使用 MyBatis 选择整数列

Question

我正在尝试使用 MyBatis 从包含整数的表中获取单个列。我假设将其提取为列表（尽管我在这一点上可能错了！）

这是我的 mapper.xml

<select id="getPersonIds" resultType="java.util.List">
    SELECT PersonId
        FROM TestDB.users
</select>

这是我的映射器界面

package com.paulmjohnson.Mappers;
import java.util.List;

public interface PersonMapper {

    List<Integer> getPersonIds();

}

我的 DAO：

public List<Integer> getPersonIds() {
        SqlSession session = MyBatisUtil.getSqlSessionFactory().openSession();
        logger.info("Getting Person IDs");
        List<Integer> personIds = session.selectList("com.paulmjohnson.Mappers.PersonMapper.getPersonIds");
        session.close();
        return personIds;
}

我怎么称呼它：

PeopleDAO peopleDAO = new PeopleDAO();
List<Integer> personIds = peopleDAO.getPersonIds();

MyBatis 似乎正在创建我期待的 SELECT 查询：

2019-12-26 09:39:06,677 [main] [DEBUG] c.p.Mappers.PersonMapper.getPersonIds - ==>  Preparing: SELECT PersonId FROM TestDB.users 
2019-12-26 09:39:06,678 [main] [DEBUG] c.p.Mappers.PersonMapper.getPersonIds - ==> Parameters: 

当我运行它时，我得到了这个：

Exception in thread "main" org.apache.ibatis.exceptions.PersistenceException: 
### Error querying database.  Cause: java.lang.UnsupportedOperationException
### The error may exist in person-mapper.xml
### The error may involve com.paulmjohnson.Mappers.PersonMapper.getPersonIds
### The error occurred while handling results
### SQL: SELECT PersonId             FROM TestDB.users
### Cause: java.lang.UnsupportedOperationException
    at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:30)
    at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:149)
    at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:140)
    at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:135)
    at com.paulmjohnson.DAO.PeopleDAO.getPersonIds(PeopleDAO.java:82)
    at com.paulmjohnson.UsingBuckets.main(UsingBuckets.java:105)
Caused by: java.lang.UnsupportedOperationException
    at org.apache.ibatis.reflection.wrapper.CollectionWrapper.findProperty(CollectionWrapper.java:48)
    at org.apache.ibatis.reflection.MetaObject.findProperty(MetaObject.java:85)
    at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.createAutomaticMappings(DefaultResultSetHandler.java:493)
    at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.applyAutomaticMappings(DefaultResultSetHandler.java:517)
    at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.getRowValue(DefaultResultSetHandler.java:402)
    at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.handleRowValuesForSimpleResultMap(DefaultResultSetHandler.java:354)
    at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.handleRowValues(DefaultResultSetHandler.java:328)
    at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.handleResultSet(DefaultResultSetHandler.java:301)
    at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.handleResultSets(DefaultResultSetHandler.java:194)
    at org.apache.ibatis.executor.statement.PreparedStatementHandler.query(PreparedStatementHandler.java:65)
    at org.apache.ibatis.executor.statement.RoutingStatementHandler.query(RoutingStatementHandler.java:79)
    at org.apache.ibatis.executor.SimpleExecutor.doQuery(SimpleExecutor.java:63)
    at org.apache.ibatis.executor.BaseExecutor.queryFromDatabase(BaseExecutor.java:324)
    at org.apache.ibatis.executor.BaseExecutor.query(BaseExecutor.java:156)
    at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:109)
    at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:83)
    at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:147)
    ... 4 more

我很确定我在这里做了一些非常愚蠢的事情！

Answer 1

在你的 mapper.xml 中有 resultType="java.util.List" 你应该使用 resultType="java.lang.Integer" 代替。