MyBatis 的 <foreach> 标签用来迭代集合或数组。为了正确使用 <foreach> 标签，我们先看看标签的 DTD 定义，如下：
<!ELEMENT foreach (#PCDATA | include | trim | where | set | foreach | choose | if | bind)*> <!ATTLIST foreach collection CDATA #REQUIRED item CDATA #IMPLIED index CDATA #IMPLIED open CDATA #IMPLIED close CDATA #IMPLIED separator CDATA #IMPLIED >
<foreach> 标签中允许出现 <include>、<trim>、<where>、<set>、<foreach>、<choose>、<if>、<bind> 等标签。支持六个属性，分别如下：
collection：指定待迭代的集合名称，通常是 @Param("collectionName") 中指定的 collectionName；也可以是 bean.collectionName。
item：迭代集合过程中，当前迭代的值
index：迭代索引，从 0 开始
open：将指定的值添加到 <foreach> 标签拼接内容的前面，如：in (1,2,3,4,5) 中的 “(” 符号
close：将指定的值追加到 <foreach> 标签拼接内容的后面，如：in (1,2,3,4,5) 中的 “)” 符号
separator：指定两个元素之前使用什么字符进行分割，如：in (1,2,3,4,5) 中的 “,”
下面通过示例介绍怎样使用 <foreach> 标签构建 IN 条件（样例：select * from users where user_id in (1,2,3,4,5)），比如：
<select id="selectPostIn" resultType="domain.blog.Post">
SELECT * FROM POST P
WHERE ID in
<foreach item="item" index="index" collection="list"
open="(" separator="," close=")">
#{item}
</foreach>
</select><foreach> 标签是非常强大的，它允许你指定一个集合，声明集合项和索引变量，它们可以用在元素体内。它也允许你指定开放和关闭的字符串，在迭代之间放置分隔符。这个标签是很智能的，它不会偶然地附加多余的分隔符。
注意：你可以传递一个 List 实例或者数组作为参数对象传给 MyBatis。当你这么做的时候， MyBatis 会自动将它包装在一个 Map 中，用名称作为键。 List 实例将会以 “list” 作为键，而数组实例将会以 “array” 作为键。
（1）MyBatis 配置文件 mybatis-cfg.xml 内容如下：
<?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>
<properties resource="database.properties"/>
<environments default="MySqlDatabase" >
<environment id="MySqlDatabase" >
<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>
<mapper resource="com/hxstrive/mybatis/dynamic_sql/demo6/UserMapper.xml" />
</mappers>
</configuration>（2）实体 UserBean.java 代码如下：
package com.hxstrive.mybatis.dynamic_sql.demo6;
public class UserBean {
private Integer userId;
private String name;
private String sex;
private Integer age;
// 忽略 getter 和 setter 方法
@Override
public String toString() {
return "UserBean{" +
"userId=" + userId +
", name='" + name + '\'' +
", sex='" + sex + '\'' +
", age=" + age +
'}';
}
}（3）Mapper 定义如下
a、UserMapper.java
package com.hxstrive.mybatis.dynamic_sql.demo6;
import org.apache.ibatis.annotations.Param;
import java.util.List;
public interface UserMapper {
List<UserBean> getUserList(@Param("ids") List<Integer> userIdList);
}b、UserMapper.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.hxstrive.mybatis.dynamic_sql.demo6.UserMapper">
<!-- 映射结果 -->
<resultMap id="RESULT_MAP" type="com.hxstrive.mybatis.dynamic_sql.demo6.UserBean">
<id column="user_id" jdbcType="INTEGER" property="userId" />
<result column="name" jdbcType="VARCHAR" property="name" />
<result column="sex" jdbcType="VARCHAR" property="sex" />
<result column="age" jdbcType="INTEGER" property="age" />
</resultMap>
<select id="getUserList" resultMap="RESULT_MAP">
select `user_id`, `name`, `age`, `sex` from `user`
<where>
`user_id` in
<foreach collection="ids" item="item" index="index"
open="(" separator="," close=")">
#{item}
</foreach>
</where>
</select>
</mapper>（4）客户端代码如下：
package com.hxstrive.mybatis.dynamic_sql.demo6;
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.InputStream;
import java.util.Arrays;
import java.util.List;
public class DynamicSqlDemo {
public static void main(String[] args) throws Exception {
String cfgName = "com/hxstrive/mybatis/dynamic_sql/demo6/mybatis-cfg.xml";
InputStream input = Resources.getResourceAsStream(cfgName);
SqlSessionFactoryBuilder factoryBuilder = new SqlSessionFactoryBuilder();
SqlSessionFactory sqlFactory = factoryBuilder.build(input);
System.out.println("\n\n示例1：");
getUserList(sqlFactory);
}
private static void getUserList(SqlSessionFactory sqlFactory) {
SqlSession sqlSession = sqlFactory.openSession(true);
UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
List<UserBean> userBeanList = userMapper.getUserList(
Arrays.asList(new Integer[]{1, 3}));
for(UserBean userBean : userBeanList) {
System.out.println(userBean);
}
}
}运行客户端代码，将输出如下结果：
2020-09-11 12:49:58,800 DEBUG [org.apache.ibatis.logging.LogFactory] - Logging initialized using 'class org.apache.ibatis.logging.slf4j.Slf4jImpl' adapter.
2020-09-11 12:49:58,840 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - PooledDataSource forcefully closed/removed all connections.
2020-09-11 12:49:58,841 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - PooledDataSource forcefully closed/removed all connections.
2020-09-11 12:49:58,841 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - PooledDataSource forcefully closed/removed all connections.
2020-09-11 12:49:58,841 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - PooledDataSource forcefully closed/removed all connections.
示例1：
2020-09-11 12:49:59,105 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Opening JDBC Connection
2020-09-11 12:49:59,619 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - Created connection 710239027.
2020-09-11 12:49:59,623 DEBUG [com.hxstrive.mybatis.dynamic_sql.demo6.UserMapper.getUserList] - ooo Using Connection [com.mysql.jdbc.JDBC4Connection@2a556333]
2020-09-11 12:49:59,624 DEBUG [com.hxstrive.mybatis.dynamic_sql.demo6.UserMapper.getUserList] - ==> Preparing: select `user_id`, `name`, `age`, `sex` from `user` WHERE `user_id` in ( ? , ? )
2020-09-11 12:49:59,677 DEBUG [com.hxstrive.mybatis.dynamic_sql.demo6.UserMapper.getUserList] - ==> Parameters: 1(Integer), 3(Integer)
2020-09-11 12:49:59,785 DEBUG [com.hxstrive.mybatis.dynamic_sql.demo6.UserMapper.getUserList] - <== Total: 2
UserBean{userId=1, name='赫仑', sex='男', age=27}
UserBean{userId=3, name='叶星云', sex='女', age=31}上面示例中，将执行如下 SQL：
select `user_id`, `name`, `age`, `sex` from `user` WHERE `user_id` in ( ? , ? )
该 SQL 语句将返回两条数据。
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