MyBatis入门

MyBatis

MyBatis 是一款优秀的持久层框架，它支持自定义 SQL、存储过程以及高级映射。MyBatis 免除了几乎所有的 JDBC 代码以及设置参数和获取结果集的工作。MyBatis 可以通过简单的 XML 或注解来配置和映射原始类型、接口和 Java POJO（Plain Old Java Objects，普通老式 Java 对象）为数据库中的记录。

ORMapping: Object Relationship Mapping 对象关系映射
对象指⾯向对象，关系指关系型数据库
Java 到 MySQL 的映射，开发者可以以⾯向对象的思想来管理数据库。

如何使用

• 新建Maven工程，pom.xml

view code<dependencies>
    <dependency>
        <groupId>org.mybatis</groupId>
        <artifactId>mybatis</artifactId>
        <version>3.4.5</version>
    </dependency>
    <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>8.0.11</version>
    </dependency>
    <dependency>
        <groupId>org.projectlombok</groupId>
        <artifactId>lombok</artifactId>
        <version>1.18.6</version>
        <scope>provided</scope>
    </dependency>
</dependencies>

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

• 新建数据表

view codeuse mybatis;
create table t_account(
    id int primary key auto_increment,
    username varchar(11),
    password varchar(11),
    age int
)

• 新建数据表对应的实体类Account

view codepackage com.oeong.entity;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

@Data  
@AllArgsConstructor // 有参构造
@NoArgsConstructor  // 无参构造
public class Account {
    private long id;
    private String username;
    private String password;
    private int age;
}

• 创建MyBatis的配置文件config.xml，文件名可自定义

view code<?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>
    <!--  MyBatis运行环境  -->
    <environments default="development">
        <environment id="development">

            <transactionManager type="JDBC"></transactionManager>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url"
                          value="jdbc:mysql://localhost:3306/mybatis?serverTimezone=GMT%2B8"/>
                <property name="username" value="root"/>
                <property name="password" value="root"/>
            </dataSource>
        </environment>
    </environments>
</configuration>

使用原生接口

1、MyBatis 框架需要开发者⾃定义 SQL 语句，写在 Mapper.xml ⽂件中，实际开发中，会为每个实体 类创建对应的 Mapper.xml ，定义管理该对象数据的 SQL。

view code<?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="com.oeong.mapper.AccoutMapper">
    <insert id="save" parameterType="com.oeong.entity.Account">
        insert into t_account(username, password, age)
        values (#{username}, #{password}, #{age})
    </insert>
</mapper>

• namespace 通常设置为⽂件所在包+⽂件名的形式。
• insert 标签表示执⾏添加操作。
• select 标签表示执⾏查询操作。
• update 标签表示执⾏更新操作。
• delete 标签表示执⾏删除操作。
• id 是实际调⽤ MyBatis ⽅法时需要⽤到的参数。
• parameterType 是调⽤对应⽅法时参数的数据类型。

2、在全局配置文件config.xml中注册AccoutMapper.xml

view code<?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>
    <!--  MyBatis运行环境  -->
    <environments default="development">
        <environment id="development">

            <transactionManager type="JDBC"></transactionManager>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url"
                          value="jdbc:mysql://localhost:3306/mybatis?serverTimezone=GMT%2B8"/>
                <property name="username" value="root"/>
                <property name="password" value="root"/>
            </dataSource>
        </environment>
    </environments>

    <mappers>
        <mapper resource="com/oeong/mapper/AccountMapper.xml"></mapper>
    </mappers>
</configuration>

3、 调用MyBatis的原生接口执行添加操作

view codepackage com.oeong.test;

import com.oeong.entity.Account;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.InputStream;

public class Test {
    public static void main(String[] args) {
        //加载MyBatis配置⽂件
        InputStream inputStream =
                Test.class.getClassLoader().getResourceAsStream("config.xml");
        SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new
                SqlSessionFactoryBuilder();
        SqlSessionFactory sqlSessionFactory =
                sqlSessionFactoryBuilder.build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        String statement = "com.oeong.mapper.AccoutMapper.save";
        Account account = new Account(1L,"张三","123123",22);
        sqlSession.insert(statement,account);
        sqlSession.commit();
    }
}

