"#{}"占位符

作用

• 传参大部分使用"#{}"，在数据库底层使用的是：PreparedStatement预编译处理对象
• 数据库底层被解析为"?"，用来传值，是安全的数据库访问，可以防止sql注入
• 通过在SqlMapConfig.xml添加日志输出配置，在后文测试输出的结果中可以验证

<?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>

    <!-- 设置日志输出-->
    <settings>
        <setting name="logImpl" value="STDOUT_LOGGING"/>
    </settings>

</configuration>

写法

• "#{}"里参数的写法，要参考parameterType的类型
• 如果parameterType的类型是简单类型(8种基本类型(封装) + String类型)，则"#{}"里变量名称任意

    <!--
        //根据用户主键查取用户信息
        User getById();
    -->

    <select id="getById" resultType="user" parameterType="int">
        select
            id, username, birthday, sex, address
        from
            users
        where
            id=#{asYouLike}
    </select>

• 如果parameterType的类型是实体类的类型，则"#{}"里只能是类中成员变量的名称，而且区分大小写

//User实体类中的属性

public class User {
    private Integer id;
    private String userName;
    private Date birthday;
    private String sex;
    private String address;
}

   <!--
        //向用户表中增加用户信息
        int insert(User user);
   -->

    <insert id="insert" parameterType="user">
        insert into
                users(username, birthday, sex, address)
        values(#{userName}, #{birthday}, #{sex}, #{address})
    </insert>

"${}"占位符

用于字符串的拼接和字符串的替换

字符串拼接

作用

• 一般用于模糊查询，建议少用，因为存在sql注入风险

写法

• "${}"中参数名称的写法，分两种情况，与"#{}"的两种情况相同，可参考之
• 注意：对于parameterType的类型是简单类型时，"${}"里变量名称随便写，但是分版本
  • 如果是3.5.1及以下版本，只能写"value"

模糊查询示例

• 未优化前，存在sql注入风险

    <!--
        //根据用户名模糊查询用户信息
        List<User> getByName(String name);
    -->

    <select id="getByName" parameterType="string" resultType="user">
        select
            id, username, birthday, sex, address
        from
            users
        where username like '%${name}%'
    </select>

• 优化后，使用"#{}"接受传参，底层是preparedStatement预编译对象，可以防止sql注入

    <!--
        //优化后的模糊查询
        List<User> getByNameBetter(String name);
    -->

    <select id="getByNameBetter" parameterType="string" resultType="user">
        select
            id, username, birthday, sex, address
        from
            users
        where username like concat('%', #{name}, '%')
    </select>

字符串替换("${}"的主要作用)

需求：在users表中，根据地址或者用户名模糊查询用户信息

sql语句：

select * from users where username like '%模糊查询条件%'

select * from users where address like '%模糊查询条件%'

存在的问题：两条sql语句的结构在本质上是相同的，写两条语句十分冗余，可以采用替换列名的方式进行优化

UsersMapper.java接口

package com.example.mapper;

import com.example.pojo.User;
import org.apache.ibatis.annotations.Param;

import java.util.List;

/**
 * 数据访问层的接口，定义对数据库完成的CRUD的操作
 */
public interface UsersMapper {
    
    //根据用户名或者地址模糊查询
    List<User> getByNameOrAddress(
            @Param("colName")
            String colName,
            @Param("userName")
            String userName
    );
}

• 接口分析：当接口中的方法的参数有多个时，用注解标识参数，sql标签可通过注解中声明的参数名获取参数

UsersMapper.xml文件

<?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.example.mapper.UsersMapper">

    <!--
        //根据用户名或者地址模糊查询
        List<User> getByNameOrAddress(
                @Param("colName")
                String colName,
                @Param("userName")
                String userName);
    -->
    
    <select id="getByNameOrAddress" resultType="user">
        select
            id, username, birthday, sex, address
        from
            users
        where
            ${colName} like concat('%', #{userName}, '%')
    </select>

</mapper>

• sql标签分析
  • 当标签对应的接口中的方法有多个参数时，标签中的入参类型，即：parameterType，取消不写，通过方法中注解的参数名称获取参数
  • 用于替换时只可以使用"${}"，应该放在like前。"#{}"用来传值，应该放在like后用来占位传值

