今天写代码的时候遇到一个非常傻的错误，代码目的：是在一个表里，通过subject_id来分类查找学科详情，正确返回如下：

表结构如下：

原因如下

问题：

第一应该返回次数是2，

但是只执行了一次subject_id=3，并且报了一个null空异常，非常的奇怪

错误log： 

Caused by: org.apache.ibatis.exceptions.PersistenceException: 
### Error querying database.  Cause: java.lang.UnsupportedOperationException
### The error may exist in file [F:\Huaicai-GXDZ\hucais-customized-channe\target\classes\mapper\KnowledgeCategoryMapper.xml]
### The error may involve com.hucais.infra.mapper.KnowledgeCategoryMapper.selectName
### The error occurred while handling results
### SQL: select category_name from customized_knowledge_category where subject_id = ?
### Cause: java.lang.UnsupportedOperationException
    at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:30) ~[mybatis-3.5.6.jar:3.5.6]
    at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:149) ~[mybatis-3.5.6.jar:3.5.6]
    at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:140) ~[mybatis-3.5.6.jar:3.5.6]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_211]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_211]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_211]
    at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_211]
    at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:427) ~[mybatis-spring-2.0.6.jar:2.0.6]
    ... 155 common frames omitted
Caused by: java.lang.UnsupportedOperationException: null
    at org.apache.ibatis.reflection.wrapper.CollectionWrapper.findProperty(CollectionWrapper.java:48) ~[mybatis-3.5.6.jar:3.5.6]
    at org.apache.ibatis.reflection.MetaObject.findProperty(MetaObject.java:85) ~[mybatis-3.5.6.jar:3.5.6]
    at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.createAutomaticMappings(DefaultResultSetHandler.java:549) ~[hzero-starter-mybatis-mapper-1.11.1.RELEASE.jar:3.5.6]
    at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.applyAutomaticMappings(DefaultResultSetHandler.java:573) ~[hzero-starter-mybatis-mapper-1.11.1.RELEASE.jar:3.5.6]
    at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.getRowValue(DefaultResultSetHandler.java:392) ~[hzero-starter-mybatis-mapper-1.11.1.RELEASE.jar:3.5.6]
    at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.handleRowValuesForSimpleResultMap(DefaultResultSetHandler.java:344) ~[hzero-starter-mybatis-mapper-1.11.1.RELEASE.jar:3.5.6]
    at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.handleRowValues(DefaultResultSetHandler.java:318) ~[hzero-starter-mybatis-mapper-1.11.1.RELEASE.jar:3.5.6]
    at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.handleResultSet(DefaultResultSetHandler.java:291) ~[hzero-starter-mybatis-mapper-1.11.1.RELEASE.jar:3.5.6]
    at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.handleResultSets(DefaultResultSetHandler.java:184) ~[hzero-starter-mybatis-mapper-1.11.1.RELEASE.jar:3.5.6]
    at org.apache.ibatis.executor.statement.PreparedStatementHandler.query(PreparedStatementHandler.java:65) ~[mybatis-3.5.6.jar:3.5.6]
    at org.apache.ibatis.executor.statement.RoutingStatementHandler.query(RoutingStatementHandler.java:79) ~[mybatis-3.5.6.jar:3.5.6]
    at org.apache.ibatis.executor.SimpleExecutor.doQuery(SimpleExecutor.java:63) ~[mybatis-3.5.6.jar:3.5.6]
    at org.apache.ibatis.executor.BaseExecutor.queryFromDatabase(BaseExecutor.java:325) ~[mybatis-3.5.6.jar:3.5.6]
    at org.apache.ibatis.executor.BaseExecutor.query(BaseExecutor.java:156) ~[mybatis-3.5.6.jar:3.5.6]
    at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:109) ~[mybatis-3.5.6.jar:3.5.6]
    at org.apache.ibatis.executor.loader.ResultLoader.selectList(ResultLoader.java:81) ~[mybatis-3.5.6.jar:3.5.6]
    at org.apache.ibatis.executor.loader.ResultLoader.loadResult(ResultLoader.java:70) ~[mybatis-3.5.6.jar:3.5.6]
    at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.getNestedQueryMappingValue(DefaultResultSetHandler.java:816) ~[hzero-starter-mybatis-mapper-1.11.1.RELEASE.jar:3.5.6]
