[Java] CopyOnWriteArraySet serialization fails with ClassCastException

[vybhavjs]

Search before asking

• I had searched in the issues and found no similar issues.

Version

OS: macOS 26.1
Fory: 0.13.1
Java : java 21.0.8 2025-07-15 LTS

Component(s)

Java

Minimal reproduce step

/**
 * SPDX-License-Identifier: MIT
 * Copyright (c) 2025 Apple Inc.
 */

import org.apache.fory.Fory;
import org.apache.fory.config.Language;

import java.util.Set;
import java.util.concurrent.CopyOnWriteArraySet;

public class CopyOnWriteArraySetTest {
    public static void main(String[] args) {
        Fory fory = Fory.builder()
                .withLanguage(Language.JAVA)
                .requireClassRegistration(false)
                .build();

        Set<String> set = new CopyOnWriteArraySet<>();
        set.add("Value1");
        set.add("Value2");

        byte[] serialized = fory.serialize(set);
        CopyOnWriteArraySet deserialised = fory.deserialize(serialized, CopyOnWriteArraySet.class);
        System.out.println("deserialised=" + deserialised);
    }
}

What did you expect to see?

Fory should be able to serialize java.util.concurrent.CopyOnWriteArraySet objects.

What did you see instead?

Serialization fails with a ClassCastException.

View full stack trace

Exception in thread "main" org.apache.fory.exception.SerializationException: java.lang.ClassCastException: class org.apache.fory.collection.CollectionSnapshot cannot be cast to class java.util.List (org.apache.fory.collection.CollectionSnapshot is in unnamed module of loader 'app'; java.util.List is in module java.base of loader 'bootstrap')
	at org.apache.fory.Fory.processSerializationError(Fory.java:362)
	at org.apache.fory.Fory.serialize(Fory.java:331)
	at org.apache.fory.Fory.serialize(Fory.java:278)
	at com.fory_tests.CopyOnWriteArraySetTest.main(CopyOnWriteArraySetTest.java:19)
Caused by: java.lang.ClassCastException: class org.apache.fory.collection.CollectionSnapshot cannot be cast to class java.util.List (org.apache.fory.collection.CollectionSnapshot is in unnamed module of loader 'app'; java.util.List is in module java.base of loader 'bootstrap')
	at org.apache.fory.builder.CopyOnWriteArraySetForyCodec_0.writeFields$(CopyOnWriteArraySetForyCodec_0.java:113)
	at org.apache.fory.builder.CopyOnWriteArraySetForyCodec_0.write(CopyOnWriteArraySetForyCodec_0.java:224)
	at org.apache.fory.serializer.collection.CollectionSerializers$DefaultJavaCollectionSerializer.write(CollectionSerializers.java:729)
	at org.apache.fory.Fory.writeData(Fory.java:654)
	at org.apache.fory.Fory.write(Fory.java:405)
	at org.apache.fory.Fory.serialize(Fory.java:325)

Anything Else?

There seems to be a more fundamental issue with CopyOnWriteArrayListSerializer and how it works with codegen. For any object which has a CopyOnWriteArrayList field (like CopyOnWriteArraySet), serialization fails with the same error because the generated codec attempts to cast CollectionSnapshot to List.

Are you willing to submit a PR?

• I'm willing to submit a PR!

[vybhavjs]

@chaokunyang Apologies for the very late response - I was waiting on internal approvals at work.

The PR addresses CopyOnWriteArraySet serialisation, but as mentioned in the "Anything Else" section, it seems like this issue is broader than that - serialisation still fails for any object with a CopyOnWriteArrayList field (CopyOnWriteArraySet is one example of this, which is now fixed). I confirmed this with the 0.14.0 snapshot that included the fix. This is a simple test to reproduce the error :

    public static class NestedCopyOnWriteArrayList {
        private final CopyOnWriteArrayList<String> list;

        public NestedCopyOnWriteArrayList(CopyOnWriteArrayList<String> list) {
            this.list = list;
        }
    }

    public static void main(String[] args) {
        Fory fory = Fory.builder()
                .withLanguage(Language.JAVA)
                .requireClassRegistration(false)
                .build();

        final CopyOnWriteArrayList<String> list = new CopyOnWriteArrayList<>(Arrays.asList("a", "b", "c"));
        NestedCopyOnWriteArrayList nestedObject = new NestedCopyOnWriteArrayList(list);
        byte[] serialised = fory.serialize(nestedObject);
        NestedCopyOnWriteArrayList deserialised = fory.deserialize(serialised, NestedCopyOnWriteArrayList.class);
        System.out.println("deserialised=" + deserialised);
    }

I'm not sure if that should continue to be tracked with this issue OR if we need another one - I think this issue makes sense, but please advise.

I'd like to submit a PR to try and fix this.