mybatis提供了用于数组和集合循环遍历的foreach元素。

1.foreach foreach元素常用属性见下表： |属性|含义| |-|-| |item|集合中元素迭代时的别名，该参数为必选。| |index|在list和数组中,index是元素的序号，在map中，index是元素的key，该参数可选。| |open|foreach代码的开始符号，一般是(和close=")"合用。常用在in(),values()时。该参数可选。| |separator|元素之间的分隔符，例如在in()的时候，separator=","会自动在元素中间用“,“隔开，避免手动输入逗号导致sql错误，如in(1,2,)这样。该参数可选。| |close|foreach代码的关闭符号，一般是)和open="("合用。常用在in(),values()时。该参数可选。| |collection|要做foreach的对象，作为入参时，List对象默认用"list"代替作为键，数组对象有"array"代替作为键，Map对象没有默认的键。当然在作为入参时可以使用@Param("keyName")来设置键，设置keyName后，list,array将会失效。 |

2.foreach迭代数组

映射文件：

<?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.mapper.usersMapper">

    <!--foreach迭代数组-->
    <select id="getUsersByIds" parameterType="java.util.Arrays" resultType="Users">
        select * from users where uid in
        <foreach collection="array" item="uid" index="index" open="(" separator="," close=")">
            #{uid}
        </foreach>
    </select>
</mapper>

测试类：

    @Test
    public void testQueryUsersByIds(){
        SqlSession session = null;
        Integer[] ids = {1,2,4};
        try {
            session = SqlSessionFactoryUtil.openSqlSession();
            List<Users> usersList = session.selectList("getUsersByIds",ids);
            usersList.forEach(System.out::println);
        } catch (Exception ex) {
            ex.printStackTrace();
        } finally {
            if (session != null) {
                session.close();
            }
        }
    }

3.foreach迭代List

映射文件：

<?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.mapper.usersMapper">

   <!--foreach迭代List-->
    <select id="getUsersByIds" parameterType="java.util.List" resultType="Users">
        select * from users where uid in
        <foreach collection="list" item="uid" index="index" open="(" separator="," close=")">
            #{uid}
        </foreach>
    </select>
</mapper>

测试类：

    @Test
    public void testQueryUsersByIds(){
        SqlSession session = null;
        List<Integer> list = new ArrayList<Integer>();
        list.add(1);
        list.add(2);
        list.add(4);
        try {
            session = SqlSessionFactoryUtil.openSqlSession();
            List<Users> usersList = session.selectList("getUsersByIds",list);
            usersList.forEach(System.out::println);
        } catch (Exception ex) {
            ex.printStackTrace();
        } finally {
            if (session != null) {
                session.close();
            }
        }
    }

4.foreach迭代Set

映射文件：

<?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.mapper.usersMapper">

   <!--foreach迭代Set-->
    <select id="getUsersByIds" parameterType="java.util.Set" resultType="Users">
        select * from users where uid in
        <foreach collection="collection" item="uid" index="index" open="(" separator="," close=")">
            #{uid}
        </foreach>
    </select>
</mapper>

测试类：

    @Test
    public void testQueryUsersByIds(){
        SqlSession session = null;
        Set<Integer> set= new HashSet<Integer>();
        set.add(1);
        set.add(2);
        set.add(4);
        try {
            session = SqlSessionFactoryUtil.openSqlSession();
            List<Users> usersList = session.selectList("getUsersByIds",set);
            usersList.forEach(System.out::println);
        } catch (Exception ex) {
            ex.printStackTrace();
        } finally {
            if (session != null) {
                session.close();
            }
        }
    }

5.foreach迭代Map

根据用户类型和用户编号两个查询条件查询用户资料。

映射文件：

<?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.mapper.usersMapper">

   <!--foreach迭代Map-->
    <select id="getUsersByTypeAndIds" parameterType="java.util.Map" resultType="Users">
        select * from users where type=#{type} and uid in
        <foreach collection="ids" item="uid" index="index" open="(" separator="," close=")">
            #{uid}
        </foreach>
    </select>
</mapper>

测试类：

    @Test
    public void testQueryUsersByTypeAndIds(){
        SqlSession session = null;
        List<Integer> idList = new ArrayList<Integer>();
        idList.add(1);
        idList.add(2);
        idList.add(4);
        Map<String,Object> map = new HashMap<String,Object>();
        map.put("type","student");
        map.put("ids",idList);
        try {
            session = SqlSessionFactoryUtil.openSqlSession();
            List<Users> usersList = session.selectList("getUsersByTypeAndIds",map);
            usersList.forEach(System.out::println);
        } catch (Exception ex) {
            ex.printStackTrace();
        } finally {
            if (session != null) {
                session.close();
            }
        }
    }