Add serializers for unmodifiable and synchronized collections

[theigl]

This PR adds serializers for JDK unmodifiable and synchronized collections.

This has been one of the most requested features for Kryo, but I have so far rejected all requests and redirected users to the kryo-serializers project because it is not possible to implement these serializers without reflection. Since this comes up again and again an we already have the same issue with the ClosureSerializer that has been part of Kryo for many years, I decided to give up my opposition.

The serializers need to be registered manually for now to maintain backwards compatibility, but will be added as default serializers in Kryo 6.

Note: The unsafe logic will probably have to be replaced with Varhandles for JDK24+.

#689
#885
#909
#913
#997
#1022

[nastra]

@theigl thanks for working on this. Do you already know which version might be shipped with this feature?

[theigl]

Hi @nastra. I'm still unhappy about merging this because it uses terminally deprecated code in Unsafe, but I think I will include it in Kryo 5.7.0. The release will probably happen in June or July.

[Copilot]

Pull request overview

This PR introduces built-in (manually-registered) Kryo serializers for JDK Collections.*unmodifiable* and Collections.*synchronized* wrappers, enabling round-trip serialization/copy without relying on default reflective construction of these wrapper types.

Changes:

• Added UnmodifiableCollectionSerializers and SynchronizedCollectionSerializers that unwrap the delegate collection/map via Unsafe field offsets and re-wrap on read/copy.
• Added JUnit tests covering round-trip and copy behavior for common unmodifiable/synchronized wrapper variants.
• Exposed small UnsafeUtil helpers (objectFieldOffset, getObject) used by the new serializers.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 10 comments.

Show a summary per file

File	Description
src/com/esotericsoftware/kryo/unsafe/UnsafeUtil.java	Adds Unsafe helper wrappers used to read internal wrapper delegate fields.
src/com/esotericsoftware/kryo/serializers/UnmodifiableCollectionSerializers.java	New serializers for Collections.unmodifiable* wrappers with manual registration/default-serializer helpers.
src/com/esotericsoftware/kryo/serializers/SynchronizedCollectionSerializers.java	New serializers for Collections.synchronized* wrappers, including synchronized write behavior.
test/com/esotericsoftware/kryo/serializers/UnmodifiableCollectionsSerializersTest.java	New round-trip/copy test coverage for unmodifiable wrappers.
test/com/esotericsoftware/kryo/serializers/SynchronizedCollectionSerializersTest.java	New round-trip/copy test coverage for synchronized wrappers.

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