通过Mapper代理实现自定义接口

• 自定义接口，定义相关业务方法
• 编写与方法相对应的Mapper.xml

1、自定义接口

view codepackage com.oeong.repository;

import com.oeong.entity.Account;

import java.util.List;

public interface AccountRepository {
    public int save(Account account);
    public int update(Account account);
    public int deleteById(long id);
    public List<Account> findAll();
    public Account findById(long id);
}

2、创建接⼝对应的 Mapper.xml，定义接⼝⽅法对应的 SQL 语句。

statement 标签可根据 SQL 执⾏的业务选择 insert、delete、update、select。MyBatis 框架会根据规则⾃动创建接⼝实现类的代理对象。

规则：

• Mapper.xml 中 namespace 为接⼝的全类名。
• Mapper.xml 中 statement 的 id 为接⼝中对应的⽅法名。
• Mapper.xml 中 statement 的 parameterType 和接⼝中对应⽅法的参数类型⼀致。
• Mapper.xml 中 statement 的 resultType 和接⼝中对应⽅法的返回值类型⼀致。

view code<?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="com.oeong.repository.AccountRepository">
    <insert id="save" parameterType="com.oeong.entity.Account">
        insert into t_account(username, password, age) value (#{username}, #{password}, #{age})
    </insert>
    <update id="update" parameterType="com.oeong.entity.Account">
        update t_account set username=#{username}, password=#{password}, age=#{age} where id=#{id}
    </update>
    <delete id="deleteById" parameterType="long">
        delete from t_account where id=#{id}
    </delete>
    <select id="findAll" resultType="com.oeong.entity.Account">
        select * from t_account
    </select>
    <select id="findById" parameterType="long" resultType="com.oeong.entity.Account">
        select * from t_account where id = #{id}
    </select>
</mapper>

3、在config.xml 中注册 AccountRepository.xml

view code<mappers>
    <mapper resource="com/oeong/mapper/AccountMapper.xml"></mapper>
    <mapper resource="com/oeong/mapper/AccountRepository.xml"></mapper>
</mappers>

4、调用接口的代理对象完成相关的业务操作

view codepackage com.oeong.test;

import com.oeong.entity.Account;
import com.oeong.repository.AccountRepository;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.InputStream;
import java.util.List;

public class Test2 {
    public static void main(String[] args) {
        // 加载MyBatis配置⽂件
        InputStream inputStream = Test2.class.getClassLoader().getResourceAsStream("config.xml");
        SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();
        SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        // 获取实现接口的代理对象
        AccountRepository accountRepository = sqlSession.getMapper(AccountRepository.class);

        // 查询全部对象
//        List<Account> list = accountRepository.findAll();
//        for (Account account : list) {
//            System.out.println(account);
//        }

        // 添加对象
//        Account account = new Account(3L, "李四", "111222", 23);
//        int result = accountRepository.save(account);
//        sqlSession.commit(); // 对数据产生变化，都需要commit

        // 通过id查询对象
//        Account account = accountRepository.findById(3L);
//        System.out.println(account);

        // 修改对象
//        Account account = accountRepository.findById(3L);
//        account.setUsername("小红");
//        account.setAge(18);
//        int result = accountRepository.update(account);
//        sqlSession.commit();
//        System.out.println(result);

        // 通过id删除对象
//        int result = accountRepository.deleteById(3L);
//        System.out.println(result);
//        sqlSession.commit();

        sqlSession.close();
    }
}

Mapper.xml

statement 标签：select、update、delete、insert 分别对应查询、修改、删除、添加操作。

parameterType：参数数据类型

1、基本数据类型，通过 id 查询 Account

view code<select id="findById" parameterType="long" resultType="com.oeong.entity.Account">
     select * from t_account where id = #{id}
</select>

2、String 类型，通过 name 查询 Account

view code<select id="findByName" parameterType="java.lang.String" resultType="com.oeong.entity.Account">
	select * from t_account where username = #{username}
</select>

3、 包装类

view code<select id="findById2" parameterType="java.lang.Long"
resultType="com.oeong.entity.Account">
     select * from t_account where id = #{id}
</select>

4、多个参数，通过 name 和 age 查询 Account

view code<select id="findByNameAndAge" resultType="com.oeong.entity.Account">
     select * from t_account where username = #{arg0} and age = #{arg1}
</select>

