Class loading deadlock with JsonElementTypeAdapter #2739
Description
At Google we import Gson from HEAD periodically. After the last such import, we had reports of a server appearing to get stuck, which we were able to track down to a class loading deadlock introduced by #2727. In short, what happens is that Thread 1 calls GsonBuilder.registerTypeHierarchyAdapter which triggers a load of the TypeAdapters class here. That in turn requires JsonElementTypeAdapter to be loaded for a static final field here. Meanwhile, Thread 2 calls GsonBuilder.create, which triggers a load of JsonElementTypeAdapter here. That involves calling TypeAdapters.newTypeHierarchyFactory here, which requires TypeAdapters to be loaded. So Thread 1 is loading TypeAdapters and waiting for JsonElementTypeAdapter to be loaded, while Thread 2 is loading JsonElementTypeAdapter and waiting for TypeAdapters to be loaded.
The deadlock might be due to my request on that PR that the static final fields in TypeAdapters be preserved, because some existing projects are depending on them.
The fix is straightforward, in any case. Instead of JsonElementTypeAdapter and EnumTypeAdapter being public classes that are referenced directly from the Gson class, we should go back to having that class reference static final fields in TypeAdapters. Then every classloading path goes through TypeAdapters and the deadlock is averted.
Fortunately this bug didn't make it into any release, so most users will be unaffected.