Gson 2.8,.6 fails to serialize/deserialize SparseArray on Android 10

[bamibestelt]

I'm using Gson in my @TypeConverter class to convert SparseArray objects into json String to be saved in Room database. Here is how I do it:

`class RoomTypeConverter {

@TypeConverter
fun sparseArrayToString(sparseArray: SparseArray<String>) : String = Gson().toJson(sparseArray)

@TypeConverter
fun stringToSparseArray(string: String) : SparseArray<String> = Gson().fromJson(string, object: TypeToken<SparseArray<String>>() {}.type)

@TypeConverter
fun sparseIntArrayToString(sparseIntArray: SparseIntArray) : String = Gson().toJson(sparseIntArray)

@TypeConverter
fun stringToSparseIntArray(string: String) : SparseIntArray = Gson().fromJson(string, SparseIntArray::class.java)

}`

The TypeConverter works fine for Android 9 and below as SparseArray successfully converted as JSON string, but fails to work properly on Android 10 as the JSON string saved in database are just empty JSON "{}".

[lyubomyr-shaydariv]

You should not (de)serialize objects of classes you don't control and don't know it's structure. What you should do: convert your sparse array to a DTO object that is Gson-friendly (simple built-ins like List, Map). Why: Gson does not support the Android runtime classes directly, and mostly picks up a type adapter that (de)serializes the object field-by-field including its private fields that are not excluded (neither transient, nor excluded explicitly). You probably need to implement a custom type adapter too for the reasons of backwards compatibility. See more: #1573 #1613

[ozmium]

The exact same problem happens with Json-IO: It works fine on Android 9 and below, but on Android 10/11, Json-IO will save a SparseArray as an empty {"@type":"android.util.SparseArray"} with no object mappings.

The solution/workaround is to use HashMap<Long, Object> instead of SparseArray<Object>. This will work on all Android versions from Android 4 to Android 11, with correct serialization and deserialization. And HashMap has a similar interface to SparseArray.

Note that HashMap<Integer, Object> does not work with Json-IO on Android 10/11 (see jdereg/json-io#141), but it may work with Gson.

[Marcono1234]

Closing this issue because as mentioned in #1660 (comment) you should not rely on reflection-based serialization for third-party classes.