mybatis（上）

2024-03-27 09:04:46

什么是mybatis

持久层框架
支持定制化SQL，存储过程和高级映射
几乎避免了所有的JDBC代码

如何获取mybatis

maven仓库
Github

持久层

数据持久化

持久化就是将程序的数据从瞬时状态，转化成持久状态
内存数据：断电即失
数据库，io文件持久化

mybatis优点：

sql和代码分离，提高了程序的可维护性
提供了映射标签，支持对象与数据库的orm字段关系映射
支持动态sql

编写程序

导依赖包，mybatis、mysql、junit、注意静态资源到处问题

<?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>org.example</groupId>
    <artifactId>MyPractice</artifactId>
    <version>1.0-SNAPSHOT</version>

    <packaging>jar</packaging>

    <!--版本锁定-->
    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.source>1.7</maven.compiler.source>
        <maven.compiler.target>1.7</maven.compiler.target>
        <spring.version>5.2.2.RELEASE</spring.version>
    </properties>

    <dependencies>

        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.11</version>
            <scope>test</scope>
        </dependency>

        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>1.16.10</version>
        </dependency>

        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.22</version>
        </dependency>

        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.4.1</version>
        </dependency>
    </dependencies>
    <!--    静态资源导出问题-->
    <build>
    <resources>
        <resource>
            <directory>src/main/java</directory>
            <includes>
                <include>**/*.properties</include>
                <include>**/*.xml</include>
            </includes>
            <filtering>true</filtering>
        </resource>
        <resource>
            <directory>src/main/resources</directory>
            <includes>
                <include>**/*.properties</include>
                <include>**/*.xml</include>
            </includes>
            <filtering>true</filtering>
        </resource>
    </resources>
    </build>
</project>

mybatis配置文件，sqlSesisionConfig.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">

<configuration>

    <!-- 配置包的别名，这样我们在mapper中定义时，就不需要使用类的全限定名称，只需要使用类名即可 -->
    <typeAliases>
        <package name="mybatis.pojo"/>
    </typeAliases>

    <!-- 数据库配置 -->
    <environments default="mysql">
        <environment id="mysql">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/my?serverTimezone=UTC&amp;useUnicode=true&amp;characterEncoding=utf8"/>
                <property name="username" value="root"/>
                <property name="password" value="666666"/>
            </dataSource>
        </environment>
    </environments>

    <!-- mybatis的映射配置文件 -->
    <mappers>
        <mapper resource="mybatis/dao/adminMapper.xml"/>
    </mappers>

</configuration>

pojo实体类

package mybatis.pojo;

/**
 *作者: 柯神_
 *时间: 15:09
 *日期: 2020/11/9
 *User: 蜘蛛啊全是腿儿
**/

public class Admin {

  private long adminId;
  private String adminName;
  private String adminPassword;


  public long getAdminId() {
    return adminId;
  }

  public void setAdminId(long adminId) {
    this.adminId = adminId;
  }


  public String getAdminName() {
    return adminName;
  }

  public void setAdminName(String adminName) {
    this.adminName = adminName;
  }

  public String getAdminPassword() {
    return adminPassword;
  }

  public void setAdminPassword(String adminPassword) {
    this.adminPassword = adminPassword;
  }

  @Override
  public String toString() {
    return "Admin{" +
            "adminId=" + adminId +
            ", adminName='" + adminName + '\'' +
            ", adminPassword='" + adminPassword + '\'' +
            '}';
  }
}

dao接口

public interface AdminDao {
    List<Admin> find();
}

mapper映射文件

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="mybatis.dao.AdminDao">
    <select id="find" resultType="admin" >
        select * from admins
    </select>
</mapper>

测试

    @Test
    public void mybatis(){

        try {
            //通过Resources读取配置文件
            InputStream inputStream = Resources.getResourceAsStream("sqlSessionConfig.xml");
            //通过配置文件去建立一个SqlSessionFactory工厂
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
            //SqlSessionFactory工厂生产SqlSession对象
            SqlSession sqlSession = sqlSessionFactory.openSession(true);
            //通过SqlSession对象，拿到动态代理对象
            AdminDao adminDao = sqlSession.getMapper(AdminDao.class);
            List<Admin> admins = adminDao.find();
            System.out.println(admins);
        } catch (IOException e) {
            e.printStackTrace();
        }

