Support for JDK17

[dkroehan]

Describe the bug
It seems that Kryo is currently not compatible with JDK17. When calling the default constructor it fails with:

java.lang.reflect.InaccessibleObjectException: Unable to make private java.nio.DirectByteBuffer(long,int) accessible: module java.base does not "opens java.nio" to unnamed module @61322f9d
	at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:354)
	at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:297)
	at java.base/java.lang.reflect.Constructor.checkCanSetAccessible(Constructor.java:188)
	at java.base/java.lang.reflect.Constructor.setAccessible(Constructor.java:181)
	at com.esotericsoftware.kryo.unsafe.UnsafeUtil.<clinit>(UnsafeUtil.java:100)
	at java.base/java.lang.Class.forName0(Native Method)
	at java.base/java.lang.Class.forName(Class.java:467)
	at com.esotericsoftware.kryo.util.Util.<clinit>(Util.java:47)
	at com.esotericsoftware.kryo.serializers.OptionalSerializers.addDefaultSerializers(OptionalSerializers.java:38)
	at com.esotericsoftware.kryo.Kryo.<init>(Kryo.java:230)
	at com.esotericsoftware.kryo.Kryo.<init>(Kryo.java:168)

To Reproduce
Kryo kryo = new Kryo();

Environment:

• JDK Version: OpenJDK 17.0.2 (Corretto-17.0.2.8.1)
• Kryo Version: 5.3.0

Additional context
In my case I am also registering some standard classes.
It seems to work for classes like:

• java.util.Locale
• java.time.Instant
• java.util.HashMap

But it fails for java.util.regex.Pattern due to:

java.lang.reflect.InaccessibleObjectException: Unable to make field private java.lang.String java.util.regex.Pattern.pattern accessible: module java.base does not "opens java.util.regex" to unnamed module @185a6e9

What would be the proper way to fix this?

[theigl]

Your specific case can be solved by using a custom RegexSerializer that doesn't rely on reflection.

Alternatively, you can add JVM arguments for opening up the modules in question. See this comment.

I think it makes sense to include the RegexSerializer and UUIDSerializer directly in Kryo, so those two classes can be serialized on JDK 17 without opening any modules.

[theigl]

I cannot reproduce your first issue. I just downloaded and ran my tests on Corretto to make sure it isn't a JVM specific problem.

[dkroehan]

Thank you for the quick response.

I did set a breakpoint at AccessibleObject#checkCanSetAccessible at line 354 where an InaccessibleObjectException is created for debugging purposes.
This is where I found: Unable to make private java.nio.DirectByteBuffer(long,int) accessible: module java.base does not "opens java.nio" to unnamed module @61322f9d

This is caused by the Util static initializer block at: https://github.com/EsotericSoftware/kryo/blob/master/src/com/esotericsoftware/kryo/util/Util.java#L47
This calls UnsafeUtil with the static initializer at: https://github.com/EsotericSoftware/kryo/blob/master/src/com/esotericsoftware/kryo/unsafe/UnsafeUtil.java#L99

The InaccessibleObjectException is caught in line 101, which causes the directByteBufferConstructor to be null. I didn't pay attention that the Exception is actually not bubbling up.

Now I'm wondering if this is expected behaviour or a failure with java 17.

[dkroehan]

I tried the RegexSerializer using:

kryo.register(Pattern.class, new RegexSerializer());

It seems to work.

FYI: UnmodifiableCollectionsSerializer from kryo-serializers also lacks compatibility. There is already an open issue, which I just updated:
#magro/kryo-serializers#131 (comment)

[theigl]

Now I'm wondering if this is expected behaviour or a failure with java 17.

This is expected behavior and has no effect unless you want to use UnsafeByteBuffers for Input/Output. If you want to use those, you have to open the module up for reflection.

FYI: UnmodifiableCollectionsSerializer from kryo-serializers also lacks compatibility. There is already an open issue, which I just updated:

AFAIK it is impossible to write a non-reflective version of a serializer for UnmodifiableCollection that covers all potential cases. So these serializers will probably never make it into Kryo and you will have to add add-opens arguments if you need to serialize them.

[theigl]

Kryo is currently missing built-in, safe serializers for the following JDK classes:

• java.net.URI
• java.util.UUID
• java.util.regex.Pattern
• java.util.concurrent.atomic.AtomicBoolean
• java.util.concurrent.atomic.AtomicInteger
• java.util.concurrent.atomic.AtomicLong
• java.util.concurrent.atomic.AtomicReference

These serializers are trivial to implement, but cannot be registered in Kryo 5 as default serializers because it would break backwards compatibility. I will add them to DefaultSerializers so users can register them, but they will only be registered by default in Kryo 6.