MyBatis 中 ${}和 #{}的正确使用方法(千万不要乱用)

所属分类：软件编程 / java 阅读数： 35

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1、#{}是预编译处理，MyBatis在处理#{ }时，它会将sql中的#{ }替换为？，然后调用PreparedStatement的set方法来赋值，传入字符串后，会在值两边加上单引号，如上面的值 “4,44,514”就会变成“ ‘4,44,514' ”；

2、是字符串替换，在处理是字符串替换，MyBatis在处理时,它会将sql中的{}是字符串替换，在处理{ }是字符串替换， MyBatis在处理{ }时,它会将sql中的是字符串替换，在处理是字符串替换，MyBatis在处理时,它会将sql中的{ }替换为变量的值，传入的数据不会加两边加上单引号。

注意：使用${ }会导致sql注入，不利于系统的安全性！SQL注入：就是通过把SQL命令插入到Web表单提交或输入域名或页面请求的查询字符串，最终达到欺骗服务器执行恶意的SQL命令。常见的有匿名登录（在登录框输入恶意的字符串）、借助异常获取数据库信息等

package com.xiaobu.mapper;

import com.xiaobu.base.mapper.MyMapper;
import com.xiaobu.entity.Country;

import java.util.List;

/**
 * @author xiaobu
 * @version JDK1.8.0_171
 * @date on 2018/11/27 19:21
 * @description V1.0
 */
public interface CountryMapper extends MyMapper<Country> {
 /**
  * 功能描述:通过#{}来进行查询
  *
  * @param ids id
  * @return java.util.List<com.xiaobu.entity.Country>
  * @author xiaobu
  * @date 2019/7/26 11:53
  * @version 1.0
  */
 List<Country> findList(String ids);

 /**
  * 功能描述:通过${}来进行查询
  *
  * @param ids id
  * @return java.util.List<com.xiaobu.entity.Country>
  * @author xiaobu
  * @date 2019/7/26 11:53
  * @version 1.0
  */
 List<Country> findList2(String ids);

 /**
  * 功能描述: 通过foreach来进行查询
  *
  * @param ids id
  * @return java.util.List<com.xiaobu.entity.Country>
  * @author xiaobu
  * @date 2019/7/26 11:53
  * @version 1.0
  */
 List<Country> findListByForEach(List<Integer> ids);
}

<?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.xiaobu.mapper.CountryMapper">

 <select id="findList" resultType="com.xiaobu.entity.Country">
  select * from country where id in (#{ids} )
 </select>


 <select id="findList2" resultType="com.xiaobu.entity.Country">
  select * from country where id in (${ids} )
 </select>
 
 <select id="findListByForEach" parameterType="List" resultType="com.xiaobu.entity.Country">
  select * from country where id in
  <foreach collection="list" index="index" item="item" open="(" separator="," close=")">
   #{item}
  </foreach>
 </select>
</mapper>

@Test
 public void countTotal(){
  //统计总数 SELECT COUNT(Id) FROM country
  Example example = new Example(City.class);
  int count =countryMapper.selectCountByExample(example);
  System.out.println("count = " + count);

  //按条件查询 SELECT COUNT(Id) FROM country
  Country country = new Country();
  //country.setCountryname("1234");
  int conunt2 = countryMapper.selectCount(country);
  System.out.println("conunt2 = " + conunt2);
 }

 @Test
 public void findList(){
  //Preparing: select * from country where id in ( '1,2,3')
  List<Country> countries = countryMapper.findList("1,2,3");
  //countries = [Country(countryname=Angola, countrycode=AO)]
  System.out.println("countries = " + countries);
  //报错 There is no getter for property named 'ids' in 'class java.lang.String
  List<Country> countries2 = countryMapper.findList2("1,2,3");
  System.out.println("countries2 = " + countries2);
 }

 @Test
 public void findListByForeach(){
  //Preparing: select * from country where id in ( ? , ? , ? )
  //Parameters: 1(Integer), 2(Integer), 3(Integer)
  List<Integer> list = new ArrayList<>(3);
  list.add(1);
  list.add(2);
  list.add(3);
  List<Country> countries2 = countryMapper.findListByForEach(list);
  System.out.println("countries2 = " + countries2);
 }