在编写程序的过程中可能遇到的问题

映射文件没有注册在mybatis的配置文件中

<!-- mybatis的映射配置文件 -->
    <mappers>
        <mapper resource="mybatis/dao/adminMapper.xml"/>
    </mappers>

绑定接口错误

<mapper namespace="mybatis.dao.AdminDao">

方法名不对

<!--id对应dao接口中的方法名-->
<select id="find" resultType="admin" >

maven资源导出问题

    <!--    静态资源导出问题-->
    <build>
    <resources>
        <resource>
            <directory>src/main/java</directory>
            <includes>
                <include>**/*.properties</include>
                <include>**/*.xml</include>
            </includes>
            <filtering>true</filtering>
        </resource>
        <resource>
            <directory>src/main/resources</directory>
            <includes>
                <include>**/*.properties</include>
                <include>**/*.xml</include>
            </includes>
            <filtering>true</filtering>
        </resource>
    </resources>
    </build>

万能map

当数据库中的字段过多，应该优先考虑使用Map，key可以不必和类中的属性名一致

	int addByMap(Map<String,Object> map);

    <insert id="addByMap" parameterType="map">
        insert into admins (adminId,adminName,adminPassword) values(#{a},#{b},#{c})
    </insert>

	Map<String,Object> map = new HashMap<>();
            map.put("a","6");
            map.put("b","强强");
            map.put("c","999999");
            int add = adminDao.addByMap(map);
            if (add>0){
                System.out.println(map.get("b") + "插入成功！");
            }

模糊查询

1.在执行代码的时候传递通配符% %

List<Admins> admins = adminDao.getLike("%神%");

    <select id="getLike" resultType="admins" >
        select * from admins where adminName like #{value}
    </select>

2.在sql拼接中使用通配符

List<Admins> admins = adminDao.getLike("神");

    <select id="getLike" resultType="admins" >
        select * from admins where adminName like "%"#{value}"%"
    </select>

配置文件中

环境配置（environments），可以配置多个环境，但是只能选取一个

    <environments default="mysql">
        <environment id="mysql">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/my?serverTimezone=UTC&amp;useUnicode=true&amp;characterEncoding=utf8"/>
                <property name="username" value="root"/>
                <property name="password" value="666666"/>
            </dataSource>
        </environment>
        <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/my?serverTimezone=UTC&amp;useUnicode=true&amp;characterEncoding=utf8"/>
                <property name="username" value="admin"/>
                <property name="password" value="99999"/>
            </dataSource>
        </environment>
    </environments>

属性（properties），可以引入外部文件，优先使用外部文件

    <properties resource="db.properties">
        <property name="username" value="root"/>
        <property name="password" value="666666"/>
    </properties>

取别名（typeAliases）

package，为这个包下的所有类取别名，默认当前类名并且首字母变为小写

    <typeAliases>
        <package name="mybatis.pojo"/>
    </typeAliases>

typeAlias，为当前类取别名

    <typeAliases>
        <typeAlias type="admin" alias="com.my.pojo.Admin"/>
        <typeAlias type="admin2" alias="com.my.pojo.Admin2"/>
    </typeAliases>

3.注解，Alias（" "）标注在类上，为当前类取别名

@Alias("adc")
public class Admin{
    ___________
}

设置（settings）

	<settings>
<!--        缓存是否开启，默认是开启的-->
        <setting name="cacheEnable" value="true"/>
<!--        懒加载，默认为false-->
        <setting name="lazyLoadingEnable" value="false"/>
<!--        开启驼峰命名规则，默认为false-->
        <setting name="mapUnderscoreToCamelCase" value="false"/>
<!--        指定mybatis日志的具体实现-->
        <setting name="logImpl" value="LOG4J"/>
    </settings>

