文章目录
Mybatis
概述
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mybatis是什么？有什么特点？
它是一款半自动的ORM持久层框架，具有较高的SQL灵活性，支持高级映射(一对一，一对多)，动态SQL，延迟加载和缓存等特性，但它的数据库无关性较低
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什么是ORM？
Object Relation Mapping，对象关系映射。对象指的是Java对象，关系指的是数据库中的关系模型，对象关系映射，指的就是在Java对象和数据库的关系模型之间建立一种对应关系，比如用一个Java的Student类，去对应数据库中的一张student表，类中的属性和表中的列一一对应。Student类就对应student表，一个Student对象就对应student表中的一行数据
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为什么mybatis是半自动的ORM框架？
用mybatis进行开发，需要手动编写SQL语句。而全自动的ORM框架，如hibernate，则不需要编写SQL语句。用hibernate开发，只需要定义好ORM映射关系，就可以直接进行CRUD操作了。由于mybatis需要手写SQL语句，所以它有较高的灵活性，可以根据需要，自由地对SQL进行定制，也因为要手写SQL，当要切换数据库时，SQL语句可能就要重写，因为不同的数据库有不同的方言(Dialect)，所以mybatis的数据库无关性低。虽然mybatis需要手写SQL，但相比JDBC，它提供了输入映射和输出映射，可以很方便地进行SQL参数设置，以及结果集封装。并且还提供了关联查询和动态SQL等功能，极大地提升了开发的效率。并且它的学习成本也比hibernate低很多
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快速入门
只需要通过如下几个步骤，即可用mybatis快速进行持久层的开发
- 编写全局配置文件
- 编写mapper映射文件
- 加载全局配置文件，生成SqlSessionFactory
- 创建SqlSession，调用mapper映射文件中的SQL语句来执行CRUD操作
原生开发示例
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在本地虚拟机mysql上创建一个库yogurt，并在里面创建一张student表
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打开IDEA，创建一个maven项目
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导入依赖的jar包
<dependencies> <dependency> <groupId>mysql</groupId> <artifactId>mysql-connector-java</artifactId> <version>5.1.10</version> </dependency> <dependency> <groupId>org.mybatis</groupId> <artifactId>mybatis</artifactId> <version>3.4.6</version> </dependency> <dependency> <groupId>org.projectlombok</groupId> <artifactId>lombok</artifactId> <version>1.18.12</version> <scope>provided</scope> </dependency> <dependency> <groupId>junit</groupId> <artifactId>junit</artifactId> <version>4.10</version> <scope>test</scope> </dependency> </dependencies>- 1
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创建一个po类
package com.yogurt.po;
import lombok.*;
@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
@ToString
public class Student {
<span class="token keyword">private</span> Integer id<span class="token punctuation">;</span>
<span class="token keyword">private</span> String name<span class="token punctuation">;</span>
<span class="token keyword">private</span> Integer score<span class="token punctuation">;</span>
<span class="token keyword">private</span> Integer age<span class="token punctuation">;</span>
<span class="token keyword">private</span> Integer gender<span class="token punctuation">;</span>
}
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编写mapper映射文件(编写SQL)
<!-- StudentMapper.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=“test”>
<select id=“findAll” resultType=“com.yogurt.po.Student”>
SELECT * FROM student;
</select><span class="token tag"><span class="token tag"><span class="token punctuation"><</span>insert</span> <span class="token attr-name">id</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>insert<span class="token punctuation">"</span></span> <span class="token attr-name">parameterType</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>com.yogurt.po.Student<span class="token punctuation">"</span></span><span class="token punctuation">></span></span> INSERT INTO student (name,score,age,gender) VALUES (#{name},#{score},#{age},#{gender}); <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>insert</span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>delete</span> <span class="token attr-name">id</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>delete<span class="token punctuation">"</span></span> <span class="token attr-name">parameterType</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>int<span class="token punctuation">"</span></span><span class="token punctuation">></span></span> DELETE FROM student WHERE id = #{id}; <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>delete</span><span class="token punctuation">></span></span></mapper>
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编写数据源properties文件
db.url=jdbc:mysql://192.168.183.129:3306/yogurt?characterEncoding=utf8 db.user=root db.password=root db.driver=com.mysql.jdbc.Driver- 1
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编写全局配置文件(主要是配置数据源信息)
<?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> <!-- 配置文件信息 --> <properties resource="properties/db.properties"></properties><span class="token tag"><span class="token tag"><span class="token punctuation"><</span>environments</span> <span class="token attr-name">default</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>development<span class="token punctuation">"</span></span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>environment</span> <span class="token attr-name">id</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>development<span class="token punctuation">"</span></span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>transactionManager</span> <span class="token attr-name">type</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>JDBC<span class="token punctuation">"</span></span><span class="token punctuation">/></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>dataSource</span> <span class="token attr-name">type</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>POOLED<span class="token punctuation">"</span></span><span class="token punctuation">></span></span> <span class="token comment"><!-- 从配置文件中加载属性 --></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>property</span> <span class="token attr-name">name</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>driver<span class="token punctuation">"</span></span> <span class="token attr-name">value</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>${db.driver}<span class="token punctuation">"</span></span><span class="token punctuation">/></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>property</span> <span class="token attr-name">name</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>url<span class="token punctuation">"</span></span> <span class="token attr-name">value</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>${db.url}<span class="token punctuation">"</span></span><span class="token punctuation">/></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>property</span> <span class="token attr-name">name</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>username<span class="token punctuation">"</span></span> <span class="token attr-name">value</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>${db.user}<span class="token punctuation">"</span></span><span class="token punctuation">/></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>property</span> <span class="token attr-name">name</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>password<span class="token punctuation">"</span></span> <span class="token attr-name">value</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>${db.password}<span class="token punctuation">"</span></span><span class="token punctuation">/></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>dataSource</span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>environment</span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>environments</span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>mappers</span><span class="token punctuation">></span></span> <span class="token comment"><!-- 加载前面编写的SQL语句的文件 --></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>mapper</span> <span class="token attr-name">resource</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>StudentMapper.xml<span class="token punctuation">"</span></span><span class="token punctuation">/></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>mappers</span><span class="token punctuation">></span></span> </configuration>
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编写dao类
package com.yogurt.dao; import com.yogurt.po.Student;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;import java.io.IOException;
