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source/_posts/Mybatis基本介绍.md
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# Mybatis
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***
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## Maven引入Mybatis
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版本号最好去Maven Repository中查找
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```xml
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<properties>
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<mybatis.version>3.5.7</mybatis.version>
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</properties>
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<dependencies>
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<dependency>
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<groupId>org.mybatis</groupId>
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<artifactId>mybatis</artifactId>
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<version>${mybatis.version}</version>
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</dependency>
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</dependencies>
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```
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## Mybatis配置文件 *mybatis-config.xml*
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```xml
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<?xml version="1.0" encoding="UTF-8" ?>
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<!DOCTYPE configuration
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PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
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"http://mybatis.org/dtd/mybatis-3-config.dtd">
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<configuration>
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<typeAliases>
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<!--<typeAlias type="com.crx.entity.User" alias="user"></typeAlias>-->
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<!-- 默认值就是类型首字母小写 -->
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<package name="com.wyd.mybatis20210702.entity"/>
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</typeAliases>
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<environments default="development">
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<environment id="development">
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<transactionManager type="JDBC"></transactionManager>
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<dataSource type="POOLED">
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<property name="driver" value="com.mysql.jdbc.Driver"/>
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<property name="url" value="jdbc:mysql://localhost:3306/ssm?useSSL=false&useUnicode=true&characterEncoding=utf8"/>
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<property name="username" value="root"/>
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<property name="password" value="123456"/>
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</dataSource>
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</environment>
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</environments>
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<!-- 注册mapper映射文件 -->
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<mappers>
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<mapper resource="mapper/UserMapper.xml"></mapper>
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<!-- 注册接口 -->
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<!--<mapper class="com.wyd.dao.UserMapper2"></mapper>-->
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</mappers>
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</configuration>
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```
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## Springboot中 *application.yml*
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```yaml
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spring:
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datasource:
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driver-class-name: com.mysql.cj.jdbc.Driver
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url: jdbc:mysql://localhost:3306/ssm?useSSL=false&useUnicode=true&characterEncoding=utf8&serverTimezone=Asia/Shanghai
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username: root
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password: 123456
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mybatis:
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type-aliases-package: com.wyd.mybatis20210702.entity
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mapper-locations: classpath*:/mapper/*.xml
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```
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## Mybatis interface *UserMapper*
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```java
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package com.wyd.mybatis20210702.dao;
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import com.wyd.mybatis20210702.entity.User;
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public interface UserMapper {
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User selectUserById(Integer id);
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int insertUser(User user);
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int updateUser(User user);
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int deleteUser(Integer id);
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}
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```
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每个Mapper接口都有对应的xml映射文件，如果idea安装有MybatisX插件可以单击类名 ALT+ENTER 快捷键即可创建对应xml映射文件
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映射文件夹一般命名为mapper 存放于resources文件夹下
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在Mapper接口每声明一个方法可以通过MybatisX创建对应的映射，但命名需遵循一定规范，否则需自己选择crud
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## Mybatis xml映射文件 ***UserMapper.xml***
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```xml
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<?xml version="1.0" encoding="UTF-8" ?>
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<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
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<mapper namespace="com.wyd.mybatis20210702.dao.UserMapper">
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<select id="selectUserById" resultType="user">
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SELECT id,username,PASSWORD,birthday FROM USER
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</select>
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<insert id="insertUser" parameterType="com.crx.entity.User">
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insert into user (username,password) values (#{username},#{password})
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</insert>
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<update id="updateUser" parameterType="com.crx.entity.User">
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update user set username = #{username},password = #{password},birthday = #{birthday} where id = #{id}
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</update>
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<delete id="deleteUser">
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delete from user where id = #{id}
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</delete>
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</mapper>
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```
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可以发现xml映射文件头跟mybatis-config.xml配置文件头很像
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就有一个单词不同
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一个是config 一个是mapper，这决定了这个xml文件可以写什么语句
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每个参数都接#{}，{}里就写接口中的参数名
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重点来了
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### #{}和${}的区别
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学过jdbc知道有Statement和PreparedStatement
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一个是固定好的sql语句，一个是预处理语句
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PreparedStatement #{}就很像 ? 后面接参数名，这就很好地防止了 Sql Injection 也就是sql'注入
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我这些sql语句都没有写${}就是因为防止sql注入，开发时严谨使用
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1、Statement和PreparedStatement区别是一样
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2、#代表使用的底层是PreparedStatement。占位符的写法，预加载
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3、$代表使用的底层是Statement。字符串的拼接的写法。SQL注入
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那么什么时候使用${}呢？最多的是模糊查询，但是也不推荐这样用
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```xml
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<select id="selectAllUsersByUsername" parameterType="string" resultType="com.crx.entity.User">
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select id,username,password,birthday from user where username like '%${username}%'
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</select>
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```
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为了拼接上参数就必须使用${}来固化sql
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应该在Java代码上来实现模糊查询
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```java
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String username = ;
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if(username != null && !"".equals(username.trim())){
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username = "%" + username + "%";
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}else{
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username = "%%";
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}
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```
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这样就可以避免sql注入
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---
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resultType 就是 返回的数据类型，有点儿Java方法中的return 不过这是返回的类型而不是变量名
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parameterType 就是参数类型，实体类要写包名类名
