1.介紹

foreach

動態 SQL 的另外一個常用的必要操作是需要對一個集合進行遍歷，通常是在構建 IN 條件語句的時候。比如：

<selectid="selectPostIn"resultType="domain.blog.Post">
  SELECT *
  FROM POST P
  WHERE ID in
  <foreachitem="item"index="index"collection="list"open="("separator=","close
 
=")">
        #{item}
  </foreach></select>

foreach 元素的功能是非常強大的，它允許你指定一個集合，宣告可以用在元素體內的集合項和索引變數。它也允許你指定開閉匹配的字串以及在迭代中間放置分隔符。這個元素是很智慧的，因此它不會偶然地附加多餘的分隔符。

注意 你可以將任何可迭代物件（如列表、集合等）和任何的字典或者陣列物件傳遞給foreach作為集合引數。當使用可迭代物件或者陣列時，index是當前迭代的次數，item的值是本次迭代獲取的元素。當使用字典（或者Map.Entry物件的集合）時，index是鍵，item是值。

到此我們已經完成了涉及 XML 配置檔案和 XML 對映檔案的討論。下一部分將詳細探討 Java API，這樣才能從已建立的對映中獲取最大利益。

2.用法

3.測試例子

參考：https://www.cnblogs.com/fangyu19900812/p/6046209.html

mapper：

package com.example.demo.mapper;

import java.util.List;
import java.util.Map;

import com.example.demo.entity.UserEntity;

public interface UserMapper {
	List<UserEntity> getAll();
	
	void insert(UserEntity user);
	
	List<UserEntity> testForeachOne(List<Integer> ids);
	
	List<UserEntity> testForeachTwo(int[] ids);
	
	List<UserEntity> testForeachThree(Map<String,Object> map);
}
 

測試類：

package com.example.demo;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;

import com.example.demo.entity.UserEntity;
import com.example.demo.mapper.UserMapper;

@RunWith(SpringRunner.class)
@SpringBootTest
public class TestForeach {
	
	@Autowired
	UserMapper mapper;
	
	@Test
	public void testAll() {
		List<UserEntity> list = mapper.getAll();
		for(UserEntity user:list) {
			System.out.print(user.toString());
		}
	}
	
	
	@Test
	public void testForeachOne() throws Exception{
		List ids = new ArrayList();
		ids.add(37);
		ids.add(38);
		ids.add(39);
		List<UserEntity> users = mapper.testForeachOne(ids);
		for(UserEntity user:users) {
			System.out.println(user);
		}
	}
	
	@Test
	public void testForeachTwo() throws Exception{
		int[] ids = new int[] {37,38,39};
		List<UserEntity> users = mapper.testForeachTwo(ids);
		for(UserEntity user:users) {
			System.out.println(user);
		}
	}
	
	@Test
	public void testForeachThree() throws Exception{
		List ids = new ArrayList();
		ids.add(37);
		ids.add(38);
		ids.add(39);
		Map map = new HashMap();
		map.put("ids", ids);
		List<UserEntity> users = mapper.testForeachThree(map);
		for(UserEntity user:users) {
			System.out.println(user);
		}
	}
}

mapper.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.demo.mapper.UserMapper">
	<resultMap id="BaseResultMap" type="com.example.demo.entity.UserEntity">
		<id column="id" property="id" jdbcType="BIGINT" />
		<result column="userName" property="userName" jdbcType="VARCHAR" />
		<result column="passWord" property="password" jdbcType="VARCHAR" />
		<result column="user_sex" property="userSex"
			javaType="com.example.demo.enums.UserSexEnum" />
		<result column="nick_name" property="nickName" jdbcType="VARCHAR" />
	</resultMap>

	<sql id="Base_Column_List"> 
		id, userName, passWord, user_sex, nick_name
	</sql>

	<select id="getAll" resultMap="BaseResultMap">
		SELECT
		<include refid="Base_Column_List" />
		FROM users
	</select>

	
	<!-- 測試foreach  list -->
	<select id="testForeachOne" parameterType="java.util.List"
		resultMap="BaseResultMap">
		select * from users where id in
		<foreach collection="list" index="index" item="item" open="("
			separator="," close=")">
			#{item}
		</foreach>
	</select>
	
	<!-- 測試foreach  array -->
	<select id="testForeachTwo" parameterType="java.util.List"
		resultMap="BaseResultMap">
		select * from users where id in
		<foreach collection="array" index="index" item="item" open="("
			separator="," close=")">
			#{item}
		</foreach>
	</select>
	
	<!-- 測試foreach  map -->
	<select id="testForeachThree" parameterType="java.util.HashMap"
		resultMap="BaseResultMap">
		select * from users where id in
		<foreach collection="ids" index="index" item="item" open="("
			separator="," close=")">
			#{item}
		</foreach>
	</select>
</mapper>

資料庫：

DROP TABLE IF EXISTS `users`;
CREATE TABLE `users` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT COMMENT '主鍵id',
  `userName` varchar(32) DEFAULT NULL COMMENT '使用者名稱',
  `passWord` varchar(32) DEFAULT NULL COMMENT '密碼',
  `user_sex` varchar(32) DEFAULT NULL,
  `nick_name` varchar(32) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=28 DEFAULT CHARSET=utf8;

注意事項：

問題一：

Caused by: org.apache.ibatis.type.TypeException: Could not resolve type alias 'BaseResultMap'.  Cause: java.lang.ClassNotFoundException: Cannot find class: BaseResultMap
	at org.apache.ibatis.type.TypeAliasRegistry.resolveAlias(TypeAliasRegistry.java:120) ~[mybatis-3.4.0.jar:3.4.0]
	at org.apache.ibatis.builder.BaseBuilder.resolveAlias(BaseBuilder.java:149) ~[mybatis-3.4.0.jar:3.4.0]
	at org.apache.ibatis.builder.BaseBuilder.resolveClass(BaseBuilder.java:116) ~[mybatis-3.4.0.jar:3.4.0]
	... 73 common frames omitted
Caused by: java.lang.ClassNotFoundException: Cannot find class: BaseResultMap
	at org.apache.ibatis.io.ClassLoaderWrapper.classForName(ClassLoaderWrapper.java:200) ~[mybatis-3.4.0.jar:3.4.0]
	at org.apache.ibatis.io.ClassLoaderWrapper.classForName(ClassLoaderWrapper.java:89) ~[mybatis-3.4.0.jar:3.4.0]
	at org.apache.ibatis.io.Resources.classForName(Resources.java:261) ~[mybatis-3.4.0.jar:3.4.0]
	at org.apache.ibatis.type.TypeAliasRegistry.resolveAlias(TypeAliasRegistry.java:116) ~[mybatis-3.4.0.jar:3.4.0]
	... 75 common frames omitted

問題二：

原始碼：