import java.io.InputStream;
import java.util.List;public class StudentDao {
<span class="token keyword">private</span> SqlSessionFactory sqlSessionFactory<span class="token punctuation">;</span> <span class="token keyword">public</span> <span class="token function">StudentDao</span><span class="token punctuation">(</span>String configPath<span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span> InputStream inputStream <span class="token operator">=</span> Resources<span class="token punctuation">.</span><span class="token function">getResourceAsStream</span><span class="token punctuation">(</span>configPath<span class="token punctuation">)</span><span class="token punctuation">;</span> sqlSessionFactory <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">SqlSessionFactoryBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">build</span><span class="token punctuation">(</span>inputStream<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token keyword">public</span> List<span class="token generics function"><span class="token punctuation"><</span>Student<span class="token punctuation">></span></span> <span class="token function">findAll</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span> SqlSession sqlSession <span class="token operator">=</span> sqlSessionFactory<span class="token punctuation">.</span><span class="token function">openSession</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> List<span class="token generics function"><span class="token punctuation"><</span>Student<span class="token punctuation">></span></span> studentList <span class="token operator">=</span> sqlSession<span class="token punctuation">.</span><span class="token function">selectList</span><span class="token punctuation">(</span><span class="token string">"findAll"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> sqlSession<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">return</span> studentList<span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token keyword">public</span> <span class="token keyword">int</span> <span class="token function">addStudent</span><span class="token punctuation">(</span>Student student<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span> SqlSession sqlSession <span class="token operator">=</span> sqlSessionFactory<span class="token punctuation">.</span><span class="token function">openSession</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">int</span> rowsAffected <span class="token operator">=</span> sqlSession<span class="token punctuation">.</span><span class="token function">insert</span><span class="token punctuation">(</span><span class="token string">"insert"</span><span class="token punctuation">,</span> student<span class="token punctuation">)</span><span class="token punctuation">;</span> sqlSession<span class="token punctuation">.</span><span class="token function">commit</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> sqlSession<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">return</span> rowsAffected<span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token keyword">public</span> <span class="token keyword">int</span> <span class="token function">deleteStudent</span><span class="token punctuation">(</span><span class="token keyword">int</span> id<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span> SqlSession sqlSession <span class="token operator">=</span> sqlSessionFactory<span class="token punctuation">.</span><span class="token function">openSession</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">int</span> rowsAffected <span class="token operator">=</span> sqlSession<span class="token punctuation">.</span><span class="token function">delete</span><span class="token punctuation">(</span><span class="token string">"delete"</span><span class="token punctuation">,</span>id<span class="token punctuation">)</span><span class="token punctuation">;</span> sqlSession<span class="token punctuation">.</span><span class="token function">commit</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> sqlSession<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">return</span> rowsAffected<span class="token punctuation">;</span> <span class="token punctuation">}</span>}
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测试
public class SimpleTest {<span class="token keyword">private</span> StudentDao studentDao<span class="token punctuation">;</span> <span class="token annotation punctuation">@Before</span> <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">init</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span> studentDao <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">StudentDao</span><span class="token punctuation">(</span><span class="token string">"mybatis-config.xml"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token annotation punctuation">@Test</span> <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">insertTest</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span> Student student <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Student</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> student<span class="token punctuation">.</span><span class="token function">setName</span><span class="token punctuation">(</span><span class="token string">"yogurt"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> student<span class="token punctuation">.</span><span class="token function">setAge</span><span class="token punctuation">(</span><span class="token number">24</span><span class="token punctuation">)</span><span class="token punctuation">;</span> student<span class="token punctuation">.</span><span class="token function">setGender</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span> student<span class="token punctuation">.</span><span class="token function">setScore</span><span class="token punctuation">(</span><span class="token number">100</span><span class="token punctuation">)</span><span class="token punctuation">;</span> studentDao<span class="token punctuation">.</span><span class="token function">addStudent</span><span class="token punctuation">(</span>student<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token annotation punctuation">@Test</span> <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">findAllTest</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span> List<span class="token generics function"><span class="token punctuation"><</span>Student<span class="token punctuation">></span></span> all <span class="token operator">=</span> studentDao<span class="token punctuation">.</span><span class="token function">findAll</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> all<span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span>System<span class="token punctuation">.</span>out<span class="token operator">:</span><span class="token operator">:</span>println<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> }
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总结：
- 编写mapper.xml，书写SQL，并定义好SQL的输入参数，和输出参数
- 编写全局配置文件，配置数据源，以及要加载的mapper.xml文件
- 通过全局配置文件，创建SqlSessionFactory
- 每次进行CRUD时，通过SqlSessionFactory创建一个SqlSession
- 调用SqlSession上的
selectOne，selectList，insert，delete，update等方法，传入mapper.xml中SQL标签的id，以及输入参数
注意要点
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全局配置文件中，各个标签要按照如下顺序进行配置，因为mybatis加载配置文件的源码中是按照这个顺序进行解析的
<configuration> <!-- 配置顺序如下 propertiessettings typeAliases typeHandlers objectFactory plugins environments environment transactionManager dataSource mappers --></span>
</configuration>
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各个子标签说明如下
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<properties>一般将数据源的信息单独放在一个properties文件中,然后用这个标签引入,在下面environment标签中，就可以用
${}占位符快速获取数据源的信息 -
<settings>用来开启或关闭mybatis的一些特性，比如可以用
<setting name="lazyLoadingEnabled" value="true"/>来开启延迟加载，可以用<settings name="cacheEnabled" value="true"/>来开启二级缓存 -
<typeAliases>在mapper.xml中需要使用
