Got *object is not an instance of declaring class*

[hantsy]

Today I tried to create a simple Spring application using the following stack:

• Spring Boot 2.7.3
• Kotlin Coroutines
• graphql-kotlin 6.2.2

The example project codes: https://github.com/hantsy/spring-graphql-sample/tree/master/graphql-kotlin

When run the application in playground or run the simple tests, I got an exception like this.

I really do not know where is wrong, the exception does not provides any useful info.

java.lang.IllegalArgumentException: object is not an instance of declaring class
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77) ~[na:na]
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
	at java.base/java.lang.reflect.Method.invoke(Method.java:568) ~[na:na]
	at kotlin.reflect.jvm.internal.calls.CallerImpl$Method.callMethod(CallerImpl.kt:97) ~[kotlin-reflect-1.6.21.jar:1.6.21-release-334(1.6.21)]
	at kotlin.reflect.jvm.internal.calls.CallerImpl$Method$Instance.call(CallerImpl.kt:113) ~[kotlin-reflect-1.6.21.jar:1.6.21-release-334(1.6.21)]
	at kotlin.reflect.jvm.internal.KCallableImpl.call(KCallableImpl.kt:108) ~[kotlin-reflect-1.6.21.jar:1.6.21-release-334(1.6.21)]
	at com.expediagroup.graphql.generator.execution.PropertyDataFetcher.get(PropertyDataFetcher.kt:35) ~[graphql-kotlin-schema-generator-6.2.2.jar:6.2.2]
	at graphql.execution.ExecutionStrategy.fetchField(ExecutionStrategy.java:279) ~[graphql-java-18.3.jar:na]
	at graphql.execution.ExecutionStrategy.resolveFieldWithInfo(ExecutionStrategy.java:210) ~[graphql-java-18.3.jar:na]
	at graphql.execution.AsyncExecutionStrategy.execute(AsyncExecutionStrategy.java:60) ~[graphql-java-18.3.jar:na]
	at graphql.execution.ExecutionStrategy.completeValueForObject(ExecutionStrategy.java:667) ~[graphql-java-18.3.jar:na]
	at graphql.execution.ExecutionStrategy.completeValue(ExecutionStrategy.java:454) ~[graphql-java-18.3.jar:na]
	at graphql.execution.ExecutionStrategy.completeField(ExecutionStrategy.java:404) ~[graphql-java-18.3.jar:na]
	at graphql.execution.ExecutionStrategy.lambda$resolveFieldWithInfo$1(ExecutionStrategy.java:212) ~[graphql-java-18.3.jar:na]
	at java.base/java.util.concurrent.CompletableFuture.uniApplyNow(CompletableFuture.java:684) ~[na:na]
	at java.base/java.util.concurrent.CompletableFuture.uniApplyStage(CompletableFuture.java:662) ~[na:na]
	at java.base/java.util.concurrent.CompletableFuture.thenApply(CompletableFuture.java:2168) ~[na:na]
	at graphql.execution.ExecutionStrategy.resolveFieldWithInfo(ExecutionStrategy.java:211) ~[graphql-java-18.3.jar:na]
	at graphql.execution.AsyncExecutionStrategy.execute(AsyncExecutionStrategy.java:60) ~[graphql-java-18.3.jar:na]
	at graphql.execution.Execution.executeOperation(Execution.java:159) ~[graphql-java-18.3.jar:na]
	at graphql.execution.Execution.execute(Execution.java:105) ~[graphql-java-18.3.jar:na]
	at graphql.GraphQL.execute(GraphQL.java:641) ~[graphql-java-18.3.jar:na]
	at graphql.GraphQL.lambda$parseValidateAndExecute$11(GraphQL.java:561) ~[graphql-java-18.3.jar:na]
	at java.base/java.util.concurrent.CompletableFuture.uniComposeStage(CompletableFuture.java:1187) ~[na:na]
	at java.base/java.util.concurrent.CompletableFuture.thenCompose(CompletableFuture.java:2309) ~[na:na]
	at graphql.GraphQL.parseValidateAndExecute(GraphQL.java:556) ~[graphql-java-18.3.jar:na]
	at graphql.GraphQL.executeAsync(GraphQL.java:524) ~[graphql-java-18.3.jar:na]
	at com.expediagroup.graphql.server.execution.GraphQLRequestHandler.execute(GraphQLRequestHandler.kt:99) ~[graphql-kotlin-server-6.2.2.jar:6.2.2]
	at com.expediagroup.graphql.server.execution.GraphQLRequestHandler.executeRequest$suspendImpl(GraphQLRequestHandler.kt:78) ~[graphql-kotlin-server-6.2.2.jar:6.2.2]
	at com.expediagroup.graphql.server.execution.GraphQLRequestHandler.executeRequest(GraphQLRequestHandler.kt) ~[graphql-kotlin-server-6.2.2.jar:6.2.2]
	at com.expediagroup.graphql.server.execution.GraphQLServer$execute$2.invokeSuspend(GraphQLServer.kt:61) ~[graphql-kotlin-server-6.2.2.jar:6.2.2]
	at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33) ~[kotlin-stdlib-1.6.21.jar:1.6.21-release-334(1.6.21)]
	at kotlinx.coroutines.DispatchedTaskKt.resume(DispatchedTask.kt:234) ~[kotlinx-coroutines-core-jvm-1.6.4.jar:na]
	at kotlinx.coroutines.DispatchedTaskKt.resumeUnconfined(DispatchedTask.kt:190) ~[kotlinx-coroutines-core-jvm-1.6.4.jar:na]
	at kotlinx.coroutines.DispatchedTaskKt.dispatch(DispatchedTask.kt:161) ~[kotlinx-coroutines-core-jvm-1.6.4.jar:na]
