InaccessibleObjectException with JDK 16

GSon (2.8.6) has trouble with JDK 16, seemingly due to https://bugs.openjdk.java.net/browse/JDK-8256358 and https://bugs.openjdk.java.net/browse/JDK-8260600 which prevent access through reflection to JDK internals.

This leads to these errors (for example when trying to serialize an empty list constructed with Collections.emptyList()) :

java.lang.reflect.InaccessibleObjectException: Unable to make private java.util.Collections$EmptyList() accessible: module java.base does not "opens java.util" to unnamed module @ffaa6af
	at com.google.gson.internal.reflect.UnsafeReflectionAccessor.makeAccessible(UnsafeReflectionAccessor.java:44)
	at com.google.gson.internal.ConstructorConstructor.newDefaultConstructor(ConstructorConstructor.java:103)
	at com.google.gson.internal.ConstructorConstructor.get(ConstructorConstructor.java:85)
	at com.google.gson.internal.bind.CollectionTypeAdapterFactory.create(CollectionTypeAdapterFactory.java:54)
	at com.google.gson.Gson.getDelegateAdapter(Gson.java:541)
	at io.gsonfire.gson.WrapTypeAdapterFactory.create(WrapTypeAdapterFactory.java:26)
	at com.google.gson.Gson.getAdapter(Gson.java:458)
	at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:56)
	at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.write(MapTypeAdapterFactory.java:208)
	at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.write(MapTypeAdapterFactory.java:145)
	at com.google.gson.Gson.toJson(Gson.java:704)
	at com.google.gson.Gson.toJson(Gson.java:683)
...

I would be happy to contribute but I do not even know how to fix this in Gson. Any ideas ?

[Marcono1234]

@masbaehr, that is unrelated to this issue. As the package name suggests (com.google.gson.internal) LinkedTreeMap is an internal Map implementation of Gson. This issue has nothing to do with JDK 16; 2.8.6 added a module declaration which enforces that classes from the internal package cannot be used. For your case should be able to simply use Map instead:

Map translationMapR = gson.fromJson(reader, Map.class);

(Also note that for better type safety should should probably use TypeToken.)

[tomparle]

@Marcono1234 Sure, I wanted to mark it as “workaround available” but could not find how, other than closing it. Reopened !