How to register an InstanceCreator with Gson in Kotlin?

Question

I can use Code 1 to save MutableList to json string using Gson correctly, But I get the error when I try to restore MutableList

Answer 1

Gson is having a hard time deserialising polymorphic objects as in your MutableList. Here's what you need to do:

1. Add the RuntimeTypeAdapterFactory.java to your project manually (does not seem to be part of gson library). See also this answer.

2. Change your code to use the factory

   Create Gson instance:

   val adapter = RuntimeTypeAdapterFactory
           .of(DeviceDef::class.java)
           .registerSubtype(BluetoothDef::class.java)
           .registerSubtype(WiFiDef::class.java)
   
   val gson = GsonBuilder().setPrettyPrinting().registerTypeAdapterFactory(adapter).create()
   

3. register each of your subtypes in the factory and it will work as intended :)