5、Java Bean

view code<update id="update" parameterType="com.oeong.entity.Account">
	update t_account set username = #{username},password = #{password},age = #{age} where id = #{id}
</update>

resultType：结果类型

1、基本数据类型，统计 Account 总数

view code<select id="count" resultType="int">
     select count(id) from t_account
</select>

2、包装类，统计 Account 总数

view code<select id="count2" resultType="java.lang.Integer">
     select count(id) from t_account
</select>

3、String 类型，通过 id 查询 Account 的 name

view code<select id="findNameById" resultType="java.lang.String">
     select username from t_account where id = #{id}
</select>

4、Java Bean

view code<select id="findById" parameterType="long" resultType="com.oeong.entity.Account">
     select * from t_account where id = #{id}
</select>

级联查询

一对多 association

SQL

view codeCREATE TABLE `student` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(30) DEFAULT NULL,
  `cid` int(10) unsigned DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `cid` (`cid`),
  CONSTRAINT `student_ibfk_1` FOREIGN KEY (`cid`) REFERENCES `classes` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8mb4;

CREATE TABLE `classes` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(30) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8mb4;

Student

view codepackage com.oeong.entity;
import lombok.Data;
@Data
public class Student {
	private long id;
    private String name;
    private Classes classes;
}

Classes

view codepackage com.oeong.entity;
import lombok.Data;
import java.util.List;
@Data
public class Classes {
    private long id;
    private String name;
    private List<Student> students;
}

StudentRepository

view codepackage com.oeong.repository;
import com.oeong.entity.Student;
public interface StudentRepository {
     public Student findById(long id);
}

StudentRepository.xml

association javaType

view code<?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="com.oeong.repository.StudentRepository">
    <resultMap id="studentMap" type="com.oeong.entity.Student">
        <id column="id" property="id"></id>
        <result column="name" property="name"></result>
        <association property="classes" javaType="com.oeong.entity.Classes">
            <id column="cid" property="id"></id>
            <result column="cname" property="name"></result>
        </association>
    </resultMap>
    <select id="findById" parameterType="long" resultMap="studentMap">
        select s.id,s.name,c.id as cid,c.name as cname from student s,classes c
        where s.id = #{id} and s.cid = c.id
    </select>
</mapper>

resultMap 结果集

id：主键 result：普通列

column：数据列 property：JavaBean属性

ClassesRepository

view codepackage com.oeong.repository;
import com.oeong.entity.Classes;
public interface ClassesRepository {
     public Classes findById(long id);
}

ClassesRepository.xml

collection ofType

view code<?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="com.oeong.repository.ClassesRepository">
    <resultMap id="classesMap" type="com.oeong.entity.Classes">
        <id column="cid" property="id"></id>
        <result column="cname" property="name"></result>
        <collection property="students" ofType="com.oeong.entity.Student">
            <id column="id" property="id"/>
            <result column="name" property="name"/>
        </collection>
    </resultMap>
    <select id="findById" parameterType="long" resultMap="classesMap">
        select s.id,s.name,c.id as cid,c.name as cname from student s,classes c
        where c.id = #{id} and s.cid = c.id
    </select>
</mapper>

多对多 collection

SQL

view codeCREATE TABLE `goods` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(30) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8mb4;

CREATE TABLE `customer` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(30) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8mb4;

CREATE TABLE `customer_goods` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `cid` int(10) unsigned DEFAULT NULL,
  `gid` int(10) unsigned DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `cid` (`cid`),
  KEY `gid` (`gid`),
  CONSTRAINT `customer_goods_ibfk_1` FOREIGN KEY (`cid`) REFERENCES `customer` (`id`),
  CONSTRAINT `customer_goods_ibfk_2` FOREIGN KEY (`gid`) REFERENCES `goods` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8mb4;

Customer

view codepackage com.oeong.entity;
import lombok.Data;
import java.util.List;
@Data
public class Customer {
     private long id;
     private String name;
     private List<Goods> goods;
}

Goods

view codepackage com.oeong.entity;
import lombok.Data;
import java.util.List;
@Data
public class Goods {
     private long id;
     private String name;
     private List<Customer> customers;
}

CustomerRepository

view codepackage com.oeong.repository;
import com.oeong.entity.Customer;
public interface CustomerRepository {
     public Customer findById(long id);
}

