foreach用法--对于list和array
foreach的主要用在构建in条件中，它可以在SQL语句中进行迭代一个集合。
foreach元素的属性主要有 item，index，collection，open，separator，close。
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，用于表示在迭代过程中，每次迭代到的位置，open表示该语句以什么开始，separator表示在每次进行迭代之间以什么符号作为分隔 符，close表示以什么结束。
在使用foreach的时候最关键的也是最容易出错的就是collection属性，该属性是必须指定的，但是在不同情况 下，该属性的值是不一样的，主要有一下3种情况：
1.如果传入的是单参数且参数类型是一个List的时候，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数组的时候，collection的属性值为array
3.如果使用Map封装了，collection的属性值为对应的Key
4.parameterType为list，java.util.List，java.util.ArrayList均可以，切记不可以为Array.
建表语句和表的数据，以及范例如下
第1种
DAO:
public List<User> findByIdList(List<String> idList);SQL:
<select id="findByIdList" resultMap="resultMap" parameterType="list"> <!-- parameterType="java.util.List" 也正确 -->
SELECT
<include refid="columnSql"></include>
FROM t_user
WHERE id in
<foreach collection="list" item="id" open="(" separator="," close=")">
#{id}
</foreach>
</select>2018-02-04 14:44:18,015 -[DEBUG] method:[com.springmvc.dao.UserDao.findByIdList (66277ms)] - ==>
Preparing: SELECT id, username, password, sex FROM t_user WHERE id in ( ? , ? , ? )
2018-02-04 14:44:18,015 -[DEBUG] method:[com.springmvc.dao.UserDao.findByIdList (66277ms)] - ==>
Parameters: 1(String), 2(String), 3(String)
2018-02-04 14:44:18,062 -[DEBUG] method:[com.springmvc.dao.UserDao.findByIdList (66324ms)] - <==
Total: 3
第2种
DAO:
public List<User> findByIdList(List<String> idList);SQL:
<select id="findByIdList" resultMap="resultMap" parameterType="list"> <!-- parameterType="java.util.List" 也正确 -->
SELECT
<include refid="columnSql"></include>
FROM t_user
WHERE id in
<foreach collection="list" item="item123" open="(" separator="," close=")">
#{item123}
</foreach>
</select>2018-02-04 14:48:25,654 -[DEBUG] method:[com.springmvc.dao.UserDao.findByIdList (16876ms)] - ==>
Preparing: SELECT id, username, password, sex FROM t_user WHERE id in ( ? , ? , ? )
2018-02-04 14:48:25,654 -[DEBUG] method:[com.springmvc.dao.UserDao.findByIdList (16876ms)] - ==>
Parameters: 1(String), 2(String), 3(String)
2018-02-04 14:48:25,654 -[DEBUG] method:[com.springmvc.dao.UserDao.findByIdList (16876ms)] - <==
Total: 3
第3种
DAO:
public List<User> findByIdList(List<String> idList);SQL:
<select id="findByIdList" resultMap="resultMap" parameterType="java.util.List">
SELECT
<include refid="columnSql"></include>
FROM t_user
WHERE id in
<foreach collection="List" item="item123" open="(" separator="," close=")"> <!-- 这里collection="List"报错 -->
#{item123}
</foreach>
</select>org.mybatis.spring.MyBatisSystemException: nested exception is org.apache.ibatis.binding.BindingException:
Parameter 'List' not found. Available parameters are [list]
第4种
SERVICE:
public List<User> findByIdList(List<String> idList) {
Map<String, Object> map = new HashMap<String, Object>();
map.put("List123", idList);
return userDao.findByIdList(map);
}DAO:
public List<User> findByIdList(Map<String, Object> map);SQL:
<select id="findByIdList" resultMap="resultMap" parameterType="java.util.List">
SELECT
<include refid="columnSql"></include>
FROM t_user
WHERE id in
<foreach collection="List123" item="item123" open="(" separator="," close=")"> <!-- 使用Map封装，key是List123 -->
#{item123}
</foreach>
