MyBatis1-入门


MyBatis

是一个对JDBC的封装,是一个数据持久化框架

优点

减少代码量,降低维护成本,讲SQL写到XML中,降低耦合度,支持动态SQL语句,支持标签映射,

缺点

SQL编写工作量大,SQL语句依赖数据库,导致移植性较差

核心接口和类

SqlSessionFactoryBuilder - SqlSessionFactory - SqlSession

开发方式

使用原生接口或者用Mapper代理实现自定义接口

实际操作

  • 导入依赖
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<dependency>
<groupId>org.mybatis</groupId>
<artifactId>mybatis</artifactId>
<version>3.4.5</version>
</dependency>
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<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>
  • 创建表
  • 创建表对应的实体类
  • 创建MyBatis配置文件MyBatis.xml
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<environments default="development">
<environment id="development">
<!-- 配置JDBC事务管理 -->
<transactionManager type="JDBC"></transactionManager>
<!-- POOLED配置JDBC数据源连接池 -->
<dataSource type="POOLED">
<property name="driver" value="com.mysql.cj.jdbc.Driver"></property>
<property name="url" value="jdbc:mysql://localhost:3306/mybatis?useUnicode=true&amp;characterEncoding=UTF-8"></property>
<property name="username" value="root"></property>
<property name="password" value="123456"></property>
</dataSource>
</environment>
<environment id="other">
<!-- write-->
</environment>
</environments>
  • 创建Mapper.xml
    namespace 设置为文件所在包+文件名,insert表示执行insert,id是参数,parameterType是类型
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    <?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.wsx.mapper.AccoutMapper">

    <insert id="save" parameterType="com.wsx.entity.Account">
    insert into t_account(username,password,age) values(#{username},#{password},#{age})
    </insert>

    </mapper>
  • 注册SQL语句
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    <!-- 注册AccountMapper.xml -->
    <mappers>
    <mapper resource="com/wsx/mapper/AccountMapper.xml"></mapper>
    <mapper resource="com/wsx/repository/AccountRepository.xml"></mapper>
    <mapper resource="com/wsx/repository/StudentRepository.xml"></mapper>
    <mapper resource="com/wsx/repository/ClassesRepository.xml"/>
    <mapper resource="com/wsx/repository/CustomerRepository.xml"/>
    <mapper resource="com/wsx/repository/GoodsRepository.xml"/>
    </mappers>
  • 调用接口
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//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.wsx.mapper.AccoutMapper.save";
Account account = new Account(1L,"张三","123123",22);
sqlSession.insert(statement,account);
sqlSession.commit();
sqlSession.close();

自定义接口

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public interface AccountRepository {
public int save(Account account);
public int update(Account account);
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<insert id="save" parameterType="com.wsx.entity.Account">
insert into t_account(username,password,age) values(#{username},#{password},#{age})
</insert>
<update id="update" parameterType="com.wsx.entity.Account">
update t_account set username = #{username},password = #{password},age = #{age} where id = #{id}
</update>
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InputStream inputStream = Test.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);
accountRepository.()

传入多个变量

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

查询深入

多对一

比方这个, 我们查询出来有多个id,这时候,student对象的id会匹配第一个,name也是,所以查询出来classes就null了
我们需要使用resultMap来处理

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@Data
public class Classes {
private long id;
private String name;
private List<Student> students;
}

@Data
public class Student {
private long id;
private String name;
private Classes classes;
}
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<resultMap id="studentMap" type="com.wsx.entity.Student">
<id column="id" property="id"></id>
<result column="name" property="name"></result>
<association property="classes" javaType="com.wsx.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>

一对多

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<resultMap id="classesMap" type="com.wsx.entity.Classes">
<id column="cid" property="id"></id>
<result column="cname" property="name"></result>
<collection property="students" ofType="com.wsx.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>

逆向工程

传统开发中,实体类、自定义Mapper接口、Mapper.xml 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,减少工作量

MBG

MyBatis Generator , 一个专门为MyBatis框架开发者定制的代码生成器

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<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>

配置文件

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<?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?useUnicode=true&amp;characterEncoding=UTF-8"
userId="root"
password="123456"
></jdbcConnection>
<javaModelGenerator targetPackage="com.wsx.entity" targetProject="./src/main/java"></javaModelGenerator>
<sqlMapGenerator targetPackage="com.wsx.repository" targetProject="./src/main/java"></sqlMapGenerator>
<javaClientGenerator type="XMLMAPPER" targetPackage="com.wsx.repository" targetProject="./src/main/java"></javaClientGenerator>
<table tableName="t_user" domainObjectName="User"></table>
</context>
</generatorConfiguration>

1、jdbcConnection 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)。
生成代码如下,这个代码不重要,跑就完事了, 他就给我们生成了很多sql

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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

先配置

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

然后把多表关联查询拆分成多个单表查询, 调用接口的时候再去查询,不用就不查询,比方有图书表和图书类型表,我们查到了一些类型,这就是一个单表查询,如果你不去调用getBook,接下来的查询就不会进行, 慎用,这个东西讲一次查询拆成了多次,这可能导致性能降低

MyBatis缓存

我干嘛不用redis?

一级缓存

默认开启且不可关闭,是SqlSession级别的,用HashMap实现,不同的SqlSession之间的数据区域互不影响, 当执行了DML操作(insert,update,delete),就要清空缓存

二级缓存

多个SqlSession共用同一个缓存,依然是HashMap实现
在config.xml中开启

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<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>

在Mapper.xml 中加入

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<cache></cache>

实体类要实现序列化(Serializable)

第三方缓存

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<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>

配置第三方缓存,ehcache.xml

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<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>

在config.xml中开启(同上)
最后在mapper.xml中配置 缓存失效间隔、回收策略, 这里就不需要实现序列化了

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<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>

动态SQL

where 控制 and是否删除,
if 控制是否存在,
这里我不深入了…会用就行

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<select id="findByAccount" parameterType="com.wsx.entity.Account" resultType="com.wsx.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>

参考资料

4小时学会MyBatis
你真的懂了mybatis延迟加载吗?


文章作者: fightinggg
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