CustomerRepository.xml

view code<?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="com.oeong.repository.CustomerRepository">
    <resultMap id="customerMap" type="com.oeong.entity.Customer">
        <id column="cid" property="id"></id>
        <result column="cname" property="name"></result>
        <collection property="goods" ofType="com.oeong.entity.Goods">
            <id column="gid" property="id"/>
            <result column="gname" property="name"/>
        </collection>
    </resultMap>
    <select id="findById" parameterType="long" resultMap="customerMap">
        select c.id cid,c.name cname,g.id gid,g.name gname from customer c,goods g,
        customer_goods cg where c.id = #{id} and cg.cid = c.id and cg.gid = g.id
    </select>
</mapper>

GoodsRepository

view codepackage com.oeong.repository;
import com.oeong.entity.Goods;
public interface GoodsRepository {
     public Goods findById(long id);
}

GoodsRepository.xml

view code<?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="com.oeong.repository.CustomerRepository">
    <resultMap id="customerMap" type="com.oeong.entity.Customer">
        <id column="cid" property="id"></id>
        <result column="cname" property="name"></result>
        <collection property="goods" ofType="com.oeong.entity.Goods">
            <id column="gid" property="id"/>
            <result column="gname" property="name"/>
        </collection>
    </resultMap>
    <select id="findById" parameterType="long" resultMap="customerMap">
        select c.id cid,c.name cname,g.id gid,g.name gname from customer c,goods g,
        customer_goods cg where c.id = #{id} and cg.cid = c.id and cg.gid = g.id
    </select>
</mapper>

逆向工程

MyBatis Generator，简称 MBG，是⼀个专⻔为 MyBatis 框架开发者定制的代码⽣成器，可⾃动⽣成 MyBatis 框架所需的实体类、Mapper 接⼝、Mapper.xml，⽀持基本的 CRUD 操作，但是⼀些相对复杂的 SQL 需要开发者⾃⼰来完成。

MyBatis 框架需要：实体类、⾃定义 Mapper 接⼝、Mapper.xml

传统的开发中上述的三个组件需要开发者⼿动创建，逆向⼯程可以帮助开发者来⾃动创建三个组件，减 轻开发者的⼯作量，提⾼⼯作效率。

如何使用

• 新建Maven工程，pom.xml

view code<dependencies>
  <dependency>
    <groupId>org.mybatis</groupId>
    <artifactId>mybatis</artifactId>
    <version>3.4.5</version>
  </dependency>
  <dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
    <version>8.0.11</version>
  </dependency>
  <dependency>
    <groupId>org.mybatis.generator</groupId>
    <artifactId>mybatis-generator-core</artifactId>
    <version>1.3.2</version>
  </dependency>
</dependencies>

• 创建MBG配置文件generatorConfig.xml

  1. jdbcConnnection 配置数据库连接信息
  2. javaModelGenerator 配置JavaBean的生成策略
  3. sqlMapGenerator 配置SQL映射文件生成策略
  4. javaClientGenerator 配置Mapper接口的生成策略
  5. table 配置目标数据表（tableName：表名，domainObjectName：JavaBean类名）

view code<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE generatorConfiguration
        PUBLIC "-//mybatis.org//DTD MyBatis Generator Configuration 1.0//EN"
        "http://mybatis.org/dtd/mybatis-generator-config_1_0.dtd">
<generatorConfiguration>
    <context id="testTables" targetRuntime="MyBatis3">
        <jdbcConnection driverClass="com.mysql.cj.jdbc.Driver"
                        connectionURL="jdbc:mysql://localhost:3306/mybatis?serverTimezone=GMT%2B8"
                        userId="root"
                        password="root"
        ></jdbcConnection>
        <javaModelGenerator targetPackage="com.oeong.entity" targetProject="./src/main/java"></javaModelGenerator>
        <sqlMapGenerator targetPackage="com.oeong.repository" targetProject="./src/main/java"></sqlMapGenerator>
        <javaClientGenerator type="XMLMAPPER" targetPackage="com.oeong.repository" targetProject="./src/main/java"></javaClientGenerator>
        <table tableName="t_account" domainObjectName="Account"></table>
    </context>
</generatorConfiguration>