parameterType和resultType属性来配置SQL语句的输入参数类型和输出参数类型，类必须要写上全限定名，比如一个SQL的返回值映射为Student类，则resultType属性要写com.yogurt.po.Student，这太长了，所以可以用别名来简化书写，比如<typeAliases> <typeAlias type="com.yogurt.po.Student" alias="student"/> </typeAliases>- 1
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之后就可以在
resultType上直接写student，mybatis会根据别名配置自动找到对应的类。当然，如果想要一次性给某个包下的所有类设置别名，可以用如下的方式
<typeAliases> <package name="com.yogurt.po"/> </typeAliases>- 1
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如此，指定包下的所有类，都会以简单类名的小写形式，作为它的别名
另外，对于基本的Java类型 -> 8大基本类型以及包装类，以及String类型，mybatis提供了默认的别名，别名为其简单类名的小写，比如原本需要写
java.lang.String，其实可以简写为string -
<typeHandlers>用于处理Java类型和Jdbc类型之间的转换，mybatis有许多内置的TypeHandler，比如StringTypeHandler，会处理Java类型String和Jdbc类型CHAR和VARCHAR。这个标签用的不多
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<objectFactory>mybatis会根据
resultType或resultMap的属性来将查询得到的结果封装成对应的Java类，它有一个默认的DefaultObjectFactory，用于创建对象实例，这个标签用的也不多 -
<plugins>可以用来配置mybatis的插件，比如在开发中经常需要对查询结果进行分页，就需要用到pageHelper分页插件，这些插件就是通过这个标签进行配置的。在mybatis底层，运用了责任链模式+动态代理去实现插件的功能
<!-- PageHelper 分页插件 --> <plugins> <plugin interceptor="com.github.pagehelper.PageInterceptor"> <property name="helperDialect" value="mysql"/> </plugin> </plugins>- 1
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<environments>用来配置数据源
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<mappers>用来配置mapper.xml映射文件，这些xml文件里都是SQL语句
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mapper.xml的SQL语句中的占位符
${}和#{}一般会采用
#{}，#{}在mybatis中，最后会被解析为?，其实就是Jdbc的PreparedStatement中的?占位符，它有预编译的过程，会对输入参数进行类型解析(如果入参是String类型，设置参数时会自动加上引号)，可以防止SQL注入，如果parameterType属性指定的入参类型是简单类型的话(简单类型指的是8种java原始类型再加一个String)，#{}中的变量名可以任意，如果入参类型是pojo，比如是Student类public class Student{ private String name; private Integer age; //setter/getter }- 1
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那么
#{name}表示取入参对象Student中的name属性，#{age}表示取age属性，这个过程是通过反射来做的，这不同于${}，${}取对象的属性使用的是OGNL(Object Graph Navigation Language)表达式而
${}，一般会用在模糊查询的情景，比如SELECT * FROM student WHERE name like '%${name}%';它的处理阶段在
#{}之前，它不会做参数类型解析，而仅仅是做了字符串的拼接，若入参的Student对象的name属性为zhangsan，则上面那条SQL最终被解析为SELECT * FROM student WHERE name like '%zhangsan%';而如果此时用的是
SELECT * FROM student WHERE name like '%#{name}%';这条SQL最终就会变成SELECT * FROM student WHERE name like '%'zhangsan'%';所以模糊查询只能用${},虽然普通的入参也可以用${},但由于${}不会做类型解析，就存在SQL注入的风险，比如SELECT * FROM user WHERE name = '${name}' AND password = '${password}'我可以让一个user对象的password属性为
'OR '1' = '1，最终的SQL就变成了SELECT * FROM user WHERE name = 'yogurt' AND password = ''OR '1' = '1'，因为OR '1' = '1'恒成立，这样攻击者在不需要知道用户名和密码的情况下，也能够完成登录验证另外,对于pojo的入参，
${}中获取对象属性的语法和#{}几乎一样，但${}在mybatis底层是通过OGNL表达式语言进行处理的，这跟#{}的反射处理有所不同对于简单类型(8种java原始类型再加一个String)的入参，
${}中参数的名字必须是value，例子如下<select id="fuzzyCount" parameterType="string" resultType="int"> SELECT count(1) FROM `user` WHERE name like '%${value}%' </select>- 1
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为什么简单类型的变量名必须为value呢？因为mybatis源码中写死的value，哈哈
上面其实是比较原始的开发方式，我们需要编写dao类，针对mapper.xml中的每个SQL标签，做一次封装，SQL标签的id要以字符串的形式传递给SqlSession的相关方法，容易出错，非常不方便；为了简化开发，mybatis提供了mapper接口代理的开发方式，不需要再编写dao类，只需要编写一个mapper接口，一个mapper的接口和一个mapper.xml相对应，只需要调用SqlSession对象上的
getMapper()，传入mapper接口的class信息，即可获得一个mapper代理对象，直接调用mapper接口中的方法，即相当于调用mapper.xml中的各个SQL标签，此时就不需要指定SQL标签的id字符串了，mapper接口中的一个方法，就对应了mapper.xml中的一个SQL标签基于Mapper代理的示例
全局配置文件和mapper.xml文件是最基本的配置，仍然需要。不过，这次我们不编写dao类，我们直接创建一个mapper接口
package com.yogurt.mapper;import com.yogurt.po.Student;
import java.util.List;
public interface StudentMapper {
List<span class="token generics function"><span class="token punctuation"><</span>Student<span class="token punctuation">></span></span> <span class="token function">findAll</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">int</span> <span class="token function">insert</span><span class="token punctuation">(</span>Student student<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">int</span> <span class="token function">delete</span><span class="token punctuation">(</span>Integer id<span class="token punctuation">)</span><span class="token punctuation">;</span> List<span class="token generics function"><span class="token punctuation"><</span>Student<span class="token punctuation">></span></span> <span class="token function">findByName</span><span class="token punctuation">(</span>String value<span class="token punctuation">)</span><span class="token punctuation">;</span>}
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而我们的mapper.xml文件如下
<?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.yogurt.mapper.StudentMapper”>
<select id=“findAll” resultType=“com.yogurt.po.Student”>
SELECT * FROM student;
</select><span class="token tag"><span class="token tag"><span class="token punctuation"><</span>insert</span> <span class="token attr-name">id</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>insert<span class="token punctuation">"</span></span> <span class="token attr-name">parameterType</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>com.yogurt.po.Student<span class="token punctuation">"</span></span><span class="token punctuation">></span></span> INSERT INTO student (name,score,age,gender) VALUES (#{name},#{score},#{age},#{gender}); <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>insert</span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>delete</span> <span class="token attr-name">id</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>delete<span class="token punctuation">"</span></span> <span class="token attr-name">parameterType</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>int<span class="token punctuation">"</span></span><span class="token punctuation">></span></span> DELETE FROM student WHERE id = #{id}; <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>delete</span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>select</span> <span class="token attr-name">id</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>findByName<span class="token punctuation">"</span></span> <span class="token attr-name">parameterType</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>string<span class="token punctuation">"</span></span> <span class="token attr-name">resultType</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>student<span class="token punctuation">"</span></span><span class="token punctuation">></span></span> SELECT * FROM student WHERE name like '%${value}%'; <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>select</span><span class="token punctuation">></span></span></mapper>
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mapper接口和mapper.xml之间需要遵循一定规则，才能成功的让mybatis将mapper接口和mapper.xml绑定起来
- mapper接口的全限定名，要和mapper.xml的namespace属性一致
- mapper接口中的方法名要和mapper.xml中的SQL标签的id一致
- mapper接口中的方法入参类型，要和mapper.xml中SQL语句的入参类型一致
- mapper接口中的方法出参类型，要和mapper.xml中SQL语句的返回值类型一致
测试代码如下
public class MapperProxyTest { private SqlSessionFactory sqlSessionFactory;<span class="token annotation punctuation">@Before</span> <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">init</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span> InputStream resourceAsStream <span class="token operator">=</span> Resources<span class="token punctuation">.</span><span class="token function">getResourceAsStream</span><span class="token punctuation">(</span><span class="token string">"mybatis-config.xml"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> sqlSessionFactory <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">SqlSessionFactoryBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">build</span><span class="token punctuation">(</span>resourceAsStream<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token annotation punctuation">@Test</span> <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span> SqlSession sqlSession <span class="token operator">=</span> sqlSessionFactory<span class="token punctuation">.</span><span class="token function">openSession</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> StudentMapper mapper <span class="token operator">=</span> sqlSession<span class="token punctuation">.</span><span class="token function">getMapper</span><span class="token punctuation">(</span>StudentMapper<span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span> List<span class="token generics function"><span class="token punctuation"><</span>Student<span class="token punctuation">></span></span> studentList <span class="token operator">=</span> mapper<span class="token punctuation">.</span><span class="token function">findAll</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> studentList<span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span>System<span class="token punctuation">.</span>out<span class="token operator">:</span><span class="token operator">:</span>println<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>}