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而Java基本数据类型，比如int就直接写int就行了，引用数据类型比如Integer，String 要把首字目小写，integer，string，这是因为用到了别名，也可以像实体类一样写java.lang.Integer，java.lang.String
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## Mybatis xml映射文件特性
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动态sql
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应用场景，条件查询
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```xml
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<select id="selectUsers" resultType="com.crx.entity.User">
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select id,username,password,birthday from user
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<!-- if标签相当于多重if，只要条件全都满足，所有满足的sql代码块都会执行 -->
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<where>
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<if test="id != null">
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and id = #{id}
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</if>
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<if test="username != null">
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and username = #{username}
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</if>
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<if test="password != null">
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and password = #{password}
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</if>
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</where>
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</select>
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```
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<where>标签可动态拼接条件，如果where后条件字段都为null，则where以及后面语句都不会拼接
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<if test="">标签则相当于if判断,test等于约束条件，需要注意的是其中只要写字段名即可
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---
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```xml
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<select id="selectUsers" resultType="com.crx.entity.User">
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select id,username,password,birthday from user
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/*
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when,choose,otherwise相当于if...else if....else if
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otherwise相当于else，
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当满足第一个条件时，不再向下执行判断
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*/
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<where>
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<choose>
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<when test="username != null and username != ''">
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and username = #{username}
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</when>
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<when test="password != null and password != ''">
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and password = #{password}
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</when>
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<otherwise>
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and id = #{id}
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</otherwise>
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</choose>
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</where>
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</select>
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```
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<choose>标签有点像Java中的switch语句，其中的<when>标签则向当于case，但这个case中是带有break的，也就是说每满足一个<when test>条件则此<choose>标签后续语句都不会执行，有一点需要说的是<where>中可以有多个<choose>，满足一个<when test>则会跳出这个<choose>标签，继续执行后续语句，如果<when test>都不满足，如果有<otherwise>标签，则会执行，这有点儿像default语句了
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---
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```xml
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<select id="selectUsers" resultType="com.crx.entity.User">
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select id,username,password,birthday from user
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/*
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有时候我们需要去掉一些特殊的SQL语法，比如说and，or，where，
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此时可以使用trim标签，
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*/
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<!--<trim prefix="where" prefixOverrides="and">
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<if test="username != null and username != '' ">
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username = #{username}
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</if>
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<if test="password != null and password != ''">
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and password = #{password}
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</if>
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</trim>-->
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</select>
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```
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这个例子中，<trim>标签中的**prefix**属性会使*where* 元素会动态地在行首插入 where关键字，**prefixOverrides**属性会删掉额外的and（这些and是在使用条件语句给列赋值时引入的）
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如果<if test>都不满足则不会插入where关键字
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---
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Update同样适用动态sql
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```xml
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<update id="updateUser" parameterType="com.crx.entity.User">
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update user
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<set>
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<if test="username != null and username != '' ">
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username = #{username}
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</if>
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<if test="password != null and password != ''">
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,password = #{password}
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</if>
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where id = #{id}
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</set>
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</update>
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```
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这个例子中，*set* 元素会动态地在行首插入 SET 关键字，并会删掉额外的逗号（这些逗号是在使用条件语句给列赋值时引入的）。
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也相当于上面<trim>标签
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```xml
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<trim prefix="SET" suffixOverrides=",">
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<if test="username != null and username != '' ">
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username = #{username}
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</if>
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<if test="password != null and password != ''">
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,password = #{password}
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</if>
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where id = #{id}
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</trim>
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```
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动态 SQL 的另一个常见使用场景是对集合进行遍历
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Mapper接口
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```java
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List<User> selectUserByIds(@Param("ids") List<Integer> ids);
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```
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这里需要注意的是@Param注解必须加上，Mybatis内部解析参数时，会把接口中的方法形参列表当作一个Map
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当参数不是一个时，key是arg0,arg1,...或param1,param2,...
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```xml
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<select id="selectUserByIds" resultType="com.crx.entity.User">
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<include refid="selectUser"></include>
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where id in
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<foreach collection="ids" open="(" close=")" separator="," item="id">
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#{id}
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</foreach>
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</select>
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```
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这里看到<include>标签不要慌，这是引入公共sql语句，很像Java中的引包，多用于重复的sql语句，减少代码量，增加简洁性
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```xml
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<sql id="selectUser">
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select id,username,password,birthday from user
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</sql>
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```
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使用<sql>标签来定义公共sql，其中**id**属性是这个公共sql独有的命名，切忌命名不能重复
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引用就看上面的集合遍历实例
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再继续看集合遍历实例
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先把想要生成的sql语句写上
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select id,username,password,birthday from user where id in(1,2,3,5)
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看到in了吧，这是包含，意思是查询id 是1，2，3，5符合条件的行
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相当于 id = 1 or id =2 or id = 3 or id = 5
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这里用到了<foreach>标签是不是很熟悉，确实很像JavaScript中的forEach，这个标签中**collection**属性就是需要遍历的对象名，
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那么后面的**open**和**close**属性是干啥的，顾名思义，前者相当于前缀suffix，后者就是后缀了presuffix了，相当于这个<foreach>标签最终生成的sql语句前后都有(),**separator**属性每遍历出一个元素就会在元素后加上逗号最后的元素不加，**item**属性就是生成sql语句参数名
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也就是往#{id}中传值
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