• 创建Generator执行类

view codepackage com.oeong.test;

import org.mybatis.generator.api.MyBatisGenerator;
import org.mybatis.generator.config.Configuration;
import org.mybatis.generator.config.xml.ConfigurationParser;
import org.mybatis.generator.exception.InvalidConfigurationException;
import org.mybatis.generator.exception.XMLParserException;
import org.mybatis.generator.internal.DefaultShellCallback;

import java.io.File;
import java.io.IOException;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<String> warings = new ArrayList<String>();
        boolean overwrite = true;
        String genCig = "/generatorConfig.xml";
        File configFile = new File(Main.class.getResource(genCig).getFile());
        ConfigurationParser configurationParser = new ConfigurationParser(warings);
        Configuration configuration = null;

        try {
            configuration = configurationParser.parseConfiguration(configFile);
        } catch (IOException e) {
            e.printStackTrace();
        } catch (XMLParserException e) {
            e.printStackTrace();
        }
        DefaultShellCallback callback = new DefaultShellCallback(overwrite);
        MyBatisGenerator myBatisGenerator = null;
        try {
            myBatisGenerator = new MyBatisGenerator(configuration, callback, warings);
        } catch (InvalidConfigurationException e) {
            e.printStackTrace();
        }
        try {
            myBatisGenerator.generate(null);
        } catch (SQLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

延迟加载

• 什么是延迟加载？

延迟加载也叫懒加载、惰性加载，使⽤延迟加载可以提⾼程序的运⾏效率。它是针对于数据持久层的操作， 在某些特定的情况下去访问特定的数据库，在其他情况下可以不访问某些表，从⼀定程度上减少了 Java 应⽤与数据库的交互次数。

查询学⽣和班级的时，学⽣和班级是两张不同的表，如果当前需求只需要获取学⽣的信息，那么查询学⽣单表即可，如果需要通过学⽣获取对应的班级信息，则必须查询两张表。

不同的业务需求，需要查询不同的表，根据具体的业务需求来动态减少数据表查询的⼯作就是延迟加 载。

• config.xml中开启延迟加载

view code<settings>
    <!-- 打印SQL-->
    <setting name="logImpl" value="STDOUT_LOGGING" />
    <!-- 开启延迟加载 -->
    <setting name="lazyLoadingEnabled" value="true"/>
</settings>

• 将多表关联查询拆分成多个单表查询

StudentRepository

public Student findByIdLazy(long id);

StudentRepository.xml

view code<resultMap id="studentMapLazy" type="com.oeong.entity.Student">
    <id column="id" property="id"></id>
    <result column="name" property="name"></result>
    <association property="classes" javaType="com.oeong.entity.Classes"
                 select="com.oeong.repository.ClassesRepository.findByIdLazy" column="cid">
    </association>
</resultMap>
<select id="findByIdLazy" parameterType="long" resultMap="studentMapLazy">
    select * from student where id = #{id}
</select>

ClassesRepository

public Classes findByIdLay(long id);

ClassesRepository.xml

view code<select id="findByIdLazy" parameterType="long" resultType="com.oeong.entity.Classes">
    select * from classes where id = #{id}
</select>

Main

view code// 加载MyBatis配置⽂件
InputStream inputStream = Test3.class.getClassLoader().getResourceAsStream("config.xml");
SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();
SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(inputStream);
SqlSession sqlSession = sqlSessionFactory.openSession();
// 获取实现接口的代理对象
StudentRepository studentRepository = sqlSession.getMapper(StudentRepository.class);
Student student = studentRepository.findByIdLazy(1);
System.out.println(student.getId());
System.out.println(student.getClasses());
sqlSession.close();

Mybatis缓存

• 什么是MyBatis

使⽤缓存可以减少 Java 应⽤与数据库的交互次数，从⽽提升程序的运⾏效率。⽐如查询出 id = 1 的对 象，第⼀次查询出之后会⾃动将该对象保存到缓存中，当下⼀次查询时，直接从缓存中取出对象即可， ⽆需再次访问数据库。

• MyBatis 缓存分类

1、⼀级缓存：SqlSession 级别，默认开启，并且不能关闭。 操作数据库时需要创建 SqlSession 对象，在对象中有⼀个 HashMap ⽤于存储缓存数据，不同的 SqlSession 之间缓存数据区域是互不影响的。