</select>2018-02-04 15:10:46,794 -[DEBUG] method:[com.springmvc.dao.UserDao.findByIdList (14878ms)] - ==>
Preparing: SELECT id, username, password, sex FROM t_user WHERE id in ( ? , ? , ? )
2018-02-04 15:10:46,794 -[DEBUG] method:[com.springmvc.dao.UserDao.findByIdList (14878ms)] - ==>
Parameters: 1(String), 2(String), 3(String)
2018-02-04 15:10:46,809 -[DEBUG] method:[com.springmvc.dao.UserDao.findByIdList (14893ms)] - <==
Total: 3
第5种
SERVICE:
public List<User> findByIdList(String[] idArray) {
Map<String, Object> map = new HashMap<String, Object>();
map.put("Array123", idArray);
return userDao.findByIdList(map);
}DAO:
public List<User> findByIdList(Map<String, Object> map);SQL:
<select id="findByIdList" resultMap="resultMap" parameterType="array"> <!-- 这里使用array会报错 -->
SELECT
<include refid="columnSql"></include>
FROM t_user
WHERE id in
<foreach collection="Array123" item="item123" open="(" separator="," close=")">
#{item123}
</foreach>
</select>Caused by: org.apache.ibatis.builder.BuilderException: Error parsing Mapper XML.
Cause: org.apache.ibatis.builder.BuilderException: Error resolving class.
Cause: org.apache.ibatis.type.TypeException: Could not resolve type alias 'array'.
Cause: java.lang.ClassNotFoundException: Cannot find class: array
第6种
SERVICE:
public List<User> findByIdList(String[] idArray) {
Map<String, Object> map = new HashMap<String, Object>();
map.put("Array123", idArray);
return userDao.findByIdList(map);
}DAO:
public List<User> findByIdList(Map<String, Object> map);SQL:
<select id="findByIdList" resultMap="resultMap" parameterType="list"> <!-- parameterType="java.util.List" 或者 parameterType="java.util.ArrayList" 均正确 -->
SELECT
<include refid="columnSql"></include>
FROM t_user
WHERE id in
<foreach collection="Array123" item="item123" open="(" separator="," close=")">
#{item123}
</foreach>
</select>2018-02-04 15:23:07,928 -[DEBUG] method:[com.springmvc.dao.UserDao.findByIdList (43730ms)] - ==>
Preparing: SELECT id, username, password, sex FROM t_user WHERE id in ( ? , ? , ? )
2018-02-04 15:23:07,928 -[DEBUG] method:[com.springmvc.dao.UserDao.findByIdList (43730ms)] - ==>
Parameters: 1(String), 2(String), 3(String)
2018-02-04 15:23:07,928 -[DEBUG] method:[com.springmvc.dao.UserDao.findByIdList (43730ms)] - <==
Total: 3
第7种
DAO:
public List<User> findByIdList(String[] idArray);SQL:
<select id="findByIdList" resultMap="resultMap" parameterType="java.util.ArrayList">
SELECT
<include refid="columnSql"></include>
FROM t_user
WHERE id in
<foreach collection="array" item="item123" open="(" separator="," close=")">
#{item123}
</foreach>
</select>2018-02-04 15:32:44,682 -[DEBUG] method:[com.springmvc.dao.UserDao.findByIdList (21575ms)] - ==>
Preparing: SELECT id, username, password, sex FROM t_user WHERE id in ( ? , ? , ? )
2018-02-04 15:32:44,682 -[DEBUG] method:[com.springmvc.dao.UserDao.findByIdList (21575ms)] - ==>
Parameters: 1(String), 2(String), 3(String)
2018-02-04 15:32:44,682 -[DEBUG] method:[com.springmvc.dao.UserDao.findByIdList (21575ms)] - <==
Total: 3
<sql id="columnSql">
<trim suffixOverrides=","> <!-- suffixOverrides此时的作用是去除最后一个逗号 -->
id,
username,
password,
sex,
</trim>
</sql>以上基于JDK1.8。日志为亲测。仅为个人经验，希望能给大家一个参考，也希望大家多多支持编程网。
