Allow generic types in responses when using Moshi as JSON parser

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Description of the issue:

I’m trying to achieve a scenario that is very common, but with Moshi isn’t so trivial as I thought (I’m coming from Gson, that is simpler to use, but I’m aware of its many disadvantages).

I have endpoint definitions that use generic types, like this:

    @GET("api.json")
    suspend fun <T> test(): Result<T>

This is used because Result<T> contains a standard structure, basically as defined below:

@JsonClass(generateAdapter = true)
data class Result<T> (

    var data: T? = null,

    var timestamp: OffsetDateTime? = null
)

As you may see, it is used to track common information (like the server timestamp and, eventually, in the future, may have the ID of the request to match logs or something like that).

But the problem is that Moshi adapter for this class isn’t called how it should by Retrofit (I explain below), which results in this error:

2020-01-18 15:38:59.913 7097-7202/com.example.myapplication E/MainActivity: Error handling JSON data: 
    java.lang.IllegalArgumentException: Unable to create converter for com.example.myapplication.api.Result<T>
        for method RemoteService.test
        at retrofit2.Utils.methodError(Utils.java:53)
        at retrofit2.HttpServiceMethod.createResponseConverter(HttpServiceMethod.java:115)
        at retrofit2.HttpServiceMethod.parseAnnotations(HttpServiceMethod.java:82)
        at retrofit2.ServiceMethod.parseAnnotations(ServiceMethod.java:37)
        at retrofit2.Retrofit.loadServiceMethod(Retrofit.java:192)
        at retrofit2.Retrofit$1.invoke(Retrofit.java:149)
        at java.lang.reflect.Proxy.invoke(Proxy.java:813)
        at $Proxy4.test(Unknown Source)
        at com.example.myapplication.MainActivity.handleWithMoshiRetrofitConverter(MainActivity.kt:72)
        at com.example.myapplication.MainActivity$onCreate$1$1.invokeSuspend(MainActivity.kt:48)
        at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
        at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:56)
        at kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(CoroutineScheduler.kt:561)
        at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.executeTask(CoroutineScheduler.kt:727)
        at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.runWorker(CoroutineScheduler.kt:667)
        at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(CoroutineScheduler.kt:655)
     Caused by: java.lang.IllegalArgumentException: No JsonAdapter for T (with no annotations)
    for T data
    for com.example.myapplication.api.Result<T>
        at com.squareup.moshi.Moshi$LookupChain.exceptionWithLookupStack(Moshi.java:349)
        at com.squareup.moshi.Moshi.adapter(Moshi.java:150)
        at com.example.myapplication.api.ResultJsonAdapter.<init>(ResultJsonAdapter.kt:29)
        at java.lang.reflect.Constructor.newInstance0(Native Method)
        at java.lang.reflect.Constructor.newInstance(Constructor.java:430)
        at com.squareup.moshi.internal.Util.generatedAdapter(Util.java:540)
        at com.squareup.moshi.StandardJsonAdapters$1.create(StandardJsonAdapters.java:60)
        at com.squareup.moshi.Moshi.adapter(Moshi.java:138)
        at com.squareup.moshi.Moshi.adapter(Moshi.java:98)
        at retrofit2.converter.moshi.MoshiConverterFactory.responseBodyConverter(MoshiConverterFactory.java:91)
        at retrofit2.Retrofit.nextResponseBodyConverter(Retrofit.java:352)
        at retrofit2.Retrofit.responseBodyConverter(Retrofit.java:335)
        at retrofit2.HttpServiceMethod.createResponseConverter(HttpServiceMethod.java:113)
        	... 14 more
     Caused by: java.lang.IllegalArgumentException: No JsonAdapter for T (with no annotations)
        at com.squareup.moshi.Moshi.adapter(Moshi.java:148)
        	... 25 more

Firstly, I thought the problem was with Moshi generated adapter. But then I made a simple test code that uses Moshi directly to parse the text and then try the same with a Retrofit call. The Moshi part of handling it is a little bit confusing (maybe the generated adapter code could be improved for this kind of scenarios), but it works very well.

So I ended up thinking that this may be the way Retrofit call Moshi converter. Any thoughts or ideas?

PS: I know I could create a response model for each of my response possibilities, but I really don’t think that this would be a solution. I don’t see, at least right now, why Retrofit + Moshi couldn’t handle this kind of use case (except for the fact that it isn’t implemented yet, which seems to be the case).

[luanmm]

Thanks for the suggestion, @20ali20.

I ended up, for now, using Gson as parser. I read all over that it isn’t a good library for many reasons, but it is more flexible than Moshi in some use cases.

Will try to replace Gson at some point and your suggestion will be taken into account!

I will close the issue, as it is answered and there is no point to keep it open (I think).

Thanks for the attention of you all.

[20ali20]

Hello, @JakeWharton. Thanks for your soon reply!

There is no way to implement this, then? If there is no way to figure out T to call Moshi (or any other library), I can’t see a way to achieve it from Retrofit helpers, indeed.

1. Any suggestions on how to achieve something like that without using generics in Retrofit definitions?
2. Maybe Retrofit could warn somehow, in compilation time, that this is not a valid interface construction to map a remote service (if possible and not difficult to implement).

Thanks for the attention!

Have you tried creating an instance of the generic class with the specific T type in mind and then passing the class of that ? This way you have a new class where T is defined.

Also another option is to replace T with Any/Object and convert it after the matter