⼀级缓存的作⽤域是 SqlSession 范围的，当在同⼀个 SqlSession 中执⾏两次相同的 SQL 语句事，第⼀ 次执⾏完毕会将结果保存到缓存中，第⼆次查询时直接从缓存中获取。

需要注意的是，如果 SqlSession 执⾏了 DML 操作（insert、update、delete），MyBatis 必须将缓存 清空以保证数据的准确性。

2、⼆级缓存：Mapper 级别，默认关闭，可以开启。

使⽤⼆级缓存时，多个 SqlSession 使⽤同⼀个 Mapper 的 SQL 语句操作数据库，得到的数据会存在⼆ 级缓存区，同样是使⽤ HashMap 进⾏数据存储，相⽐较于⼀级缓存，⼆级缓存的范围更⼤，多个 SqlSession 可以共⽤⼆级缓存，⼆级缓存是跨 SqlSession 的。

⼆级缓存是多个 SqlSession 共享的，其作⽤域是 Mapper 的同⼀个 namespace，不同的 SqlSession 两次执⾏相同的 namespace 下的 SQL 语句，参数也相等，则第⼀次执⾏成功之后会将数据保存到⼆级 缓存中，第⼆次可直接从⼆级缓存中取出数据。

代码

• 一级缓存

view codepackage com.oeong.test;
import com.oeong.entity.Account;
import com.oeong.repository.AccountRepository;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import java.io.InputStream;
public class Test4 {
    public static void main(String[] args) {
        InputStream inputStream =
                Test.class.getClassLoader().getResourceAsStream("config.xml");
        SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new
                SqlSessionFactoryBuilder();
        SqlSessionFactory sqlSessionFactory =
                sqlSessionFactoryBuilder.build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        AccountRepository accountRepository =
                sqlSession.getMapper(AccountRepository.class);
        Account account = accountRepository.findById(1L);
        System.out.println(account);
        sqlSession.close();
        sqlSession = sqlSessionFactory.openSession();
        accountRepository = sqlSession.getMapper(AccountRepository.class);
        Account account1 = accountRepository.findById(1L);
        System.out.println(account1);
    }
}

• 二级缓存

1、MyBatis 自带的二级缓存

config.xml配置开启二级缓存

view code<settings>
    <!-- 打印SQL-->
    <setting name="logImpl" value="STDOUT_LOGGING" />
    <!-- 开启延迟加载 -->
    <setting name="lazyLoadingEnabled" value="true"/>
    <!-- 开启⼆级缓存 -->
    <setting name="cacheEnabled" value="true"/>
</settings>

Mapper.xml 配置二级缓存

<cache></cache>

实体类实现序列化接口

view codepackage com.oeong.entity;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import java.io.Serializable;

@Data   
@AllArgsConstructor // 有参构造
@NoArgsConstructor  // 无参构造
public class Account implements Serializable {
    private long id;
    private String username;
    private String password;
    private int age;
}

2、ehcache 二级缓存

pom.xml 添加相关依赖

view code<dependency>
     <groupId>org.mybatis</groupId>
     <artifactId>mybatis-ehcache</artifactId>
     <version>1.0.0</version>
</dependency>
<dependency>
     <groupId>net.sf.ehcache</groupId>
     <artifactId>ehcache-core</artifactId>
     <version>2.4.3</version>
</dependency>

添加ehcache.xml

view code<ehcache xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:noNamespaceSchemaLocation="../config/ehcache.xsd">
 <diskStore/>
     <defaultCache
         maxElementsInMemory="1000"
         maxElementsOnDisk="10000000"
         eternal="false"
         overflowToDisk="false"
         timeToIdleSeconds="120"
         timeToLiveSeconds="120"
         diskExpiryThreadIntervalSeconds="120"
         memoryStoreEvictionPolicy="LRU">
     </defaultCache>
</ehcache>

config.xml 开启二级缓存

view code<settings>
    <!-- 打印SQL-->
    <setting name="logImpl" value="STDOUT_LOGGING" />
    <!-- 开启延迟加载 -->
    <setting name="lazyLoadingEnabled" value="true"/>
    <!-- 开启⼆级缓存 -->
    <setting name="cacheEnabled" value="true"/>
</settings>

