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:
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1 |
public List<User> findByIdList(List<String> idList); |
SQL:
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< 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:
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1 |
public List<User> findByIdList(List<String> idList); |
SQL:
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1 2 3 4 5 6 7 8 9 |
< 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:
|
1 |
public List<User> findByIdList(List<String> idList); |
SQL:
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< 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:
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public List<User> findByIdList(List<String> idList) { Map<String, Object> map = new HashMap<String, Object>(); map.put( "List123" , idList); return userDao.findByIdList(map); } |
DAO:
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1 |
public List<User> findByIdList(Map<String, Object> map); |
SQL:
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1 2 3 4 5 6 7 8 9 |
< 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:
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public List<User> findByIdList(String[] idArray) { Map<String, Object> map = new HashMap<String, Object>(); map.put( "Array123" , idArray); return userDao.findByIdList(map); } |
DAO:
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1 |
public List<User> findByIdList(Map<String, Object> map); |
SQL:
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< 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:
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1 2 3 4 5 |
public List<User> findByIdList(String[] idArray) { Map<String, Object> map = new HashMap<String, Object>(); map.put( "Array123" , idArray); return userDao.findByIdList(map); } |
DAO:
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1 |
public List<User> findByIdList(Map<String, Object> map); |
SQL:
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1 2 3 4 5 6 7 8 9 |
< 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:
|
1 |
public List<User> findByIdList(String[] idArray); |
SQL:
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1 2 3 4 5 6 7 8 9 |
< 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
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<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。
原文链接：https://programskills.blog.csdn.net/article/details/79252955
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