Record serialization uses incorrect types

[purpledrazi]

Describe the bug
Record fields are serialized with the wrong type. It appears that Kryo is using the declared type of the field rather than the actual type of the object assigned to the field.

To Reproduce
The code below fails with the error "Class is not registered: java.lang.Number".

Registering java.lang.Number allows the serialization to succeed, but then deserialization fails because it attempts to create an instance of Number.

import java.nio.ByteBuffer;

import com.esotericsoftware.kryo.Kryo;
import com.esotericsoftware.kryo.io.ByteBufferInput;
import com.esotericsoftware.kryo.io.ByteBufferOutput;

public class KyroRecordTest {

    public static void main(String[] args) {
        var kryo = new Kryo();
        kryo.register(ExampleRecord.class);

        ByteBuffer buf = ByteBuffer.allocate(1000);

        kryo.writeObject(new ByteBufferOutput(buf), new ExampleRecord(12345));
        buf.flip();

        Object o = kryo.readObject(new ByteBufferInput(buf), ExampleRecord.class);

        System.out.println(o);

    }

    public static record ExampleRecord(Number n) {}
}

Environment:

• OS: Debian
• JDK Version: 16.0.2
• Kryo Version: 5.1.1

[theigl]

@purpledrazi: Thanks for this bug report!

This is quite an obvious bug. It seems nobody has tried serializing non-trivial records so far.

I'll look into this as soon as I can and hope I can fix it without breaking serialization compatibility.

[theigl]

@purpledrazi: I have created a PR for this issue. Unfortunately, it is not possible to solve this in a fully backwards-compatible way. But since this bug makes the record serializer basically unusable in real world applications, I decided to break compatibility and allow existing users to opt-in to the old behavior.

The fix will be part of Kryo 5.2.0, which I plan to release in the near future.