[GitHub] pzhdfy edited a comment on issue #6690: bugfix: when building materialized-view, if taskCount>1, may cause concurrentModificationException

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pzhdfy edited a comment on issue #6690: bugfix: when building materialized-view, if taskCount>1, may cause concurrentModificationException
URL: https://github.com/apache/incubator-druid/pull/6690#issuecomment-459931967
 
 
   @jihoonson 
   
   Yes, this is guarded by taskLock .
   
   This is just the WRONG USE of hashmap.
   
   if we remove an entry  from a hashmap  by remove(key), while iterating it,  this will throw a concurrentModificationException, even in a single thread.
   
   ![image](https://user-images.githubusercontent.com/2594619/52159446-ccf9db80-26df-11e9-9b98-fd9e735f7989.png)
   
   ref:https://stackoverflow.com/questions/602636/concurrentmodificationexception-and-a-hashmap
   
   ```java
   for (Map.Entry<Interval, HadoopIndexTask> entry : runningTasks.entrySet()) {
           Optional<TaskStatus> taskStatus = taskStorage.getStatus(entry.getValue().getId());
           if (!taskStatus.isPresent() || !taskStatus.get().isRunnable()) {
             runningTasks.remove(entry.getKey());
             runningVersion.remove(entry.getKey());
           }
         }
   ```

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