< foreach >标签

作用

• 用来进行循环遍历，完成循环条件的查询，批量删除，批量增加，批量更新

用法

• 包括循环查询 + 批量删除 + 批量增加 + 批量更新的用法

UsersMapper.java

package com.example.mapper;

import com.example.pojo.User;

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 {


    //循环查询
    List<User> getByIds(Integer []id_array);

    //批量删除
    int deleteByIds(Integer []id_array);

    //批量插入
    int insertBatch(List<User> users);
    
    //批量更新
    int updateBatch(List<User> users);
}

UsersMapper.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="com.example.mapper.UsersMapper">

    <!--
        //循环查询
        List<User> getByIds(Integer []id_array);
    -->
    <select id="getByIds" resultType="user">
        select
            <include refid="allColumns"/>
        from
            users
        where
            id
        in
            <foreach collection="array" item="id" open="(" close=")" separator=",">
                #{id}
            </foreach>
    </select>


    <!--
        //批量删除
        int deleteByIds(Integer []id_array);
    -->
    <delete id="deleteByIds">
        delete from
                   users
        where
            id
        in
        <foreach collection="array" item="id" separator="," open="(" close=")">
            #{id}
        </foreach>
    </delete>
    
    
    <!--
        //批量插入
        int insertBatch(List<User> users);
    -->
    <insert id="insertBatch">
        insert into
            users(username, birthday, sex, address)
        values
            <foreach collection="list" item="user" separator=",">
                (#{user.userName}, #{user.birthday}, #{user.sex}, #{user.address})
            </foreach>
    </insert>
    
    
    <!--
        //批量更新
        int updateBatch(List<User> users);
        private Integer id;
        private String userName;
        private Date birthday;
        private String sex;
        private String address;
    -->
    <update id="updateBatch">
        <foreach collection="list" item="user" separator=";">
            update
                users
            <set>
                <if test="user.userName != null and user.userName != ''">
                    username=#{user.userName},
                </if>

                <if test="user.birthday != null">
                    birthday=#{user.birthday},
                </if>

                <if test="user.sex != null and user.sex != ''">
                    sex=#{user.sex},
                </if>

                <if test="user.address != null and user.address != ''">
                    address=#{user.address},
                </if>
            </set>
            where
                id=#{user.id}
        </foreach>
    </update>
</mapper>

映射文件分析

• 当入参是数组时，parameterType可以不写，

  • 其实只有当入参类型是实体类时，才必须指明其类型，其他类型的数据皆可不写

• < foreach >标签的属性说明

  • collection：指明待遍历的数据容器的类型，可选的有三个：array，list，map
  • item：给遍历出的每个元素指定一个名称，便于在标签内使用
  • open 和 close：原先sql语句，in后面待遍历的数据放在括号中，这里通过标签属性的形式来实现
    • 可以将sql语句和标签的书写分隔开，使得代码更加直观
    • 注意：这是循环外层的括号，用来容纳所有的元素，循环内部的元素自身的括号不能用这对标签，要手动加上
  • 是否要使用 open 和 close标签的说明：

  <!-- 批量删除时使用 --> 
  <foreach collection="array" item="id" separator="," open="(" close=")">
       #{id}
   </foreach>
  
  <!-- 批量插入时未使用 -->
   <foreach collection="list" item="user" separator=",">
       
       <!-- 为元素手动添加括号 -->
       (#{user.userName}, #{user.birthday}, #{user.sex}, #{user.address})		
   </foreach>
  
  
  <!-- 不同选择的依据：
  	看被遍历的元素是否要放在同一个括号内，需要则使用open 和 close标签，否则不用
  	元素自身需要括号，则手动添加
  -->

• separator：遍历出的元素之间用什么符号分隔，mybatis框架会确保间隔符号的正确使用

  • 正确使用是指：在指定需要的间隔符后，间隔符号的个数以及出现在元素之间的位置，由mybatis自动正确放置(如果有多个元素的话)

• 进行批量更新操作时

  • 本质：由底层解析出的sql语句可知，本质执行的是多条独立的update语句
    • 这也决定了< update >标签内应该是一个 < foreach >标签，分隔符应该是";"，用来间隔多条相互独立的update语句
  • 注意：这与执行一条update语句，影响多行记录是不同的，必须在jdbc.properties文件中的url的值后新增配置：allowMultiQueries=true

  jdbc.driverClassName=com.mysql.cj.jdbc.Driver
  jdbc.url=jdbc:mysql://ip:3306/ssm?useUnicode=true&characterEncoding=utf8&allowMultiQueries=true
  jdbc.username=XXXX
  jdbc.password=YYYY

• 不修改jdbc.properties时报错
  

• 各sql标签在底层分别被解析为

//批量查询

==>  Preparing: select id, username, birthday, sex, address from users where id in ( ? , ? , ? )

//批量删除

==>  Preparing: delete from users where id in ( ? , ? )

//批量插入

==>  Preparing: insert into users(username, birthday, sex, address) values (?, ?, ?, ?) , (?, ?, ?, ?) , (?, ?, ?, ?)

//批量更新

==>  Preparing: 
update users SET username=?, birthday=?, sex=?, address=? where id=? ;
update users SET username=?, birthday=?, sex=?, address=? where id=? ; 
update users SET username=?, birthday=?, sex=?, address=? where id=?