测试代码

package com.example.mapper;

import com.example.pojo.User;
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 org.junit.After;
import org.junit.Before;
import org.junit.Test;

import java.io.IOException;
import java.io.InputStream;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.List;

public class TestUsersMapper {

    //时间刷
    SimpleDateFormat date = new SimpleDateFormat("yyyy-MM-dd");

    //SqlSession对象
    SqlSession sqlSession;

    //mybatis动态代理对象
    UsersMapper usersMapper;

    //获取SqlSession
    @Before
    public void getSqlSession() throws IOException {
        //读取核心配置文件
        InputStream in = Resources.getResourceAsStream("SqlMapConfig.xml");
        //创建SqlSessionFactory对象
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in);
        //获取SqlSession
        sqlSession = factory.openSession();
        //获取mybatis动态代理对象
        usersMapper = sqlSession.getMapper(UsersMapper.class);
    }

    //归还SqlSession
    @After
    public void closeSession(){
        sqlSession.close();
    }
    

    @Test
    public void testGetByNameOrAddress(){
        List<User> users = usersMapper.getByNameOrAddress("username", "小");
        //List<User> users = usersMapper.getByNameOrAddress("address", "市");
        users.forEach(System.out::println);
    }
}

测试输出

根据用户名模糊查询

Checking to see if class com.example.mapper.TestUsersMapper matches criteria [is assignable to Object]
Checking to see if class com.example.mapper.UsersMapper matches criteria [is assignable to Object]
Opening JDBC Connection
Created connection 1293462056.
Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@4d18aa28]
    
==>  Preparing: select id, username, birthday, sex, address from users where username like concat('%', ?, '%')
==> Parameters: 小(String)
    
<==    Columns: id, username, birthday, sex, address
<==        Row: 2, 小王, 2001-07-12, 1, 芜湖市
<==        Row: 3, 小张, 1999-02-22, 1, 长沙
<==        Row: 29, 小昕, 2001-03-14, 女, 忻州
<==      Total: 3
Users{id=2, userName='小王', birthday=Thu Jul 12 00:00:00 CST 2001, sex='1', address='芜湖市'}
Users{id=3, userName='小张', birthday=Mon Feb 22 00:00:00 CST 1999, sex='1', address='长沙'}
Users{id=29, userName='小昕', birthday=Wed Mar 14 00:00:00 CST 2001, sex='女', address='忻州'}
Resetting autocommit to true on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@4d18aa28]
Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@4d18aa28]
Returned connection 1293462056 to pool.

Process finished with exit code 0

根据地址模糊查询

Checking to see if class com.example.mapper.TestUsersMapper matches criteria [is assignable to Object]
Checking to see if class com.example.mapper.UsersMapper matches criteria [is assignable to Object]
Opening JDBC Connection
Created connection 1293462056.
Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@4d18aa28]
    
==>  Preparing: select id, username, birthday, sex, address from users where address like concat('%', ?, '%')
==> Parameters: 市(String)
    
<==    Columns: id, username, birthday, sex, address
<==        Row: 2, 小王, 2001-07-12, 1, 芜湖市
<==        Row: 7, 学委, 2001-05-13, 2, 平顶山市
<==      Total: 2
Users{id=2, userName='小王', birthday=Thu Jul 12 00:00:00 CST 2001, sex='1', address='芜湖市'}
Users{id=7, userName='学委', birthday=Sun May 13 00:00:00 CST 2001, sex='2', address='平顶山市'}
Resetting autocommit to true on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@4d18aa28]
Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@4d18aa28]
Returned connection 1293462056 to pool.

Process finished with exit code 0

测试结果分析

• sql标签在底层分别被解析为

==>  Preparing: select id, username, birthday, sex, address from users where username like concat('%', ?, '%')
==> Parameters: 小(String)

==>  Preparing: select id, username, birthday, sex, address from users where address like concat('%', ?, '%')
==> Parameters: 市(String)

• 由上可知
  • 标签中的${colName}分别被替换成了传入的"username"和"address"，起到了替换作用
  • "#{}"被解析成"?"，#{userName}分别拿到值：小(String)和市(String)，起到占位传值作用