场景说明
简单来说，我们系统中许多数据都是树状结构的，所以我定义了一个实体类父类BaseTreePO，并且想封装一个通用的树状对象的Service类，部分代码如下：
public interface TreeService<T extends BaseTreePO> extends IService<T> {
default String getCurrentMaximumChildPath(T entity, String parentPath) {
LambdaQueryWrapper<T> wrapper = new LambdaQueryWrapper<T>()
.orderByDesc(T::getPath);//在这里通过T::getPath获取字段名时报错
if (StrUtil.isEmpty(parentPath)) {
wrapper.isNull(T::getParentPath);
} else {
wrapper.eq(T::getParentPath, parentPath);
}
T maxPathChild = this.getBaseMapper().selectOne(this.doAppend(wrapper, entity));
return ObjectUtil.isEmpty(maxPathChild) ? null : maxPathChild.getPath();
}
}
如上述代码所示，在执行T::getPath时报错，报错堆栈信息如下：
com.baomidou.mybatisplus.core.exceptions.MybatisPlusException: can not find lambda cache for this entity [com.copm.ifm.base.basic.pojo.BaseTreePO]
at com.baomidou.mybatisplus.core.toolkit.ExceptionUtils.mpe(ExceptionUtils.java:49)
at com.baomidou.mybatisplus.core.toolkit.Assert.isTrue(Assert.java:38)
at com.baomidou.mybatisplus.core.toolkit.Assert.notNull(Assert.java:72)
at com.baomidou.mybatisplus.core.conditions.AbstractLambdaWrapper.tryInitCache(AbstractLambdaWrapper.java:94)
at com.baomidou.mybatisplus.core.conditions.AbstractLambdaWrapper.getColumn(AbstractLambdaWrapper.java:79)
at com.baomidou.mybatisplus.core.conditions.AbstractLambdaWrapper.columnToString(AbstractLambdaWrapper.java:62)
at com.baomidou.mybatisplus.core.conditions.AbstractLambdaWrapper.columnToString(AbstractLambdaWrapper.java:58)
at com.baomidou.mybatisplus.core.conditions.AbstractLambdaWrapper.columnToString(AbstractLambdaWrapper.java:38)
at com.baomidou.mybatisplus.core.conditions.AbstractWrapper.lambda$orderBy$82c52469$1(AbstractWrapper.java:310)
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1374)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)
at com.baomidou.mybatisplus.core.conditions.segments.OrderBySegmentList.transformList(OrderBySegmentList.java:37)
at com.baomidou.mybatisplus.core.conditions.segments.AbstractISegmentList.addAll(AbstractISegmentList.java:60)
at com.baomidou.mybatisplus.core.conditions.segments.MergeSegments.add(MergeSegments.java:50)
at com.baomidou.mybatisplus.core.conditions.AbstractWrapper.doIt(AbstractWrapper.java:469)
at com.baomidou.mybatisplus.core.conditions.AbstractWrapper.orderBy(AbstractWrapper.java:310)
at com.baomidou.mybatisplus.core.conditions.AbstractWrapper.orderBy(AbstractWrapper.java:47)
at com.baomidou.mybatisplus.core.conditions.interfaces.Func.orderByDesc(Func.java:264)
at com.baomidou.mybatisplus.core.conditions.interfaces.Func.orderByDesc(Func.java:245)
at com.copm.ifm.base.service.TreeService.getCurrentMaximumChildPath(TreeService.java:161)
解决方案
给对应的父类也单独增加一个Mapper类即可：
package com.copm.ifm.base.service.base.mapper;
import com.baomidou.mybatisplus.core.MybatisMapperAnnotationBuilder;
import com.baomidou.mybatisplus.core.mapper.BaseMapper;
import com.copm.ifm.base.basic.pojo.BaseTreePO;
/**
* 因为MP3.2+之后不会缓存实体类的父类字段信息，所以在使用泛型的Lambda表达式时会报错
* {@code MybatisPlusException: can not find lambda cache for this entity [com.copm.ifm.base.basic.pojo.BaseTreePO]}
* 原因是在执行{@link com.baomidou.mybatisplus.core.toolkit.LambdaUtils#getColumnMap(Class)}时
* {@code COLUMN_CACHE_MAP}中没有{@link BaseTreePO}的信息
*
* <p>
* 根据源码
* {@link com.baomidou.mybatisplus.core.MybatisMapperRegistry#addMapper(Class)}
* {@link MybatisMapperAnnotationBuilder#parse()}方法的加载逻辑
* <p>
* 他会将所有扫描到的mapper中的泛型({@link BaseMapper<Class>}中的Class，即实体类)的字段信息缓存到
* {@link com.baomidou.mybatisplus.core.toolkit.LambdaUtils}中的{@code COLUMN_CACHE_MAP}中。
* 但是MP3.2+之后不会加载父类的信息，所以{@code COLUMN_CACHE_MAP}中没有相关缓存，就报错了。
* <p>
* 因此我们单独为{@link BaseTreePO}添加一个的Mapper类，这样他就会缓存该类的信息了。
*/
public interface BaseTreeMapper extends BaseMapper<BaseTreePO> {
}
具体原因也写在了代码的注释上，这样别人在看到这个类的时候也知道是怎么回事了。
需要注意的是，我这里是没有加@Mapper注解的，因为我在启动类上加了@MapperScan注解。加了@MapperScan就不需要给每个Mapper类单独加@Mapper了，如果你没有加@MapperScan，则需要给他加上@Mapper。