An error was reported while writing the mapreducer.

java.lang.ClassCastException: class date2021_11_27_5.Commodity
    at java.lang.Class.asSubclass(Unknown Source)
    at org.apache.hadoop.mapred.JobConf.getOutputKeyComparator(JobConf.java:887)
    at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.init(MapTask.java:1004)
    at org.apache.hadoop.mapred.MapTask.createSortingCollector(MapTask.java:402)
    at org.apache.hadoop.mapred.MapTask.access$100(MapTask.java:81)
    at org.apache.hadoop.mapred.MapTask$NewOutputCollector.<init>(MapTask.java:698)
    at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:770)
    at org.apache.hadoop.mapred.MapTask.run(MapTask.java:341)
    at org.apache.hadoop.mapred.LocalJobRunner$Job$MapTaskRunnable.run(LocalJobRunner.java:243)
    at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
    at java.util.concurrent.FutureTask.run(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)
2021-11-29 10:00:26,301 INFO [org.apache.hadoop.mapred.LocalJobRunner] - map task executor complete.
  2021-11-29 10:00:26,302 WARN [org.apache.hadoop.mapred.LocalJobRunner] - job_local49120036_0001
  java.lang.Exception: java.io.IOException: Initialization of all the collectors failed. Error in last collector was :class date2021_11_27_5.Commodity
    at org.apache.hadoop.mapred.LocalJobRunner$Job.runTasks(LocalJobRunner.java:462)
    at org.apache.hadoop.mapred.LocalJobRunner$Job.run(LocalJobRunner.java:522)
Caused by: java.io.IOException: Initialization of all the collectors failed. Error in last collector was :class date2021_11_27_5.Commodity
    at org.apache.hadoop.mapred.MapTask.createSortingCollector(MapTask.java:415)
    at org.apache.hadoop.mapred.MapTask.access$100(MapTask.java:81)
    at org.apache.hadoop.mapred.MapTask$NewOutputCollector.<init>(MapTask.java:698)
    at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:770)
    at org.apache.hadoop.mapred.MapTask.run(MapTask.java:341)
    at org.apache.hadoop.mapred.LocalJobRunner$Job$MapTaskRunnable.run(LocalJobRunner.java:243)
    at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
    at java.util.concurrent.FutureTask.run(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassCastException: class date2021_11_27_5.Commodity
    at java.lang.Class.asSubclass(Unknown Source)
    at org.apache.hadoop.mapred.JobConf.getOutputKeyComparator(JobConf.java:887)
    at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.init(MapTask.java:1004)
    at org.apache.hadoop.mapred.MapTask.createSortingCollector(MapTask.java:402)
    ... 10 more

 

How to Solve:
This is because the custom entity class does not implement the WritableComparable<Commodity> interface, or does not override the compareTo method, resulting in a class conversion exception due to lack of serialization.
There is another possibility that
Implementing the WritableComparable<Commodity> interface instead of the WritableComparable<Commodity> interface because of the mapreducer rules
This implementation of two interfaces will result in the above error message when using a custom entity class for the value without problems, but when using a custom entity class for the key
Just reimplement the WritableComparable<Commodity> interface and guide the package
Hadoop specifies that if you implement the Writable and Comparable<Commodity> interfaces, the data of the custom entity class can only be used as value, but if you implement the WritableComparable<Commodity> interface, the data of the custom entity class can be used as both key and value.