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结果如下
这个mapper接口，mybatis会自动找到对应的mapper.xml，然后对mapper接口使用动态代理的方式生成一个代理类
基于注解的示例
如果实在看xml配置文件不顺眼，则可以考虑使用注解的开发方式，不过注解的开发方式，会将SQL语句写到代码文件中，后续的维护性和扩展性不是很好（如果想修改SQL语句，就得改代码，得重新打包部署，而如果用xml方式，则只需要修改xml，用新的xml取替换旧的xml即可）
使用注解的开发方式，也还是得有一个全局配置的xml文件，不过mapper.xml就可以省掉了，具体操作只用2步，如下
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创建一个Mapper接口
package com.yogurt.mapper; import com.yogurt.po.Student; import org.apache.ibatis.annotations.Insert; import org.apache.ibatis.annotations.Select; import java.util.List;
public interface PureStudentMapper {
<span class="token annotation punctuation">@Select</span><span class="token punctuation">(</span><span class="token string">"SELECT * FROM student"</span><span class="token punctuation">)</span> List<span class="token generics function"><span class="token punctuation"><</span>Student<span class="token punctuation">></span></span> <span class="token function">findAll</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token annotation punctuation">@Insert</span><span class="token punctuation">(</span><span class="token string">"INSERT INTO student (name,age,score,gender) VALUES (#{name},#{age},#{score},#{gender})"</span><span class="token punctuation">)</span> <span class="token keyword">int</span> <span class="token function">insert</span><span class="token punctuation">(</span>Student student<span class="token punctuation">)</span><span class="token punctuation">;</span>}
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在全局配置文件中修改
<mappers>标签，直接指定加载这个类<?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> <properties resource="properties/db.properties"></properties> <typeAliases> <package name="com.yogurt.po"/> </typeAliases> <environments default="development"> <environment id="development"> <transactionManager type="JDBC"/> <dataSource type="POOLED"> <property name="driver" value="${db.driver}"/> <property name="url" value="${db.url}"/> <property name="username" value="${db.user}"/> <property name="password" value="${db.password}"/> </dataSource> </environment> </environments><span class="token tag"><span class="token tag"><span class="token punctuation"><</span>mappers</span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>mapper</span> <span class="token attr-name">class</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>com.yogurt.mapper.PureStudentMapper<span class="token punctuation">"</span></span><span class="token punctuation">/></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>mappers</span><span class="token punctuation">></span></span> </configuration>
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测试代码如下
public class PureMapperTest {<span class="token keyword">private</span> SqlSessionFactory sqlSessionFactory<span class="token punctuation">;</span> <span class="token annotation punctuation">@Before</span> <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">init</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span> InputStream inputStream <span class="token operator">=</span> Resources<span class="token punctuation">.</span><span class="token function">getResourceAsStream</span><span class="token punctuation">(</span><span class="token string">"mybatis-config.xml"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> sqlSessionFactory <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">SqlSessionFactoryBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">build</span><span class="token punctuation">(</span>inputStream<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token annotation punctuation">@Test</span> <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span> SqlSession sqlSession <span class="token operator">=</span> sqlSessionFactory<span class="token punctuation">.</span><span class="token function">openSession</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> PureStudentMapper mapper <span class="token operator">=</span> sqlSession<span class="token punctuation">.</span><span class="token function">getMapper</span><span class="token punctuation">(</span>PureStudentMapper<span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span> mapper<span class="token punctuation">.</span><span class="token function">insert</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Student</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">,</span><span class="token string">"Tomcat"</span><span class="token punctuation">,</span><span class="token number">120</span><span class="token punctuation">,</span><span class="token number">60</span><span class="token punctuation">,</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> sqlSession<span class="token punctuation">.</span><span class="token function">commit</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> List<span class="token generics function"><span class="token punctuation"><</span>Student<span class="token punctuation">></span></span> studentList <span class="token operator">=</span> mapper<span class="token punctuation">.</span><span class="token function">findAll</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> studentList<span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span>System<span class="token punctuation">.</span>out<span class="token operator">:</span><span class="token operator">:</span>println<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>}
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结果如下
上面我们见到了在全局配置文件中，两种配置mapper的方式，分别是
<!-- 在mapper接口中使用注解 --> <mappers> <mapper class="com.yogurt.mapper.PureStudentMapper"/> </mappers><!-- 普通加载xml -->
<mappers>
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而在实际工作中，一般我们会将一张表的SQL操作封装在一个mapper.xml中，可能有许多张表需要操作，那么我们是不是要在
<mappers>标签下写多个<mapper>标签呢？其实不用，还有第三种加载mapper的方法，使用<package>标签<mappers> <package name="com.yogurt.mapper"/> </mappers>- 1
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这样就会自动加载
com.yogurt.mapper包下的所有mapper，这种方式需要将mapper接口文件和mapper.xml文件都放在com.yogurt.mapper包下，且接口文件和xml文件的文件名要一致。注意，在IDEA的maven开发环境下，maven中还需配置<resources>标签，否则maven打包不会将java源码目录下的xml文件打包进去，见下文三种加载mapper的方式总结
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<mapper resource="" />加载普通的xml文件，传入xml的相对路径(相对于类路径)
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<mapper class="" />使用mapper接口的全限定名来加载，若mapper接口采用注解方式，则不需要xml;若mapper接口没有采用注解方式，则mapper接口和xml文件的名称要相同，且在同一个目录
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<package name="" />扫描指定包下的所有mapper，若mapper接口采用注解方式，则不需要xml;若mapper接口没有采用注解方式，则mapper接口和xml文件的名称要相同，且在同一目录
注意：用后两种方式加载mapper接口和mapper.xml映射文件时，可能会报错
仔细检查了一下，mapper接口文件和xml映射文件确实放在了同一个目录下，而且文件名一致，xml映射文件的namespace也和mapper接口的全限定名对的上。为什么会这样呢？
其实是因为，对于
src/main/java源码目录下的文件，maven打包时只会将该目录下的java文件打包，而其他类型的文件都不会被打包进去，去工程目录的target目录下看看maven构建后生成的文件我们需要在pom.xml中的
<build>标签下 添加<resources>标签，指定打包时要将xml文件打包进去<build> <resources> <resource> <directory>src/main/java</directory> <includes> <include>**/*.xml</include> </includes> </resource> </resources> </build>- 1
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此时再用maven进行打包，看到对应目录下有了xml映射文件