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.lang.reflect.Array;

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();
    }

    //循环查询测试
    @Test
    public void testGetByIds(){
        Integer []id_array = {1, 3, 5};
        List<User> users = usersMapper.getByIds(id_array);
        users.forEach(System.out::println);
    }

    //批量删除测试
    @Test
    public void testDeleteByIds(){
        Integer []id_array = {3, 29};
        int num = usersMapper.deleteByIds(id_array);
        if(num == 2){
            System.out.println("批量删除成功!");
            sqlSession.commit();
        }else{
            System.out.println("批量删除失败!");
        }
    }
    
    //批量插入测试
    @Test
    public void testInsertBatch() throws ParseException {
        User u1 = new User("西决", date.parse("2001-01-01"), "男", "北京");
        User u2 = new User("南音", date.parse("2002-02-02"), "女", "北京");
        User u3 = new User("北北", date.parse("2003-03-03"), "男", "北京");

        List<User> users = new ArrayList<>();
        users.add(u1);
        users.add(u2);
        users.add(u3);

        int num = usersMapper.insertBatch(users);
        if(num == 3){
            System.out.println("批量插入成功!");
            sqlSession.commit();
        }else{
            System.out.println("批量插入失败!");
        }
    }
    
    //批量更新测试
    @Test
    public void testUpdateBatch() throws ParseException {
        User u1 = new User(31,"西决2", date.parse("2001-01-01"), "男", "北京");
        User u2 = new User(32,"南音2", date.parse("2002-02-02"), "女", "北京");
        User u3 = new User(33,"北北2", date.parse("2003-03-03"), "男", "北京");

        List<User> users = new ArrayList<>();
        users.add(u1);
        users.add(u2);
        users.add(u3);

        int num = usersMapper.updateBatch(users);
        if(num == 1){
            System.out.println("批量更新成功!");
            sqlSession.commit();
        }else{
            System.out.println("批量更新失败!");
        }
    }
}

测试代码分析(着重分析一下批量更新)

• 在执行批量更新操作时，为什么数据表的记录明明修改了3条，输出结果中更新结果的返回值却是1呢？
• 更新后的数据表，修改了3条记录

• 批量更新后的返回结果

<==    Updates: 1
批量更新成功!

• 原因：

  • 首先要区别< update >标签和update语句：在 < update >标签中有一个< foreach >标签，他循环了3条相互独立的update语句
  • 注意：每条update语句恰巧都只是修改了一条记录，所以< update >标签返回3次1后结束，因为循环3次后循环标签结束了
  • 对于如下测试代码，num其实被赋值3次，num的值是最后一次赋值的结果，本次测试恰好是：1
    • 其实笔者不太确定num是否被赋值了3，但是根据底层输出的结果可知，起作用的一定是最后一条update语句影响的记录条数
    • 不是循环的3条update语句影响条数的和，因为输出结果：是1，而不是3

   int num = usersMapper.updateBatch(users);

输出结果

//批量查询结果

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 16148478.
Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@f667fe]
==>  Preparing: select id, username, birthday, sex, address from users where id in ( ? , ? , ? )
==> Parameters: 1(Integer), 3(Integer), 5(Integer)
<==    Columns: id, username, birthday, sex, address
<==        Row: 1, 荷包蛋, 2002-08-23, 女, 黑河市
<==        Row: 3, 小张, 1999-02-22, 1, 长沙
<==        Row: 5, 段, 2001-03-10, 1, 太原
<==      Total: 3
Users{id=1, userName='荷包蛋', birthday=Fri Aug 23 00:00:00 CST 2002, sex='女', address='黑河市'}
Users{id=3, userName='小张', birthday=Mon Feb 22 00:00:00 CST 1999, sex='1', address='长沙'}
Users{id=5, userName='段', birthday=Sat Mar 10 00:00:00 CST 2001, sex='1', address='太原'}
Resetting autocommit to true on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@f667fe]
Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@f667fe]
Returned connection 16148478 to pool.

Process finished with exit code 0

//批量删除结果

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 544966217.
Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@207b8649]
==>  Preparing: delete from users where id in ( ? , ? )
==> Parameters: 3(Integer), 29(Integer)
<==    Updates: 2
批量删除成功!
Committing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@207b8649]
Resetting autocommit to true on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@207b8649]
Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@207b8649]
Returned connection 544966217 to pool.

Process finished with exit code 0

//批量增加结果

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 749604930.
Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@2cae1042]
==>  Preparing: insert into users(username, birthday, sex, address) values (?, ?, ?, ?) , (?, ?, ?, ?) , (?, ?, ?, ?)
==> Parameters: 西决(String), 2001-01-01 00:00:00.0(Timestamp), 男(String), 北京(String), 南音(String), 2002-02-02 00:00:00.0(Timestamp), 女(String), 北京(String), 北北(String), 2003-03-03 00:00:00.0(Timestamp), 男(String), 北京(String)
<==    Updates: 3
批量插入成功!
Committing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@2cae1042]
Resetting autocommit to true on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@2cae1042]
Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@2cae1042]
Returned connection 749604930 to pool.

Process finished with exit code 0

//批量更新结果

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 1718322084.
Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@666b83a4]
==>  Preparing: update users SET username=?, birthday=?, sex=?, address=? where id=? ; update users SET username=?, birthday=?, sex=?, address=? where id=? ; update users SET username=?, birthday=?, sex=?, address=? where id=?
==> Parameters: 西决2(String), 2001-01-01 00:00:00.0(Timestamp), 男(String), 北京(String), 31(Integer), 南音2(String), 2002-02-02 00:00:00.0(Timestamp), 女(String), 北京(String), 32(Integer), 北北2(String), 2003-03-03 00:00:00.0(Timestamp), 男(String), 北京(String), 33(Integer)
<==    Updates: 1
批量更新成功!
Committing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@666b83a4]
Resetting autocommit to true on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@666b83a4]
Closing JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@666b83a4]
Returned connection 1718322084 to pool.

Process finished with exit code 0