '}';
    }
}

最后，我们需要建立数据持久层的接口UserMapper及其对应的XML映射文件UserMapper.xml：

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package com.huling.mapper;

import com.huling.pojo.User;

public interface UserMapper {
    int insertSimpleUser();

    int updateSimpleUser();

    int deleteSimpleUser();

    User getSimpleUser();

}
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<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "https://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.huling.mapper.UserMapper">
    <insert id="insertSimpleUser">
        insert into t_user values (null,'tom','123456',22,'男','tom@163.com')
    </insert>

    <update id="updateSimpleUser">
        update t_user set username = 'admin' where id = 3
    </update>

    <delete id="deleteSimpleUser">
        delete from t_user where id = 3
    </delete>

    <select id="getSimpleUser" resultType="user">
        select * from t_user where id = 2
    </select>
</mapper>

二、MyBatis搭建测试环境

通过单元测试Junit技术，我们可以方便地测试MyBatis的各项功能，下面是一个简单的测试程序代码：

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public class MyBatisTest {

    /**
     * 测试MyBatis的简单功能
     */
    @Test
    public void test1() throws IOException {
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        try (SqlSession session = sqlSessionFactory.openSession(true)) {
            UserMapper mapper = session.getMapper(UserMapper.class);
            // 添加简单用户
            // int insertResult = mapper.insertSimpleUser();
            // System.out.println("insertResult = " + insertResult);
            // 修改简单用户
            // int updateResult = mapper.updateSimpleUser();
            // System.out.println("updateResult = " + updateResult);
            // 删除简单用户
            // int deleteResult = mapper.deleteSimpleUser();
            // System.out.println("deleteResult = " + deleteResult);
            // 查询简单用户
            User user = mapper.getSimpleUser();
            System.out.println("user = " + user);
        }
    }

}

image-20231124152941645

对象属性与数据库的表字段直接如何映射转换的呢，我们先做一个实验，给实体类User的构造方法和Getter、Setter方法添加打印信息，看一看会发生什么？

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package com.huling.pojo;

public class User {
    private Integer id;
    private String username;
    private String password;
    private Integer age;
    private String sex;
    private String email;

    public User() {
        System.out.println("User.User无参构造");
    }

    public User(Integer id, String username, String password, Integer age, String sex, String email) {
        System.out.println("User.User有参构造");
        this.id = id;
        this.username = username;
        this.password = password;
        this.age = age;
        this.sex = sex;
        this.email = email;
    }

    public Integer getId() {
        System.out.println("User.getId");
        return id;
    }

    public void setId(Integer id) {
        System.out.println("User.setId:" + id);
        this.id = id;
    }

    public String getUsername() {
        System.out.println("User.getUsername");
        return username;
    }

    public void setUsername(String username) {
        System.out.println("User.setUsername:" + username);
        this.username = username;
    }

    public String getPassword() {
        System.out.println("User.getPassword");
        return password;
    }

    public void setPassword(String password) {
        System.out.println("User.setPassword:" + password);
        this.password = password;
    }

    public Integer getAge() {
        System.out.println("User.getAge");
        return age;
    }

    public void setAge(Integer age) {
        System.out.println("User.setAge:" + age);
        this.age = age;
    }

    public String getSex() {
        System.out.println("User.getSex");
        return sex;
    }

    public void setSex(String sex) {
        System.out.println("User.setSex:" + sex);
        this.sex = sex;
    }

    public String getEmail() {
        System.out.println("User.getEmail");
        return email;
    }

    public void setEmail(String email) {
        System.out.println("User.setEmail:" + email);
        this.email = email;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", username='" + username + '\'' +
                ", password='" + password + '\'' +
                ", age=" + age +
                ", sex='" + sex + '\'' +
                ", email='" + email + '\'' +
                '}';
    }
}

image-20231124154251654

因此可以发现，默认从查询的行记录转换为Java对象的情况下，对于对象属性与表字段一致时，直接通过对象的Setter方法注入属性值，而对象也是先通过无参构造函数创建的实例。

其实与上面的情况相对应，当我们的方法参数中包含Java对象时，也需要通过Getter方法取出对象的属性值，下面添加一个新的Mapper测试接口方法，观察一下会发生什么？

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// UserMapper接口引入的测试方法
int insertUser(User user);

// UserMapper.xml的SQL语句实现
<insert id="insertUser">
    insert into t_user values(null,#{username},#{password},#{age},#{sex},#{email})
</insert>

// MyBatisTest测试程序引入的测试代码
int result = mapper.insertUser(new User(null,"test","test",12,"女","mail"));

image-20231124155844580