`GsonBuilder` should throw exception when trying to register adapter for `Object` or `JsonElement`

[Marcono1234]

Problem solved by the feature

GsonBuilder does not support overwriting the built-in adapters for Object and for JsonElement (and subclasses). This might not be obvious to users (see #1177).

gson/gson/src/main/java/com/google/gson/Gson.java

Lines 283 to 285 in 6d9c356

	// built-in type adapters that cannot be overridden
	factories.add(TypeAdapters.JSON_ELEMENT_FACTORY);
	factories.add(ObjectTypeAdapter.getFactory(objectToNumberStrategy));

(Side note: This only affects GsonBuilder; it is possible to overwrite these adapters when using @JsonAdapter on a field.)

Feature description

• GsonBuilder.registerTypeAdapter should throw an exception when the type is Object or JsonElement or a subclass of it
• GsonBuilder.registerTypeHierarchyAdapter should throw an exception when the type is JsonElement or a subclass of it

Additionally for both methods the documentation should be extended to specify that overwriting these adapters is not possible (but maybe mention that @JsonAdapter can be used instead).

Alternatives / workarounds

Only adjust the documentation (and Troubleshooting Guide) to mention that these adapters cannot be overwritten.

Or as proposed by #1177 let users overwrite the built-in adapters for these types.

[sachinp97]

@Marcono1234

I started looking into the solution options. In my opinion, both should throw exception in these cases as overwriting the built-in adapters might open some unexpected behaviors (including the one mentioned in #1177).
What exception is more appropriate in this case:

1. UnsupportedOperationException
2. IllegalArgumentException

[Marcono1234]

Personally I think IllegalArgumentException might be more suitable, since the argument (type for which the adapter is registered) is illegal. I think UnsupportedOperationException is more often used when a method is not supported at all, regardless of the provided arguments.

As side note: I just noticed a typo in the description of this feature request: "GsonBuilder.registerTypeAdapterFactory" should be "GsonBuilder.registerTypeAdapter" (I have fix this now).

Do you want to work on this? In that case please have a short look at the contributing guide, especially the part about the CLA.
Note that this feature request here is only a proposal at the moment; so before you waste any time on this, we would have to clarify with @eamonnmcmanus if this suggested change seems reasonable.

[sachinp97]

Sure I can work on it, I already signed CLA. Thanks for letting me know, I will wait for any clarifications needed.

[eamonnmcmanus]

It does look as if we should throw an exception rather than ignoring the type adapter that the user has supplied. It's possible that this will break some existing code, but most likely any such code wasn't doing what its authors expected anyway. I didn't find any cases in Google's code base where people were registering a type adapter for Object.class or JsonElement.class, though it's possible they are doing it through a Type variable whose value happens to be one of those. I'll need to investigate a bit more deeply to see if that's happening.

I see that at least GsonTest will need to be updated. The test is using Object.class but I think it could use any other type just as well.

I agree that the documentation should mention this. There should also be a test that confirms that the exception is indeed thrown.

Thanks for volunteering, @sachinp97!

[eamonnmcmanus]

I confirmed that no Google tests fail if I make it an error to register a type adapter for Object or JsonElement, other than GsonTest as I mentioned.

[sachinp97]

Thank you @eamonnmcmanus for confirmation, I will take a a look and proceed with the implementation!

[sachinp97]

@eamonnmcmanus, @Marcono1234
Thanks for guiding so far on this.
It seems following use cases of Parameterized Type failed after latest changes.
I added a fix for this, not sure if there are other use cases that may not be covered in the tests.
Please review the PR.

[ERROR] Errors: 
[ERROR]   CollectionTest.testUserCollectionTypeAdapter:360 » ClassCast class com.google.gson.internal.$Gson$Types$ParameterizedTypeImpl cannot be cast to class java.lang.Class (com.google.gson.internal.$Gson$Types$ParameterizedTypeImpl is in unnamed module of loader 'app'; java.lang.Class is in module java.base of loader 'bootstrap')
[ERROR]   InstanceCreatorTest.testInstanceCreatorForCollectionType:103 » ClassCast class com.google.gson.internal.$Gson$Types$ParameterizedTypeImpl cannot be cast to class java.lang.Class (com.google.gson.internal.$Gson$Types$ParameterizedTypeImpl is in unnamed module of loader 'app'; java.lang.Class is in module java.base of loader 'bootstrap')
[ERROR]   MapTest.testCustomSerializerForSpecificMapType:327 » ClassCast class com.google.gson.internal.$Gson$Types$ParameterizedTypeImpl cannot be cast to class java.lang.Class (com.google.gson.internal.$Gson$Types$ParameterizedTypeImpl is in unnamed module of loader 'app'; java.lang.Class is in module java.base of loader 'bootstrap')
[ERROR]   ParameterizedTypesTest.testParameterizedTypeDeserialization:72 » ClassCast class com.google.gson.internal.$Gson$Types$ParameterizedTypeImpl cannot be cast to class java.lang.Class (com.google.gson.internal.$Gson$Types$ParameterizedTypeImpl is in unnamed module of loader 'app'; java.lang.Class is in module java.base of loader 'bootstrap')
[ERROR]   ParameterizedTypesTest.testParameterizedTypeWithCustomSerializer:111 » ClassCast class com.google.gson.internal.$Gson$Types$ParameterizedTypeImpl cannot be cast to class java.lang.Class (com.google.gson.internal.$Gson$Types$ParameterizedTypeImpl is in unnamed module of loader 'app'; java.lang.Class is in module java.base of loader 'bootstrap')
[ERROR]   ParameterizedTypesTest.testParameterizedTypeWithReaderDeserialization:160 » ClassCast class com.google.gson.internal.$Gson$Types$ParameterizedTypeImpl cannot be cast to class java.lang.Class (com.google.gson.internal.$Gson$Types$ParameterizedTypeImpl is in unnamed module of loader 'app'; java.lang.Class is in module java.base of loader 'bootstrap')
[ERROR]   ParameterizedTypesTest.testParameterizedTypesWithCustomDeserializer:127 » ClassCast class com.google.gson.internal.$Gson$Types$ParameterizedTypeImpl cannot be cast to class java.lang.Class (com.google.gson.internal.$Gson$Types$ParameterizedTypeImpl is in unnamed module of loader 'app'; java.lang.Class is in module java.base of loader 'bootstrap')
[INFO]