此时再运行单元测试，就能正常得到结果了
应用场景
主键返回
通常我们会将数据库表的主键id设为自增。在插入一条记录时，我们不设置其主键id，而让数据库自动生成该条记录的主键id，那么在插入一条记录后，如何得到数据库自动生成的这条记录的主键id呢？有两种方式
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使用
useGeneratedKeys和keyProperty属性<insert id="insert" parameterType="com.yogurt.po.Student" useGeneratedKeys="true" keyProperty="id"> INSERT INTO student (name,score,age,gender) VALUES (#{name},#{score},#{age},#{gender}); </insert>- 1
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使用
<selectKey>子标签<insert id="insert" parameterType="com.yogurt.po.Student"> INSERT INTO student (name,score,age,gender) VALUES (#{name},#{score},#{age},#{gender}); <selectKey keyProperty="id" order="AFTER" resultType="int" > SELECT LAST_INSERT_ID(); </selectKey> </insert>- 1
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如果使用的是mysql这样的支持自增主键的数据库，可以简单的使用第一种方式；对于不支持自增主键的数据库，如oracle，则没有主键返回这一概念，而需要在插入之前先生成一个主键。此时可以用
<selectKey>标签，设置其order属性为BEFORE，并在标签体内写上生成主键的SQL语句，这样在插入之前，会先处理<selectKey>，生成主键，再执行真正的插入操作。<selectKey>标签其实就是一条SQL，这条SQL的执行，可以放在主SQL执行之前或之后，并且会将其执行得到的结果封装到入参的Java对象的指定属性上。注意<selectKey>子标签只能用在<insert>和<update>标签中。上面的LAST_INSERT_ID()实际上是MySQL提供的一个函数，可以用来获取最近插入或更新的记录的主键id。
测试代码如下
public class MapperProxyTest { private SqlSessionFactory sqlSessionFactory;<span class="token annotation punctuation">@Before</span> <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">init</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span> InputStream resourceAsStream <span class="token operator">=</span> Resources<span class="token punctuation">.</span><span class="token function">getResourceAsStream</span><span class="token punctuation">(</span><span class="token string">"mybatis-config.xml"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> sqlSessionFactory <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">SqlSessionFactoryBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">build</span><span class="token punctuation">(</span>resourceAsStream<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token annotation punctuation">@Test</span> <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span> SqlSession sqlSession <span class="token operator">=</span> sqlSessionFactory<span class="token punctuation">.</span><span class="token function">openSession</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> StudentMapper mapper <span class="token operator">=</span> sqlSession<span class="token punctuation">.</span><span class="token function">getMapper</span><span class="token punctuation">(</span>StudentMapper<span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span> Student student <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Student</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token string">"Podman"</span><span class="token punctuation">,</span> <span class="token number">130</span><span class="token punctuation">,</span> <span class="token number">15</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span> mapper<span class="token punctuation">.</span><span class="token function">insert</span><span class="token punctuation">(</span>student<span class="token punctuation">)</span><span class="token punctuation">;</span> sqlSession<span class="token punctuation">.</span><span class="token function">commit</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>student<span class="token punctuation">.</span><span class="token function">getId</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>}
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结果如下
批量查询
主要是动态SQL标签的使用，注意如果
parameterType是List的话，则在标签体内引用这个List，只能用变量名list，如果parameterType是数组，则只能用变量名array<select id="batchFind" resultType="student" parameterType="java.util.List"> SELECT * FROM student <where> <if test="list != null and list.size() > 0"> AND id in <foreach collection="list" item="id" open="(" separator="," close=")"> #{id} </foreach> </if> </where> </select>- 1
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@Test public void testBatchQuery() { SqlSession sqlSession = sqlSessionFactory.openSession(); StudentMapper mapper = sqlSession.getMapper(StudentMapper.class); List<Student> students = mapper.batchFind(Arrays.asList(1, 2, 3, 7, 9)); students.forEach(System.out::println); }- 1
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结果
动态SQL
可以根据具体的参数条件，来对SQL语句进行动态拼接。
比如在以前的开发中，由于不确定查询参数是否存在，许多人会使用类似于
where 1 = 1来作为前缀，然后后面用AND拼接要查询的参数，这样，就算要查询的参数为空，也能够正确执行查询，如果不加1 = 1，则如果查询参数为空，SQL语句就会变成SELECT * FROM student where，SQL不合法。mybatis里的动态标签主要有
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if<!-- 示例 --> <select id="find" resultType="student" parameterType="student"> SELECT * FROM student WHERE age >= 18 <if test="name != null and name != ''"> AND name like '%${name}%' </if> </select>- 1
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当满足test条件时，才会将
<if>标签内的SQL语句拼接上去 -
choose<!-- choose 和 when , otherwise 是配套标签 类似于java中的switch，只会选中满足条件的一个 --> <select id="findActiveBlogLike" resultType="Blog"> SELECT * FROM BLOG WHERE state = ‘ACTIVE’ <choose> <when test="title != null"> AND title like #{title} </when> <when test="author != null and author.name != null"> AND author_name like #{author.name} </when> <otherwise> AND featured = 1 </otherwise> </choose> </select>- 1
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trim-
where<where>标签只会在至少有一个子元素返回了SQL语句时，才会向SQL语句中添加WHERE，并且如果WHERE之后是以AND或OR开头，会自动将其删掉<select id="findActiveBlogLike" resultType="Blog"> SELECT * FROM BLOG <where> <if test="state != null"> state = #{state} </if> <if test="title != null"> AND title like #{title} </if> <if test="author != null and author.name != null"> AND author_name like #{author.name} </if> </where> </select>- 1
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<where>标签可以用<trim>标签代替<trim prefix="WHERE" prefixOverrides="AND | OR"> ... </trim>- 1
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set在至少有一个子元素返回了SQL语句时，才会向SQL语句中添加SET，并且如果SET之后是以
,开头的话，会自动将其删掉<set>标签相当于如下的<trim>标签<trim prefix="SET" prefixOverrides=","> ... </trim>- 1
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可以通过
<trim>标签更加灵活地对SQL进行定制实际上在mybatis源码，也能看到trim与set,where标签的父子关系
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foreach用来做迭代拼接的，通常会与SQL语句中的
IN查询条件结合使用<select id="batchFind" resultType="student" parameterType="list"> SELECT * FROM student WHERE id in <foreach collection="list" item="item" open="(" separator="," close=")"> #{item} </foreach> </select>- 1
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sql可将重复的SQL片段提取出来，然后在需要的地方，使用
<include>标签进行引用<select id="findUser" parameterType="user" resultType="user"> SELECT * FROM user <include refid="whereClause"/> </select>
<sql id=“whereClause”>
<where>
<if test user != null>
AND username like ‘%${user.name}%’
</if>
</where>
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bindmybatis的动态SQL都是用OGNL表达式进行解析的，如果需要创建OGNL表达式以外的变量，可以用bind标签
<select id="selectBlogsLike" resultType="Blog"> <bind name="pattern" value="'%' + _parameter.getTitle() + '%'" /> SELECT * FROM BLOG WHERE title LIKE #{pattern} </select>- 1
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缓存
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一级缓存
默认开启，同一个SqlSesion级别共享的缓存，在一个SqlSession的生命周期内，执行2次相同的SQL查询，则第二次SQL查询会直接取缓存的数据，而不走数据库，当然，若第一次和第二次相同的SQL查询之间，执行了DML(INSERT/UPDATE/DELETE)，则一级缓存会被清空，第二次查询相同SQL仍然会走数据库
一级缓存在下面情况会被清除
- 在同一个SqlSession下执行增删改操作时（不必提交），会清除一级缓存
- SqlSession提交或关闭时（关闭时会自动提交），会清除一级缓存
- 对mapper.xml中的某个CRUD标签，设置属性
flushCache=true，这样会导致该MappedStatement的一级缓存，二级缓存都失效（一个CRUD标签在mybatis中会被封装成一个MappedStatement） - 在全局配置文件中设置
<setting name="localCacheScope" value="STATEMENT"/>，这样会使一级缓存失效，二级缓存不受影响
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二级缓存
默认关闭，可通过全局配置文件中的
<settings name="cacheEnabled" value="true"/>开启二级缓存总开关，然后在某个具体的mapper.xml中增加<cache />，即开启了该mapper.xml的二级缓存。二级缓存是mapper级别的缓存，粒度比一级缓存大，多个SqlSession可以共享同一个mapper的二级缓存。注意开启二级缓存后，SqlSession需要提交，查询的数据才会被刷新到二级缓存当中
缓存的详细分析可以参考我之前的文章 => 极简mybatis缓存
关联查询
使用
