java.lang.ClassCastException: class kotlin.Result cannot be cast to class java.lang.String (kotlin.Result is in unnamed module of loader 'app'; java.lang.String is in module java.base of loader 'bootstrap')

[papo2608]

With following sample code I get above error when kotlin.Result is used with a suspend function and a runtime exception is thrown in the Retrofit call factory - this works though with e.g. sealed or data classes.

class CrashingResultCallAdapterTest {

  @get:Rule val server = MockWebServer()

  @Test
  fun crash(): Unit = runBlocking {
    val retrofit = Retrofit.Builder()
      .baseUrl(server.url("/"))
      .addConverterFactory(StringConverterFactory())
      .addCallAdapterFactory(SuspendResultCallAdapterFactory())
      .callFactory { throw IllegalStateException("some error") }
      .build()
    val service = retrofit.create(ResultService::class.java)
    server.enqueue(
      MockResponse()
        .setResponseCode(200)
        .setHeader("content-type", "text/plain")
        .setBody("bar")
    )

    assertThat(service.fooBarResult().getOrThrow()).isEqualTo("bar")
  }

  @Test
  fun success(): Unit = runBlocking {
    val retrofit = Retrofit.Builder()
      .baseUrl(server.url("/"))
      .addConverterFactory(StringConverterFactory())
      .addCallAdapterFactory(SuspendResultCallAdapterFactory())
      .build()
    val service = retrofit.create(ResultService::class.java)
    server.enqueue(
      MockResponse()
        .setResponseCode(200)
        .setHeader("content-type", "text/plain")
        .setBody("bar")
    )

    assertThat(service.fooBarResult().getOrThrow()).isEqualTo("bar")

    server.enqueue(
      MockResponse()
        .setResponseCode(404)
        .setHeader("content-type", "text/plain")
        .setBody("barerror")
    )

    assertThat(service.fooBarResult().exceptionOrNull()).isInstanceOf(HttpException::class.java)

    server.enqueue(MockResponse().setSocketPolicy(DISCONNECT_AFTER_REQUEST))

    assertThat(service.fooBarResult().exceptionOrNull()).isInstanceOf(SocketTimeoutException::class.java)
  }

  private interface ResultService {

    @GET("/")
    suspend fun fooBarResult(): Result<String>

  }

  private class StringConverterFactory : Converter.Factory() {

    override fun requestBodyConverter(
      type: Type, parameterAnnotations: Array<out Annotation>, methodAnnotations: Array<out Annotation>,
      retrofit: Retrofit
    ): Converter<*, RequestBody> {
      return Converter<String, RequestBody> { value -> value.toRequestBody("text/plain".toMediaType()) }
    }

    override fun responseBodyConverter(
      type: Type, annotations: Array<out Annotation>, retrofit: Retrofit
    ): Converter<ResponseBody, *> {
      return Converter<ResponseBody, String> { value -> value.string() }
    }

  }

  private class SuspendResultCallAdapterFactory : CallAdapter.Factory() {

    override fun get(
      returnType: Type,
      annotations: Array<out Annotation>,
      retrofit: Retrofit
    ): CallAdapter<*, *>? {

      if (getRawType(returnType) != Call::class.java) return null

      if (returnType !is ParameterizedType) return null

      val resultType: Type = getParameterUpperBound(0, returnType)
      if (getRawType(resultType) != Result::class.java || resultType !is ParameterizedType
      ) return null

      val delegate: CallAdapter<*, *> = retrofit.nextCallAdapter(this, returnType, annotations)

      return CatchingCallAdapter(delegate)
    }

    private class CatchingCallAdapter(
      private val delegate: CallAdapter<*, *>,
    ) : CallAdapter<Any, Call<Result<*>>> {

      override fun responseType(): Type = delegate.responseType()

      override fun adapt(call: Call<Any>): Call<Result<*>> = CatchingCall(call)

    }

    private class CatchingCall(
      private val delegate: Call<Any>,
    ) : Call<Result<*>> {

      override fun enqueue(callback: Callback<Result<*>>) = delegate.enqueue(object : Callback<Any> {
        override fun onResponse(call: Call<Any>, response: Response<Any>) {
          if (response.isSuccessful) {
            val body = response.body()
            callback.onResponse(this@CatchingCall, Response.success(Result.success(body)))
          } else {
            val throwable = HttpException(response)
            callback.onResponse(this@CatchingCall, Response.success(Result.failure<Any>(throwable)))
          }
        }

        override fun onFailure(call: Call<Any>, t: Throwable) {
          callback.onResponse(this@CatchingCall, Response.success(Result.failure<Any>(t)))
        }
      })

      override fun clone(): Call<Result<*>> = CatchingCall(delegate)

      override fun execute(): Response<Result<*>> =
        throw UnsupportedOperationException("Suspend function should not be blocking.")

      override fun isExecuted(): Boolean = delegate.isExecuted

      override fun cancel(): Unit = delegate.cancel()

      override fun isCanceled(): Boolean = delegate.isCanceled

      override fun request(): Request = delegate.request()

      override fun timeout(): Timeout = delegate.timeout()

    }
  }

}

I'm not sure what the root cause here is.

