Random java.util.ConcurrentModificationException: null

Question

I have a method that runs every 5 minutes and removes files from a cache

private HashMap<String, CachedFile> cache = new HashMap<>();

@Scheduled(fixedDelay = 300000)
public void deleteFileCache() {

    //remove files last accessed > 12h
    cache.entrySet().stream().filter(entry -> LocalDateTime.now().minus(12, ChronoUnit.HOURS)
            .isAfter(entry.getValue().getLastAccessed())).forEach(entry -> {
        File file = new File(tempFolder, entry.getKey());
        boolean deleted = file.delete();
    });

    //remove entries from HashMap
    cache.entrySet().removeIf(entry -> LocalDateTime.now().minus(12, ChronoUnit.HOURS)
            .isAfter(entry.getValue().getLastAccessed()));

    //if little space left remove oldest files
    long freeSpace = tempFolder.getFreeSpace();
    while (freeSpace < 6000000000L) {
        Optional<String> fileToDelete = cache.entrySet().stream()
                .min(Comparator.comparing(stringCachedFileEntry -> stringCachedFileEntry.getValue().getLastAccessed()))
                .map(Map.Entry::getKey);

        fileToDelete.ifPresent(filename -> {
            new File(tempFolder, filename).delete();
            cache.remove(filename);
        });
        freeSpace = tempFolder.getFreeSpace();
    }
}

This method fails with a ConcurrentModificationException about 2-3 times a day. I am unable to see why since the method can only run once at the same time and I am not iterating at the same time as removing from the HashMap.

It fails at line .min(Comparator.comparing(stringCachedFileEntry ->...

java.util.ConcurrentModificationException: null
        at java.util.HashMap$EntrySpliterator.forEachRemaining(HashMap.java:1704) ~[na:1.8.0_212]
        at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482) ~[na:1.8.0_212]
        at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472) ~[na:1.8.0_212]
        at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708) ~[na:1.8.0_212]
        at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[na:1.8.0_212]
        at java.util.stream.ReferencePipeline.reduce(ReferencePipeline.java:479) ~[na:1.8.0_212]
        at java.util.stream.ReferencePipeline.min(ReferencePipeline.java:520) ~[na:1.8.0_212]
        at ch.my.app.server.FileCacheService.deleteFileCache(FileCacheService.java:113) ~[classes!/:1.0-SNAPSHOT]

Answer 1

A ConcurrentModificationException is not only thrown by deletions from the iterated set while iterating. Insertions cause it quite as well. So the likely reason for yours is that your Spring boot controller writes to the map without synchronization with the deleteFileCache method.

The obvious solution is to use some thread safe map instead of HashMap, for example the ConcurrentHashMap mentioned in a couple of the comments.

For example, the documentation of Spliterator says:

… After binding a Spliterator should, on a best-effort basis, throw ConcurrentModificationException if structural interference is detected. …

It seems from your stacktrace that the min method uses a Spliterator.

Structural changes include both insertions and deletions (they probably don’t include replacing a mapping in the map where only the value is changed).

Link: Documentation of Spliterator