MyBatis 动态SQL之＜foreach＞标签

简介

foreach 标签用于循环语句，它很好的支持了数据和 List、set 接口的集合，并对此提供遍历的功能。
对于一些 SQL 语句中含有 in 条件，需要迭代条件集合来生成的情况，可以使用 foreach 来实现 SQL 条件的迭代。

select * 
from TABLE
where (id=1 or id=3 or id=4)

要查询一个表中id为1，3，4的数据，可以把需要查的这些id的数据存放到一个集合中，通过遍历这个集合来查询到这些数据，这种时候我们就用到foreach了。

foreach 标签有六个属性：item，index，collection，open， close，separator

属性	作用
item	表示集合中每一个元素进行迭代时的别名
index	指定一个名字，表示在迭代过程中每次迭代到的位置
open	表示该语句以什么开始（既然是 in 条件语句，所以必然以’('开始)
close	表示该语句以什么结束（既然是 in 条件语句，所以必然以’)'结束。
separator	表示在每次进行迭代之间以什么符号作为分隔符（既然是 in 条件语句，所以必然以‘,’作为分隔符）。
collection	该属性是必选的，但在不同情况下该属性的值是不一样的，主要有以下 3 种情况: (1)如果传入的是单参数且参数类型是一个 List，collection 属性值为 list (2)如果传入的是单参数且参数类型是一个 array 数组，collection 的属性值为 array (3)如果传入的参数是多个，需要把它们封装成一个 Map，当然单参数也可以封装成 Map。Map 的 key 是参数名，collection 属性值是传入的 List 或 array 对象在自己封装的 Map 中的 key。

语法格式

trim 一般用于去除 SQL 语句中多余的 AND 关键字、逗号，或者给 SQL 语句前拼接 where、set 等后缀，可用于选择性插入、更新、删除或者条件查询等操作。trim 语法格式如下。

<foreach item="item" index="index" collection="list|array|map key" open="(" separator="," close=")">
    参数值
foreach>

网络案例

+----+----------------+----------------------------+-----+---------+---------------------+
| id | name           | url                        | age | country | createtime          |
+----+----------------+----------------------------+-----+---------+---------------------+
|  1 | 编程帮         | https://www.biancheng.net/ |  10 | CN      | 2021-02-23 10:20:40 |
|  2 | C语言中文网    | http://c.biancheng.net/    |  12 | CN      | 2021-03-08 11:23:27 |
|  3 | 百度           | https://www.baidu.com/     |  18 | CN      | 2021-03-08 11:23:53 |
|  4 | 淘宝           | https://www.taobao.com/    |  17 | CN      | 2021-03-10 10:33:54 |
|  5 | Google         | https://www.google.com/    |  23 | US      | 2021-03-10 10:34:34 |
|  6 | GitHub         | https://github.com/        |  13 | US      | 2021-03-10 10:34:34 |
|  7 | Stack Overflow | https://stackoverflow.com/ |  16 | US      | 2021-03-10 10:34:34 |
|  8 | Yandex         | http://www.yandex.ru/      |  11 | RU      | 2021-03-10 10:34:34 |
+----+----------------+----------------------------+-----+---------+---------------------+

<select id="selectWebsite"
    parameterType="net.biancheng.po.Website"
    resultType="net.biancheng.po.Website">
    SELECT id,name,url,age,country
    FROM website WHERE age in
    <foreach item="age" index="index" collection="list" open="("
        separator="," close=")">
        #{age}
    foreach>
select>

基础环境

1.数据库准备

# 创建一个名称为t_customer的表
CREATE TABLE t_customer (
    id int(32) PRIMARY KEY AUTO_INCREMENT,
    username varchar(50),
    jobs varchar(50),
    phone varchar(16)
);
# 插入3条数据
INSERT INTO t_customer VALUES ('1', 'joy', 'teacher', '13733333333');
INSERT INTO t_customer VALUES ('2', 'jack', 'teacher', '13522222222');
INSERT INTO t_customer VALUES ('3', 'tom', 'worker', '15111111111');

2.新建项目或Module

3 pom.xml中添加

<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <parent>
        <artifactId>mybatisartifactId>
        <groupId>com.examplegroupId>
        <version>1.0-SNAPSHOTversion>
    parent>
    <modelVersion>4.0.0modelVersion>

