InaccessibleObjectException with JDK 16

[tomparle]

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]

The problem might be that the internal class ConstructorConstructor always first tries to get the constructor using reflection before using a default constructor for Collection subtypes.

Though I am not sure if this can easily be solved. Maybe the simplest solution which also keeps backward compatibility would be to change this line to handle the exception from setAccessible and return null as well:

gson/gson/src/main/java/com/google/gson/internal/ConstructorConstructor.java

Lines 124 to 126 in ceae88b

	} catch (NoSuchMethodException e) {
	return null;
	}

Though InaccessibleObjectException which is thrown there only exists since Java 9, but this project targets Java 6.
Therefore it might be necessary to catch RuntimeException (the subclass).

This is related to #1540, but not the same since here the issue is caused by Gson; Collections$EmptyList is the runtime type; the compile-time type is likely List<...> which Gson's built-in adapters would be able to handle, especially since ConstructorConstructor is only used for deserialization where only the compile-time type matters.

Note that I am not a maintainer of this project.

Edit: Having newDefaultConstructor return null if the constructor is inaccessible (as proposed above in this comment) would be a backward incompatible change and might introduce difficult to track down bugs for compile-time classes with an inaccessible constructor, because then Unsafe would be used to create the instance. Instead it would be better to create an ObjectCreator which always throws an exception, see #1902 for an implementation.

[roman-dzhadan]

@tomparle
One of many reasons, why Java introduced its modules was an attempt to encapsulate the internal JDK code and make sure that end-developers will not use it. It gives more flexibility for the Java language evolution, it allows to change the internals quickly and without obligation to support backward compatibility.

In the case of "gson" library, which uses reflection quite a lot. You still can use it with JDK16.
But you have to manually open and export the required modules with JAVA_OPTS arguments.

[tomparle]

Thank you for all your answers.
I tried a few hours but eventually could not even build the gson project, alone trying any modification.
I tried with gradle (plugin maven id not found) and Maven (ConcurrentException in bnd-maven-plugin) as well.
Unless someone kindly helps me to build the project, I unfortunately will not be able to progress any more for now.

[tomparle]

Update : I found it too hard to patch gson directly, so in a custom TypeAdapter I check if a type is not accessible (like the private class java.util.Collections$EmptyCollection), and use the corresponding accessible ArrayList instead. It works for now, so I consider it a good workaround for now.

[Marcono1234]

Would you mind leaving the issue open anyways?
That way other others might find this issue more easily, and it indicates to the maintainers that this issue is still unresolved (even if it may have a workaround).

[tomparle]

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