Mapper.xml 中配置二级缓存

view code<cache type="org.mybatis.caches.ehcache.EhcacheCache">
     <!-- 缓存创建之后，最后⼀次访问缓存的时间⾄缓存失效的时间间隔 -->
     <property name="timeToIdleSeconds" value="3600"/>
     <!-- 缓存⾃创建时间起⾄失效的时间间隔 -->
     <property name="timeToLiveSeconds" value="3600"/>
     <!-- 缓存回收策略，LRU表示移除近期使⽤最少的对象 -->
     <property name="memoryStoreEvictionPolicy" value="LRU"/>
</cache>

实体类不需要实现序列化接口

view codepublic class Account {}

动态SQL

程序可以⾃动根据业务参数来决定 SQL 的组成，使⽤动态 SQL 可简化代码的开发。

• if标签

view code<select id="findByAccount" parameterType="com.oeong.entity.Account" resultType="com.oeong.entity.Account">
    select * from t_account where
    <if test="id!=0">
        id = #{id}
    </if>
    <if test="username!=null">
        and username = #{username}
    </if>
    <if test="password!=null">
        and password = #{password}
    </if>
    <if test="age!=0">
        and age = #{age}
    </if>
</select>

if 标签可以⾃动根据表达式的结果来决定是否将对应的语句添加到 SQL 中，如果条件不成⽴则不添加， 如果条件成⽴则添加。

• where标签

view code<select id="findByAccount" parameterType="com.oeong.entity.Account" resultType="com.oeong.entity.Account">
    select * from t_account
    <where>
        <if test="id!=0">
            id = #{id}
        </if>
        <if test="username!=null">
            and username = #{username}
        </if>
        <if test="password!=null">
            and password = #{password}
        </if>
        <if test="age!=0">
            and age = #{age}
        </if>
    </where>
</select>

where 标签可以⾃动判断是否要删除语句块中的 and 关键字，如果检测到 where 直接跟 and 拼接，则 ⾃动删除 and，通常情况下 if 和 where 结合起来使⽤。

• choose、when标签

view code<select id="findByAccount" parameterType="com.oeong.entity.Account"
        resultType="com.oeong.entity.Account">
    select * from t_account
    <where>
        <choose>
            <when test="id!=0">
                id = #{id}
            </when>
            <when test="username!=null">
                and username = #{username}
            </when>
            <when test="password!=null">
                and password = #{password}
            </when>
            <when test="age!=0">
                and age = #{age}
            </when>
        </choose>
    </where>
</select>

• trim标签

trim 标签中的 prefix 和 suffix 属性会被⽤于⽣成实际的 SQL 语句，会和标签内部的语句进⾏拼接，如 果语句前后出现了 prefixOverrides 或者 suffixOverrides 属性中指定的值，MyBatis 框架会⾃动将其删 除。

view code<select id="findByAccount" parameterType="com.oeong.entity.Account"
        resultType="com.oeong.entity.Account">
    select * from t_account
    <trim prefix="where" prefixOverrides="and">
        <if test="id!=0">
            id = #{id}
        </if>
        <if test="username!=null">
            and username = #{username}
        </if>
        <if test="password!=null">
            and password = #{password}
        </if>
        <if test="age!=0">
            and age = #{age}
        </if>
    </trim>
</select>

• set标签

set 标签⽤于 update 操作，会⾃动根据参数选择⽣成 SQL 语句。

view code<update id="update" parameterType="com.oeong.entity.Account">
    update t_account
    <set>
        <if test="username!=null">
            username = #{username},
        </if>
        <if test="password!=null">
            password = #{password},
        </if>
        <if test="age!=0">
            age = #{age}
        </if>
    </set>
    where id = #{id}
</update>

view codeAccount account = new Account();
account.setId(1);
account.setAge(22);
System.out.println(accountRepository.update(account));
sqlSession.commit();

• foreach

foreach 标签可以迭代⽣成⼀系列值，这个标签主要⽤于 SQL 的 in 语句。

view code<select id="findByIds" parameterType="com.oeong.entity.Account"
        resultType="com.oeong.entity.Account">
    select * from t_account
    <where>
        <foreach collection="ids" open="id in (" close=")" item="id"
                 separator=",">
            #{id}
        </foreach>
    </where>
</select>