<resultMap>标签以及<association>和<collection>子标签，进行关联查询，比较简单，不多说延迟加载
延迟加载是结合关联查询进行应用的。也就是说，只在
<association>和<collection>标签上起作用对于关联查询，若不采用延迟加载策略，而是一次性将关联的从信息都查询出来，则在主信息比较多的情况下，会产生N+1问题，导致性能降低。比如用户信息和订单信息是一对多的关系，在查询用户信息时，设置了关联查询订单信息，如不采用延迟加载策略，假设共有100个用户，则我们查这100个用户的基本信息只需要一次SQL查询
select * from user;- 1
若开启了关联查询，且不是延迟加载，则对于这100个用户，会发出100条SQL去查用户对应的订单信息
select * from orders where u_id = 1; select * from orders where u_id = 2; .... select * from orders where u_id = 100;- 1
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而我们可能只关心id=3的用户的订单信息，则很多的关联信息是无用的，于是，采用延迟加载策略，可以按需加载从信息，在需要某个主信息对应的从信息时，再发送SQL去执行查询，而不是一次性全部查出来，这样能很好的提升性能
延迟加载默认是关闭的，可以通过全局配置文件中的
<setting name="lazyLoadingEnabled" value="true"/>来开启，开启后，所有的SELECT查询，若有关联对象，都会采用延迟加载的策略。当然，也可以对指定的某个CRUD标签单独禁用延迟加载策略，通过设置SELECT标签中的fetchType=eager，则可以关闭该标签的延迟加载。(还有一个侵入式延迟加载的概念，在配置文件中通过
<setting name="aggressiveLazyLoading" value="true">来开启，大概是说，访问主对象中的主信息时，就会触发延迟加载，将从信息查询上来，这其实并不是真正意义的延迟加载，真正意义上的延迟加载应该是访问主对象中的从信息时，才触发延迟加载，去加载从信息，侵入式延迟加载默认是关闭的，一般情况下可以不用管他)注意，延迟加载在关联查询的场景下才有意义。需要配合
<resultMap>标签下的<association>和<collecction>标签使用<!-- StudentMapper.xml --> <resultMap id="studentExt" type="com.yogurt.po.StudentExt"> <result property="id" column="id"/> <result property="name" column="name"/> <result property="score" column="score"/> <result property="age" column="age"/> <result property="gender" column="gender"/> <!-- 当延迟加载总开关开启时，resultMap下的association和collection标签中，若通过select属性指定嵌套查询的SQL，则其fetchType默认是lazy的，当在延迟加载总开关开启时，需要对个别的关联查询禁用延迟加载时，才有必要配置fetchType = eager --> <!-- column用于指定用于关联查询的列 property用于指定要封装到StudentExt中的哪个属性 javaType用于指定关联查询得到的对象 select用于指定关联查询时,调用的是哪一个DQL --> <association property="clazz" javaType="com.yogurt.po.Clazz" column="class_id" select="com.yogurt.mapper.ClassMapper.findById" fetchType="lazy"/><span class="token tag"><span class="token tag"><span class="token punctuation"></</span>resultMap</span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>select</span> <span class="token attr-name">id</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>findLazy<span class="token punctuation">"</span></span> <span class="token attr-name">parameterType</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>string<span class="token punctuation">"</span></span> <span class="token attr-name">resultMap</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>studentExt<span class="token punctuation">"</span></span><span class="token punctuation">></span></span> SELECT * FROM student WHERE name like '%${value}%'; <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>select</span><span class="token punctuation">></span></span>- 1
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<!-- com.yogurt.mapper.ClassMapper --> <select id="findById" parameterType="int" resultType="com.yogurt.po.Clazz"> SELECT * FROM class WHERE id = #{id} </select>- 1
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/** 用于封装关联查询的对象 **/ public class StudentExt{<span class="token keyword">private</span> Integer id<span class="token punctuation">;</span> <span class="token keyword">private</span> String name<span class="token punctuation">;</span> <span class="token keyword">private</span> Integer score<span class="token punctuation">;</span> <span class="token keyword">private</span> Integer age<span class="token punctuation">;</span> <span class="token keyword">private</span> Integer gender<span class="token punctuation">;</span> <span class="token comment">/** 关联对象 **/</span> <span class="token keyword">private</span> Clazz clazz<span class="token punctuation">;</span> <span class="token comment">//getter/setter</span>}
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逆向工程
mybatis官方提供了mapper自动生成工具mybatis-generator-core来针对单表，生成PO类，以及Mapper接口和mapper.xml映射文件。针对单表，可以不需要再手动编写xml配置文件和mapper接口文件了，非常方便。美中不足的是它不支持生成关联查询。一般做关联查询，就自己单独写SQL就好了。
基于IDEA的mybatis逆向工程操作步骤如下
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配置maven插件
<build> <plugins> <plugin> <groupId>org.mybatis.generator</groupId> <artifactId>mybatis-generator-maven-plugin</artifactId> <version>1.3.7</version> <configuration> <!-- 输出日志 --> <verbose>true</verbose> <overwrite>true</overwrite> </configuration> </plugin> </plugins> </build>- 1
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在resources目录下创建名为generatorConfig.xml的配置文件
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配置文件的模板如下
<?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>
<!–导入属性配置–>
<properties resource=“properties/xx.properties”></properties><span class="token comment"><!-- 指定数据库驱动的jdbc驱动jar包的位置 --></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>classPathEntry</span> <span class="token attr-name">location</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>C:\Users\Vergi\.m2\repository\mysql\mysql-connector-java\8.0.11\mysql-connector-java-8.0.11.jar<span class="token punctuation">"</span></span> <span class="token punctuation">/></span></span> <span class="token comment"><!-- context 是逆向工程的主要配置信息 --></span> <span class="token comment"><!-- id：起个名字 --></span> <span class="token comment"><!-- targetRuntime：设置生成的文件适用于那个 mybatis 版本 --></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>context</span> <span class="token attr-name">id</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>default<span class="token punctuation">"</span></span> <span class="token attr-name">targetRuntime</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>MyBatis3<span class="token punctuation">"</span></span><span class="token punctuation">></span></span> <span class="token comment"><!--optional,旨在创建class时，对注释进行控制--></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>commentGenerator</span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>property</span> <span class="token attr-name">name</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>suppressDate<span class="token punctuation">"</span></span> <span class="token attr-name">value</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>true<span class="token punctuation">"</span></span> <span class="token punctuation">/></span></span> <span class="token comment"><!-- 是否去除自动生成的注释 true：是 ： false:否 --></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>property</span> <span class="token attr-name">name</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>suppressAllComments<span class="token punctuation">"</span></span> <span class="token attr-name">value</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>true<span class="token punctuation">"</span></span> <span class="token punctuation">/></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>commentGenerator</span><span class="token punctuation">></span></span> <span class="token comment"><!--jdbc的数据库连接--></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>jdbcConnection</span> <span class="token attr-name">driverClass</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>${db.driver}<span class="token punctuation">"</span></span> <span class="token attr-name">connectionURL</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>${db.url}<span class="token punctuation">"</span></span> <span class="token attr-name">userId</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>${db.user}<span class="token punctuation">"</span></span> <span class="token attr-name">password</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>${db.password}<span class="token punctuation">"</span></span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>jdbcConnection</span><span class="token punctuation">></span></span> <span class="token comment"><!--非必须，类型处理器，在数据库类型和java类型之间的转换控制--></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>javaTypeResolver</span><span class="token punctuation">></span></span> <span class="token comment"><!-- 默认情况下数据库中的 decimal，bigInt 在 Java 对应是 sql 下的 BigDecimal 类 --></span> <span class="token comment"><!-- 不是 double 和 long 类型 --></span> <span class="token comment"><!