映射器(mappers),用来注册绑定mapper映射文件

资源路径，resource路径（推荐使用，可以随意用！）

<mappers>
    <mapper resource="mybatis/dao/adminMapper.xml"/>
    <mapper resource="mybatis/dao/adminMapper1.xml"/>
    <mapper resource="mybatis/dao/adminMapper2.xml"/>
</mappers>

url（可以说已经被淘汰，太繁琐）

<mappers>
    <mapper url="D:///Users/mybatis/dao/adminMapper.xml"/>
    <mapper url="D:///Users/mybatis/dao/adminMapper1.xml"/>
    <mapper url="D:///Users/mybatis/dao/adminMapper2.xml"/>
</mappers>

class，全限定类名(最好也不要使用，接口名和映射文件必须同名，否则会出现一些问题)

<mappers>
    <mapper class="mybatis.dao.adminMapper"/>
    <mapper class="mybatis.dao.adminMapper1"/>
    <mapper class="mybatis.dao.adminMapper2"/>
</mappers>

package，将包内的都注册为映射器 (最好也不要使用，接口名和映射文件必须同名，否则会出现一些问题)
```
<mappers>
	<package name="com.my.dao"/>
</mappers>
```

resultMap，结果集映射

解决实体类的属性名，和数据库中的字段名不一致问题

1.简单结果映射

	<resultMap id="adminMap" type="admins">
        <!--column->数据库字段名，property->实体类的属性名。-->
        <result column="adminId" property="adminId"/>
        <result column="adminName" property="adminName"/>
        <result column="adminPassword" property="password"/>
    </resultMap>

    <select id="getLike" resultMap="adminMap">
        select * from admins where adminName like "%"#{value}"%"
    </select>

2.复杂结果映射

2.1. association，一个复杂类型的关联（一个对象使用这个关联）。适用于一对一和多对一的关系

   <!--按查询嵌套处理   ①查询所有人的信息 ②根据查询出来的外键id，寻找对应的书籍信息
（有点。。。太绕了，莫名其妙的null问题）-->
   <select id="find" resultMap="AB" >
        select * from admins
    </select>
    <resultMap id="AB" type="admins">
        <result column="adminId" property="adminId"/>
        <result column="adminName" property="adminName"/>
        <result column="adminPassword" property="password"/>
        <association property="bookes" column="bookesId"
                     javaType="bookes" select="getB">
        </association>
    </resultMap>

    <select id="getB" resultType="bookes">
        select * from bookes where bookID=#{id}
    </select>
<!--  按结果嵌套处理-->
	 <select id="find" resultMap="AB" >
        select a.adminId,a.adminName,b.bookName,b.detail from admins a,bookes b
        where a.bookesId=b.bookID
    </select>
    <resultMap id="AB" type="admins">
        <result column="adminId" property="adminId"/>
        <result column="adminName" property="adminName"/>
        <result column="adminPassword" property="password"/>
        <association property="bookes" javaType="bookes">
            <result column="bookId" property="bookId"/>
            <result column="bookName" property="bookName"/>
            <result column="bookCounts" property="bookCounts"/>
            <result column="detail" property="detail"/>
        </association>

2.2. collection，一个复杂类型的集合（一个集合使用这个关联），适用于一对多和多对多的关系

<!--  按结果嵌套处理-->
 	<select id="findU" resultMap="BA" >
        select * from admins a,bookes b
        where a.bookesId=b.bookID and b.bookID=#{id}
    </select>
    <resultMap id="BA" type="bookes">
        <result column="bookId" property="bookId"/>
        <result column="bookName" property="bookName"/>
        <result column="bookCounts" property="bookCounts"/>
        <result column="detail" property="detail"/>
        <collection property="admins" ofType="admins">
            <result column="adminId" property="adminId"/>
            <result column="adminName" property="adminName"/>
            <result column="adminPassword" property="password"/>
        </collection>
    </resultMap>
<!--按照查询嵌套处理  （有点。。。太绕了，莫名其妙的null问题）-->
    <select id="findU" resultMap="BA">
        select * from bookes b
        where b.bookID=#{id}
    </select>
    <resultMap id="BA" type="bookes">
        <result column="bookId" property="bookId"/>
        <result column="bookName" property="bookName"/>
        <result column="bookCounts" property="bookCounts"/>
        <result column="detail" property="detail"/>
        <collection property="admins"
                    ofType="admins" select="getAById" column="bookId"/>
    </resultMap>

    <select id="getAById" resultType="admins">
        select * from admins where bookesId=#{bookId}
    </select>

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