    at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.getPropertyMappingValue(DefaultResultSetHandler.java:521) ~[hzero-starter-mybatis-mapper-1.11.1.RELEASE.jar:3.5.6]
    at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.applyPropertyMappings(DefaultResultSetHandler.java:497) ~[hzero-starter-mybatis-mapper-1.11.1.RELEASE.jar:3.5.6]
    at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.getRowValue(DefaultResultSetHandler.java:394) ~[hzero-starter-mybatis-mapper-1.11.1.RELEASE.jar:3.5.6]
    at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.handleRowValuesForSimpleResultMap(DefaultResultSetHandler.java:344) ~[hzero-starter-mybatis-mapper-1.11.1.RELEASE.jar:3.5.6]
    at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.handleRowValues(DefaultResultSetHandler.java:318) ~[hzero-starter-mybatis-mapper-1.11.1.RELEASE.jar:3.5.6]
    at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.handleResultSet(DefaultResultSetHandler.java:291) ~[hzero-starter-mybatis-mapper-1.11.1.RELEASE.jar:3.5.6]
    at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.handleResultSets(DefaultResultSetHandler.java:184) ~[hzero-starter-mybatis-mapper-1.11.1.RELEASE.jar:3.5.6]
    at org.apache.ibatis.executor.statement.PreparedStatementHandler.query(PreparedStatementHandler.java:65) ~[mybatis-3.5.6.jar:3.5.6]
    at org.apache.ibatis.executor.statement.RoutingStatementHandler.query(RoutingStatementHandler.java:79) ~[mybatis-3.5.6.jar:3.5.6]
    at org.apache.ibatis.executor.SimpleExecutor.doQuery(SimpleExecutor.java:63) ~[mybatis-3.5.6.jar:3.5.6]
    at org.apache.ibatis.executor.BaseExecutor.queryFromDatabase(BaseExecutor.java:325) ~[mybatis-3.5.6.jar:3.5.6]
    at org.apache.ibatis.executor.BaseExecutor.query(BaseExecutor.java:156) ~[mybatis-3.5.6.jar:3.5.6]
    at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:109) ~[mybatis-3.5.6.jar:3.5.6]
    at sun.reflect.GeneratedMethodAccessor139.invoke(Unknown Source) ~[na:na]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_211]
    at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_211]
    at org.apache.ibatis.plugin.Invocation.proceed(Invocation.java:49) ~[mybatis-3.5.6.jar:3.5.6]
    at org.hzero.boot.platform.data.permission.handler.DataCacheResultHandler.intercept(DataCacheResultHandler.java:79) ~[hzero-boot-platform-1.11.1.RELEASE.jar:1.11.1.RELEASE]
    at org.apache.ibatis.plugin.Plugin.invoke(Plugin.java:61) ~[mybatis-3.5.6.jar:3.5.6]
    at com.sun.proxy.$Proxy465.query(Unknown Source) ~[na:na]
    at sun.reflect.GeneratedMethodAccessor139.invoke(Unknown Source) ~[na:na]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_211]
    at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_211]
    at org.apache.ibatis.plugin.Plugin.invoke(Plugin.java:63) ~[mybatis-3.5.6.jar:3.5.6]
    at com.sun.proxy.$Proxy465.query(Unknown Source) ~[na:na]
    at sun.reflect.GeneratedMethodAccessor139.invoke(Unknown Source) ~[na:na]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_211]
    at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_211]
    at org.apache.ibatis.plugin.Invocation.proceed(Invocation.java:49) ~[mybatis-3.5.6.jar:3.5.6]
    at org.hzero.mybatis.parser.SqlParserInterceptor.intercept(SqlParserInterceptor.java:117) ~[hzero-starter-mybatis-mapper-1.11.1.RELEASE.jar:1.11.1.RELEASE]
    at org.apache.ibatis.plugin.Plugin.invoke(Plugin.java:61) ~[mybatis-3.5.6.jar:3.5.6]
    at com.sun.proxy.$Proxy465.query(Unknown Source) ~[na:na]
    at sun.reflect.GeneratedMethodAccessor139.invoke(Unknown Source) ~[na:na]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_211]
    at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_211]
    at org.apache.ibatis.plugin.Invocation.proceed(Invocation.java:49) ~[mybatis-3.5.6.jar:3.5.6]