-- 使用常用的基本类型代替 sql 包下的引用类型 --></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>property</span> <span class="token attr-name">name</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>forceBigDecimals<span class="token punctuation">"</span></span> <span class="token attr-name">value</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>false<span class="token punctuation">"</span></span> <span class="token punctuation">/></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>javaTypeResolver</span><span class="token punctuation">></span></span> <span class="token comment"><!-- targetPackage：生成的实体类所在的包 --></span> <span class="token comment"><!-- targetProject：生成的实体类所在的硬盘位置 --></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>javaModelGenerator</span> <span class="token attr-name">targetPackage</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>mybatis.generator.model<span class="token punctuation">"</span></span> <span class="token attr-name">targetProject</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>.\src\main\java<span class="token punctuation">"</span></span><span class="token punctuation">></span></span> <span class="token comment"><!-- 是否允许子包 --></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>property</span> <span class="token attr-name">name</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>enableSubPackages<span class="token punctuation">"</span></span> <span class="token attr-name">value</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>false<span class="token punctuation">"</span></span> <span class="token punctuation">/></span></span> <span class="token comment"><!-- 是否清理从数据库中查询出的字符串左右两边的空白字符 --></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>property</span> <span class="token attr-name">name</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>trimStrings<span class="token punctuation">"</span></span> <span class="token attr-name">value</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>true<span class="token punctuation">"</span></span> <span class="token punctuation">/></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>javaModelGenerator</span><span class="token punctuation">></span></span> <span class="token comment"><!-- targetPackage 和 targetProject：生成的 mapper.xml 文件的包和位置 --></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>sqlMapGenerator</span> <span class="token attr-name">targetPackage</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>mybatis.generator.mappers<span class="token punctuation">"</span></span> <span class="token attr-name">targetProject</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>.\src\main\resources<span class="token punctuation">"</span></span><span class="token punctuation">></span></span> <span class="token comment"><!-- 针对数据库的一个配置，是否把 schema 作为字包名 --></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>property</span> <span class="token attr-name">name</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>enableSubPackages<span class="token punctuation">"</span></span> <span class="token attr-name">value</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>false<span class="token punctuation">"</span></span> <span class="token punctuation">/></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>sqlMapGenerator</span><span class="token punctuation">></span></span> <span class="token comment"><!-- targetPackage 和 targetProject：生成的 mapper接口文件的包和位置 --></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>javaClientGenerator</span> <span class="token attr-name">type</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>XMLMAPPER<span class="token punctuation">"</span></span> <span class="token attr-name">targetPackage</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>mybatis.generator.dao<span class="token punctuation">"</span></span> <span class="token attr-name">targetProject</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>.\src\main\java<span class="token punctuation">"</span></span><span class="token punctuation">></span></span> <span class="token comment"><!-- 针对 oracle 数据库的一个配置，是否把 schema 作为子包名 --></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>property</span> <span class="token attr-name">name</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>enableSubPackages<span class="token punctuation">"</span></span> <span class="token attr-name">value</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>false<span class="token punctuation">"</span></span> <span class="token punctuation">/></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>javaClientGenerator</span><span class="token punctuation">></span></span> <span class="token comment"><!-- 这里指定要生成的表 --></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>table</span> <span class="token attr-name">tableName</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>student<span class="token punctuation">"</span></span><span class="token punctuation">/></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>table</span> <span class="token attr-name">tableName</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>product<span class="token punctuation">"</span></span><span class="token punctuation">/></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>context</span><span class="token punctuation">></span></span></generatorConfiguration>
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双击执行mybatis-generator的maven插件
执行日志如下
生成的文件如下
能看到mybatis-generator除了给我们生成了基本的PO类（上图的Student和Product），还额外生成了Example类。Example类是为了方便执行SQL时传递查询条件的。使用的示例如下
public class GeneratorTest {<span class="token keyword">private</span> SqlSessionFactory sqlSessionFactory<span class="token punctuation">;</span> <span class="token annotation punctuation">@Before</span> <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">init</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{<!-- --></span> InputStream resourceAsStream <span class="token operator">=</span> Resources<span class="token punctuation">.</span><span class="token function">getResourceAsStream</span><span class="token punctuation">(</span><span class="token string">"mysql8-config.xml"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> sqlSessionFactory <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">SqlSessionFactoryBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">build</span><span class="token punctuation">(</span>resourceAsStream<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token annotation punctuation">@Test</span> <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span> SqlSession sqlSession <span class="token operator">=</span> sqlSessionFactory<span class="token punctuation">.</span><span class="token function">openSession</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> StudentMapper mapper <span class="token operator">=</span> sqlSession<span class="token punctuation">.</span><span class="token function">getMapper</span><span class="token punctuation">(</span>StudentMapper<span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span> StudentExample example <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">StudentExample</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> StudentExample<span class="token punctuation">.</span>Criteria criteria <span class="token operator">=</span> example<span class="token punctuation">.</span><span class="token function">createCriteria</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> criteria<span class="token punctuation">.</span><span class="token function">andNameLike</span><span class="token punctuation">(</span><span class="token string">"%o%"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> List<span class="token generics function"><span class="token punctuation"><</span>Student<span class="token punctuation">></span></span> students <span class="token operator">=</span> mapper<span class="token punctuation">.</span><span class="token function">selectByExample</span><span class="token punctuation">(</span>example<span class="token punctuation">)</span><span class="token punctuation">;</span> students<span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span>System<span class="token punctuation">.</span>out<span class="token operator">:</span><span class="token operator">:</span>println<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>}