    <groupId>com.biemgroupId>
    <artifactId>dynamaicSqlartifactId>

    <properties>
        <maven.compiler.source>8maven.compiler.source>
        <maven.compiler.target>8maven.compiler.target>
    properties>

    <dependencies>
        
        <dependency>
            <groupId>org.mybatisgroupId>
            <artifactId>mybatisartifactId>
            <version>3.4.6version>
        dependency>
        
        <dependency>
            <groupId>junitgroupId>
            <artifactId>junitartifactId>
            <version>4.12version>
            <scope>testscope>
        dependency>

        
        <dependency>
            <groupId>mysqlgroupId>
            <artifactId>mysql-connector-javaartifactId>
            <version>8.0.18version>
            <scope>runtimescope>
        dependency>

        
        <dependency>
            <groupId>log4jgroupId>
            <artifactId>log4jartifactId>
            <version>1.2.17version>
        dependency>

        <dependency>
            <groupId>org.projectlombokgroupId>
            <artifactId>lombokartifactId>
            <version>1.18.16version>
        dependency>
    dependencies>
project>

4.创建package和文件夹

src/main/java/下创建package
com.biem.pojo
com.biem.mapper
com.biem.util
src/main/resources/下创建文件夹
com/biem/mapper
src/test/java下创建package
com.biem.test

5 框架配置文件

5.1 mybatis核心配置文件mybatis-config.xml

DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    
    <properties resource="jdbc.properties">properties>

    
    <settings>
        <setting name="mapUnderscoreToCamelCase" value="true"/>
    settings>

    
    <typeAliases>
        
        <package name="com.biem.pojo"/>
    typeAliases>

    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driver}"/>
                <property name="url" value="${jdbc.url}"/>
                <property name="username" value="${jdbc.username}"/>
                <property name="password" value="${jdbc.password}"/>
            dataSource>
        environment>

    environments>

    
    <mappers>
        
        <package name="com.biem.mapper"/>
    mappers>


configuration>

5.2 mybatis属性文件jdbc.properties

jdbc.driver=com.mysql.cj.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/mybatis?serverTimezone=UTC
jdbc.username=root
jdbc.password=root

5.3 log4j.xml文件

DOCTYPE log4j:configuration SYSTEM "log4j.dtd">

<log4j:configuration xmlns:log4j="http://jakarta.apache.org/log4j/">

    <appender name="STDOUT" class="org.apache.log4j.ConsoleAppender">
        <param name="Encoding" value="UTF-8"/>
        <layout class="org.apache.log4j.PatternLayout">
            <param name="ConversionPattern" value="%-5p %d{MM-dd HH:mm:ss,SSS} %m (%F:%L) \n"/>
        layout>
    appender>
    <logger name="java.sql">
        <level value="debug"/>
    logger>
    <logger name="org.apache.ibatis">
        <level value="info"/>
    logger>
    <root>
        <level value="debug"/>
        <appender-ref ref="STDOUT"/>
    root>
log4j:configuration>

6 用户配置文件

6.1 实体类

package com.biem.pojo;

import lombok.*;

/**
 * ClassName: Customer
 * Package: com.biem.pojo
 * Description:
 *
 * @Create 2023/4/5 22:17
 * @Version 1.0
 */
@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
@Builder
@ToString
public class Customer {
    private Integer id;
    private String username;
    private String jobs;
    private String phone;
}

需要在pom.xml中引入lombok，简化原来的实体类的代码

6.2 mybatis接口类

package com.biem.mapper;

/**
 * ClassName: CustomerMapper
 * Package: com.biem.mapper
 * Description:
 *
 * @Create 2023/4/5 22:19
 * @Version 1.0
 */
public interface CustomerMapper {
}

6.3 mybatis用户配置文件

DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.biem.mapper.CustomerMapper">
    

    

mapper>

6.4 mybatis工具类

package com.biem.util;

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;