[qwwdfsad]

Could you please specify Kotlin version you are using and provide the whole stacktrace?

[papo2608]

Could you please specify Kotlin version you are using and provide the whole stacktrace?

Sry, here we go:
Kotlin: 1.7.10
Kotlin coroutines: 1.6.4

(Retrofit 2.9.0 and OkHttp 4.10.0)

[ivakub]

@papo2608 Can you attach a sample project and the whole stacktrace?

[papo2608]

@ivakub sure

class kotlin.Result cannot be cast to class java.lang.String (kotlin.Result is in unnamed module of loader 'app'; java.lang.String is in module java.base of loader 'bootstrap')
java.lang.ClassCastException: class kotlin.Result cannot be cast to class java.lang.String (kotlin.Result is in unnamed module of loader 'app'; java.lang.String is in module java.base of loader 'bootstrap')
	at com.example.myapplication.CrashingResultCallAdapterTest$crash$1.invokeSuspend(CrashingResultCallAdapterTest.kt:48)
	at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
	at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:106)
	at kotlinx.coroutines.EventLoopImplBase.processNextEvent(EventLoop.common.kt:284)
	at kotlinx.coroutines.BlockingCoroutine.joinBlocking(Builders.kt:85)
	at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking(Builders.kt:59)
	at kotlinx.coroutines.BuildersKt.runBlocking(Unknown Source)
	at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking$default(Builders.kt:38)
	at kotlinx.coroutines.BuildersKt.runBlocking$default(Unknown Source)
	at com.example.myapplication.CrashingResultCallAdapterTest.crash(CrashingResultCallAdapterTest.kt:33)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.rules.ExternalResource$1.evaluate(ExternalResource.java:54)
	at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
	at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:366)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:103)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:63)
	at org.junit.runners.ParentRunner$4.run(ParentRunner.java:331)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:79)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:329)
	at org.junit.runners.ParentRunner.access$100(ParentRunner.java:66)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:293)
	at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:413)
	at org.gradle.api.internal.tasks.testing.junit.JUnitTestClassExecutor.runTestClass(JUnitTestClassExecutor.java:110)
	at org.gradle.api.internal.tasks.testing.junit.JUnitTestClassExecutor.execute(JUnitTestClassExecutor.java:58)
	at org.gradle.api.internal.tasks.testing.junit.JUnitTestClassExecutor.execute(JUnitTestClassExecutor.java:38)
	at org.gradle.api.internal.tasks.testing.junit.AbstractJUnitTestClassProcessor.processTestClass(AbstractJUnitTestClassProcessor.java:62)
	at org.gradle.api.internal.tasks.testing.SuiteTestClassProcessor.processTestClass(SuiteTestClassProcessor.java:51)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
	at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:36)
	at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:24)
	at org.gradle.internal.dispatch.ContextClassLoaderDispatch.dispatch(ContextClassLoaderDispatch.java:33)
	at org.gradle.internal.dispatch.ProxyDispatchAdapter$DispatchingInvocationHandler.invoke(ProxyDispatchAdapter.java:94)
	at com.sun.proxy.$Proxy2.processTestClass(Unknown Source)
	at org.gradle.api.internal.tasks.testing.worker.TestWorker$2.run(TestWorker.java:176)
	at org.gradle.api.internal.tasks.testing.worker.TestWorker.executeAndMaintainThreadName(TestWorker.java:129)
	at org.gradle.api.internal.tasks.testing.worker.TestWorker.execute(TestWorker.java:100)
	at org.gradle.api.internal.tasks.testing.worker.TestWorker.execute(TestWorker.java:60)
	at org.gradle.process.internal.worker.child.ActionExecutionWorker.execute(ActionExecutionWorker.java:56)
	at org.gradle.process.internal.worker.child.SystemApplicationClassLoaderWorker.call(SystemApplicationClassLoaderWorker.java:133)
	at org.gradle.process.internal.worker.child.SystemApplicationClassLoaderWorker.call(SystemApplicationClassLoaderWorker.java:71)
	at worker.org.gradle.process.internal.worker.GradleWorkerMain.run(GradleWorkerMain.java:69)
	at worker.org.gradle.process.internal.worker.GradleWorkerMain.main(GradleWorkerMain.java:74)

MyApplication.zip

[ivakub]

@papo2608 Thank you! This is a bug in the backend. I created an issue — https://youtrack.jetbrains.com/issue/KT-53559/JVM-ClassCastException-class-kotlinResult-cannot-be-cast-to-class-javalangString-with-Retrofit. Feel free to follow it.

[rohangaurat]

how to fix this the issue

[wakaztahir]

In my code I was getting this issue, My functions were suspend inline functions present in another module, After removing inline My code works now !