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结果如下
PageHelper分页插件
使用该插件，快速实现查询结果的分页，使用步骤如下
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pom.xml中配置依赖
<dependency> <groupId>com.github.pagehelper</groupId> <artifactId>pagehelper</artifactId> <version>5.1.6</version> </dependency>- 1
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mybatis全局配置文件中配置
<plugin>标签<?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> <properties resource="properties/xx.properties"></properties><span class="token tag"><span class="token tag"><span class="token punctuation"><</span>plugins</span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>plugin</span> <span class="token attr-name">interceptor</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>com.github.pagehelper.PageInterceptor<span class="token punctuation">"</span></span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>property</span> <span class="token attr-name">name</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>helperDialect<span class="token punctuation">"</span></span> <span class="token attr-name">value</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>mysql<span class="token punctuation">"</span></span><span class="token punctuation">/></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>plugin</span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>plugins</span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>environments</span> <span class="token attr-name">default</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>development<span class="token punctuation">"</span></span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>environment</span> <span class="token attr-name">id</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>development<span class="token punctuation">"</span></span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>transactionManager</span> <span class="token attr-name">type</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>JDBC<span class="token punctuation">"</span></span><span class="token punctuation">/></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>dataSource</span> <span class="token attr-name">type</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>POOLED<span class="token punctuation">"</span></span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>property</span> <span class="token attr-name">name</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>driver<span class="token punctuation">"</span></span> <span class="token attr-name">value</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>${db.driver}<span class="token punctuation">"</span></span><span class="token punctuation">/></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>property</span> <span class="token attr-name">name</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>url<span class="token punctuation">"</span></span> <span class="token attr-name">value</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>${db.url}<span class="token punctuation">"</span></span><span class="token punctuation">/></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>property</span> <span class="token attr-name">name</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>username<span class="token punctuation">"</span></span> <span class="token attr-name">value</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>${db.user}<span class="token punctuation">"</span></span><span class="token punctuation">/></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>property</span> <span class="token attr-name">name</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>password<span class="token punctuation">"</span></span> <span class="token attr-name">value</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>${db.password}<span class="token punctuation">"</span></span><span class="token punctuation">/></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>dataSource</span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>environment</span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>environments</span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>mappers</span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>package</span> <span class="token attr-name">name</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>mybatis.generator.dao<span class="token punctuation">"</span></span><span class="token punctuation">/></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>mappers</span><span class="token punctuation">></span></span>
</configuration>
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在执行查询之前，先设置分页信息
// 查询第一页,每页5条信息 PageHelper.startPage(1,3);- 1
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先看一下查所有数据
@Test public void test() { SqlSession sqlSession = sqlSessionFactory.openSession(); ProductMapper mapper = sqlSession.getMapper(ProductMapper.class); //PageHelper.startPage(1,3); List<Product> products = mapper.selectByExample(new ProductExample()); products.forEach(System.out::println); }- 1
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加上PageHelper分页
@Test public void test() { SqlSession sqlSession = sqlSessionFactory.openSession(); ProductMapper mapper = sqlSession.getMapper(ProductMapper.class); PageHelper.startPage(1,3); List<Product> products = mapper.selectByExample(new ProductExample()); products.forEach(System.out::println); }- 1
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PageHelper插件的源码分析可以查看我之前的文章 =>
极简PageHelper源码分析 Mybatis Plus
mybatis虽然非常方便，但也需要编写大量的SQL语句，于是mybatis plus就应运而生了。它是一个mybatis增强工具，为了简化开发，提高效率。搭配Spring-Boot食用简直不要太爽。
可以参考mybatis-plus官网,以及慕课网的入门教程和进阶教程
（完）
文章目录
Mybatis
概述
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mybatis是什么？有什么特点？
它是一款半自动的ORM持久层框架，具有较高的SQL灵活性，支持高级映射(一对一，一对多)，动态SQL，延迟加载和缓存等特性，但它的数据库无关性较低
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什么是ORM？
Object Relation Mapping，对象关系映射。对象指的是Java对象，关系指的是数据库中的关系模型，对象关系映射，指的就是在Java对象和数据库的关系模型之间建立一种对应关系，比如用一个Java的Student类，去对应数据库中的一张student表，类中的属性和表中的列一一对应。Student类就对应student表，一个Student对象就对应student表中的一行数据
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为什么mybatis是半自动的ORM框架？
用mybatis进行开发，需要手动编写SQL语句。而全自动的ORM框架，如hibernate，则不需要编写SQL语句。用hibernate开发，只需要定义好ORM映射关系，就可以直接进行CRUD操作了。由于mybatis需要手写SQL语句，所以它有较高的灵活性，可以根据需要，自由地对SQL进行定制，也因为要手写SQL，当要切换数据库时，SQL语句可能就要重写，因为不同的数据库有不同的方言(Dialect)，所以mybatis的数据库无关性低。虽然mybatis需要手写SQL，但相比JDBC，它提供了输入映射和输出映射，可以很方便地进行SQL参数设置，以及结果集封装。并且还提供了关联查询和动态SQL等功能，极大地提升了开发的效率。并且它的学习成本也比hibernate低很多
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快速入门
只需要通过如下几个步骤，即可用mybatis快速进行持久层的开发
- 编写全局配置文件
- 编写mapper映射文件
- 加载全局配置文件，生成SqlSessionFactory
- 创建SqlSession，调用mapper映射文件中的SQL语句来执行CRUD操作
原生开发示例
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在本地虚拟机mysql上创建一个库yogurt，并在里面创建一张student表
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打开IDEA，创建一个maven项目
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导入依赖的jar包
<dependencies> <dependency> <groupId>mysql</groupId> <artifactId>mysql-connector-java</artifactId> <version>5.1.10</version> </dependency> <dependency> <groupId>org.mybatis</groupId> <artifactId>mybatis</artifactId> <version>3.4.6</version> </dependency> <dependency> <groupId>org.projectlombok</groupId> <artifactId>lombok</artifactId> <version>1.18.12</version> <scope>provided</scope> </dependency> <dependency> <groupId>junit</groupId> <artifactId>junit</artifactId> <version>4.10</version> <scope>test</scope> </dependency> </dependencies>- 1
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创建一个po类
package com.yogurt.po;