/**
 * ClassName: MybatisUtil
 * Package: com.biem.utils
 * Description:
 *
 * @Create 2023/4/5 22:23
 * @Version 1.0
 */
public class MybatisUtil {
    //利用static(静态)属于类不属于对象,且全局唯一
    private static SqlSessionFactory sqlSessionFactory = null;
    //利用静态块在初始化类时实例化sqlSessionFactory
    static {
        InputStream is= null;
        try {
            is = Resources.getResourceAsStream("mybatis-config.xml");
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(is);
        } catch (IOException e) {
            e.printStackTrace();
            throw new ExceptionInInitializerError(e);
        }
    }

    /**
     * openSession 创建一个新的SqlSession对象
     * @return SqlSession对象
     */
    public static SqlSession openSession(boolean autoCommit){
        return sqlSessionFactory.openSession(autoCommit);
    }

    public static SqlSession openSession(){
        return sqlSessionFactory.openSession();
    }
    /**
     * 释放一个有效的SqlSession对象
     * @param session 准备释放SqlSession对象
     */
    public static void closeSession(SqlSession session){
        if(session != null){
            session.close();
        }
    }
}

项目结构如下

foreach元素迭代数组

1 com.biem.mapper.CustomerMapper.class中添加

public List<Customer> findCustomerByArray(int [] ids);

2 com/biem/mapper/CustomerMapper.xml中添加

    
    <select id="findCustomerByArray" parameterType="java.util.Arrays" resultType="customer">
        select * from t_customer where id in
        <foreach item = "id" index = "index" collection="array" open="(" separator="," close=")">
            #{id}
        foreach>
    select>

3.功能测试 com.biem.test.TestMybatis.java

    @Test
    public void testFindCustomerByArray(){
        // 通过工具类获取SqlSession对象
        SqlSession session = MybatisUtil.openSession();
        int[] ids={1,3};
        CustomerMapper mapper = session.getMapper(CustomerMapper.class);
        List<Customer> customers = mapper.findCustomerByArray(ids);
        System.out.println("customers = " + customers);
        // 关闭SqlSession
        session.close();
    }

foreach元素迭代List

1 com.biem.mapper.CustomerMapper.class中添加

public List<Customer> findCustomerByList(List<Integer> ids);

2 com/biem/mapper/CustomerMapper.xml中添加

    
    <select id="findCustomerByList" parameterType="java.util.List" resultType="customer">
        select * from t_customer where id in
        <foreach item = "id" index = "index" collection="list" open="(" separator="," close=")">
            #{id}
        foreach>
    select>

3.功能测试 com.biem.test.TestMybatis.java

@Test
    public void testFindCustomerByList(){
        // 通过工具类获取SqlSession对象
        SqlSession session = MybatisUtil.openSession();
        List<Integer> ids = new ArrayList<>();
        ids.add(1);
        ids.add(2);
        CustomerMapper mapper = session.getMapper(CustomerMapper.class);
        List<Customer> customers = mapper.findCustomerByList(ids);
        System.out.println("customers = " + customers);
        // 关闭SqlSession
        session.close();
    }

foreach元素迭代Map

1 com.biem.mapper.CustomerMapper.class中添加

public List<Customer> findCustomerByMap(Map<String, Object> map);

2 com/biem/mapper/CustomerMapper.xml中添加

    
    <select id="findCustomerByMap" parameterType="java.util.Map" resultType="customer">
        select * from t_customer where jobs=#{jobs} and id in
        <foreach item = "ids" index = "index" collection="id" open="(" separator="," close=")">
            #{ids}
        foreach>
    select>

解析：使用foreach标签迭代Map集合，由于传入的参数是Map类型，所以在SQL中需要根据key分别获得相应的value值，在以上语句中，#{jobs}获取Map集合中key为jobs的value的值，而collection="id"则获取的是Map集合中key="id"所对应的value的值。

3.功能测试 com.biem.test.TestMybatis.java

    @Test
    public void testFindCustomerByMap(){
        // 通过工具类获取SqlSession对象
        SqlSession session = MybatisUtil.openSession();
        List<Integer> ids = new ArrayList<>();
        ids.add(1);
        ids.add(2);
        ids.add(3);
        Map<String, Object> maps = new HashMap<>();
        maps.put("id", ids);
        maps.put("jobs","teacher");
        CustomerMapper mapper = session.getMapper(CustomerMapper.class);
        List<Customer> customers = mapper.findCustomerByMap(maps);
        System.out.println("customers = " + customers);
        // 关闭SqlSession
        session.close();
    }