    at org.hzero.mybatis.security.DataSecurityInterceptor.intercept(DataSecurityInterceptor.java:87) ~[hzero-starter-mybatis-mapper-1.11.1.RELEASE.jar:1.11.1.RELEASE]
    at org.apache.ibatis.plugin.Plugin.invoke(Plugin.java:61) ~[mybatis-3.5.6.jar:3.5.6]
    at com.sun.proxy.$Proxy465.query(Unknown Source) ~[na:na]
    at sun.reflect.GeneratedMethodAccessor139.invoke(Unknown Source) ~[na:na]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_211]
    at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_211]
    at org.apache.ibatis.plugin.Invocation.proceed(Invocation.java:49) ~[mybatis-3.5.6.jar:3.5.6]
    at org.hzero.mybatis.security.SecurityTokenInterceptor.intercept(SecurityTokenInterceptor.java:36) ~[hzero-starter-mybatis-mapper-1.11.1.RELEASE.jar:1.11.1.RELEASE]
    at org.apache.ibatis.plugin.Plugin.invoke(Plugin.java:61) ~[mybatis-3.5.6.jar:3.5.6]
    at com.sun.proxy.$Proxy465.query(Unknown Source) ~[na:na]
    at sun.reflect.GeneratedMethodAccessor139.invoke(Unknown Source) ~[na:na]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_211]
    at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_211]
    at org.apache.ibatis.plugin.Invocation.proceed(Invocation.java:49) ~[mybatis-3.5.6.jar:3.5.6]
    at org.hzero.mybatis.security.CrossSchemaInterceptor.intercept(CrossSchemaInterceptor.java:74) ~[hzero-starter-mybatis-mapper-1.11.1.RELEASE.jar:1.11.1.RELEASE]
    at org.apache.ibatis.plugin.Plugin.invoke(Plugin.java:61) ~[mybatis-3.5.6.jar:3.5.6]
    at com.sun.proxy.$Proxy465.query(Unknown Source) ~[na:na]
    at io.choerodon.mybatis.pagehelper.PageInterceptor$DoPage.invoke(PageInterceptor.java:255) ~[hzero-starter-mybatis-mapper-1.11.1.RELEASE.jar:1.11.1.RELEASE]
    at io.choerodon.mybatis.pagehelper.PageInterceptor.intercept(PageInterceptor.java:173) ~[hzero-starter-mybatis-mapper-1.11.1.RELEASE.jar:1.11.1.RELEASE]
    at org.apache.ibatis.plugin.Plugin.invoke(Plugin.java:61) ~[mybatis-3.5.6.jar:3.5.6]
    at com.sun.proxy.$Proxy465.query(Unknown Source) ~[na:na]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_211]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_211]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_211]
    at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_211]
    at org.apache.ibatis.plugin.Invocation.proceed(Invocation.java:49) ~[mybatis-3.5.6.jar:3.5.6]
    at org.hzero.boot.platform.lov.fuzzy.LovSearchSqlInterceptor.intercept(LovSearchSqlInterceptor.java:42) ~[hzero-boot-platform-1.11.1.RELEASE.jar:1.11.1.RELEASE]
    at org.apache.ibatis.plugin.Plugin.invoke(Plugin.java:61) ~[mybatis-3.5.6.jar:3.5.6]
    at com.sun.proxy.$Proxy465.query(Unknown Source) ~[na:na]
    at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:147) ~[mybatis-3.5.6.jar:3.5.6]
    ... 161 common frames omitted

VO代码 ：

@Id @GeneratedValue @JsonSerialize(using = ToStringSerializer.class) private Long id;  @NotNull private String subjectId;  private String subjectIdMeaning;  private Integer status;  private String statusMeaning;  @NotBlank private List categoryName;

mapper.xml代码：

     ckc.id,     ckc.subject_id,     ckc.tenant_id,     ckc.category_name,     ckc.status,     ckc.delete_flag,     ckc.creation_date,     ckc.created_by,     ckc.last_updated_by,     ckc.last_update_date,     ckc.object_version_number                               select category_name from customized_knowledge_category where subject_id = #{subjectId}        select          from customized_knowledge_category ckc              and ckc.delete_flag = 0                      and ckc.category_name like concat('%',#{categoryName,jdbcType = VARCHAR},'%')                   group by ckc.subject_id 

解决办法：

经过很长时间思考和debug，终于找到原因：

竟然是这个resultType类型设置错误，改为String即可

，当然这里是要跟你设置的返回类的变量的类型相关，到底是什么原因导致，我的技术太弱没有办法解决，仅仅提供自己的小小解决办法，可能会有所帮助！