	at kotlinx.coroutines.CancellableContinuationImpl.dispatchResume(CancellableContinuationImpl.kt:397) ~[kotlinx-coroutines-core-jvm-1.6.4.jar:na]
	at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl(CancellableContinuationImpl.kt:431) ~[kotlinx-coroutines-core-jvm-1.6.4.jar:na]
	at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl$default(CancellableContinuationImpl.kt:420) ~[kotlinx-coroutines-core-jvm-1.6.4.jar:na]
	at kotlinx.coroutines.CancellableContinuationImpl.resumeWith(CancellableContinuationImpl.kt:328) ~[kotlinx-coroutines-core-jvm-1.6.4.jar:na]
	at kotlinx.coroutines.reactive.AwaitKt$awaitOne$2$1.onNext(Await.kt:242) ~[kotlinx-coroutines-reactive-1.6.4.jar:na]
	at reactor.core.publisher.StrictSubscriber.onNext(StrictSubscriber.java:89) ~[reactor-core-3.4.22.jar:3.4.22]
	at reactor.core.publisher.FluxContextWrite$ContextWriteSubscriber.onNext(FluxContextWrite.java:107) ~[reactor-core-3.4.22.jar:3.4.22]
	at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:79) ~[reactor-core-3.4.22.jar:3.4.22]
	at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:79) ~[reactor-core-3.4.22.jar:3.4.22]
	at reactor.core.publisher.Operators$MonoSubscriber.complete(Operators.java:1816) ~[reactor-core-3.4.22.jar:3.4.22]
	at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:151) ~[reactor-core-3.4.22.jar:3.4.22]
	at reactor.core.publisher.FluxContextWrite$ContextWriteSubscriber.onNext(FluxContextWrite.java:107) ~[reactor-core-3.4.22.jar:3.4.22]
	at reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onNext(FluxMapFuseable.java:299) ~[reactor-core-3.4.22.jar:3.4.22]
	at reactor.core.publisher.FluxFilterFuseable$FilterFuseableConditionalSubscriber.onNext(FluxFilterFuseable.java:337) ~[reactor-core-3.4.22.jar:3.4.22]
	at reactor.core.publisher.Operators$MonoSubscriber.complete(Operators.java:1816) ~[reactor-core-3.4.22.jar:3.4.22]
	at reactor.core.publisher.MonoCollect$CollectSubscriber.onComplete(MonoCollect.java:160) ~[reactor-core-3.4.22.jar:3.4.22]
	at reactor.core.publisher.FluxMap$MapSubscriber.onComplete(FluxMap.java:144) ~[reactor-core-3.4.22.jar:3.4.22]
	at reactor.core.publisher.FluxPeek$PeekSubscriber.onComplete(FluxPeek.java:260) ~[reactor-core-3.4.22.jar:3.4.22]
	at reactor.core.publisher.FluxMap$MapSubscriber.onComplete(FluxMap.java:144) ~[reactor-core-3.4.22.jar:3.4.22]
	at reactor.netty.channel.FluxReceive.onInboundComplete(FluxReceive.java:400) ~[reactor-netty-core-1.0.22.jar:1.0.22]
	at reactor.netty.channel.ChannelOperations.onInboundComplete(ChannelOperations.java:419) ~[reactor-netty-core-1.0.22.jar:1.0.22]
	at reactor.netty.http.server.HttpServerOperations.onInboundNext(HttpServerOperations.java:600) ~[reactor-netty-http-1.0.22.jar:1.0.22]
	at reactor.netty.channel.ChannelOperationsHandler.channelRead(ChannelOperationsHandler.java:93) ~[reactor-netty-core-1.0.22.jar:1.0.22]
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) ~[netty-transport-4.1.79.Final.jar:4.1.79.Final]
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) ~[netty-transport-4.1.79.Final.jar:4.1.79.Final]
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357) ~[netty-transport-4.1.79.Final.jar:4.1.79.Final]
	at reactor.netty.http.server.HttpTrafficHandler.channelRead(HttpTrafficHandler.java:258) ~[reactor-netty-http-1.0.22.jar:1.0.22]
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) ~[netty-transport-4.1.79.Final.jar:4.1.79.Final]
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) ~[netty-transport-4.1.79.Final.jar:4.1.79.Final]
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357) ~[netty-transport-4.1.79.Final.jar:4.1.79.Final]
	at io.netty.channel.CombinedChannelDuplexHandler$DelegatingChannelHandlerContext.fireChannelRead(CombinedChannelDuplexHandler.java:436) ~[netty-transport-4.1.79.Final.jar:4.1.79.Final]
	at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:327) ~[netty-codec-4.1.79.Final.jar:4.1.79.Final]
	at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:299) ~[netty-codec-4.1.79.Final.jar:4.1.79.Final]
	at io.netty.channel.CombinedChannelDuplexHandler.channelRead(CombinedChannelDuplexHandler.java:251) ~[netty-transport-4.1.79.Final.jar:4.1.79.Final]
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) ~[netty-transport-4.1.79.Final.jar:4.1.79.Final]
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) ~[netty-transport-4.1.79.Final.jar:4.1.79.Final]
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357) ~[netty-transport-4.1.79.Final.jar:4.1.79.Final]
	at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1410) ~[netty-transport-4.1.79.Final.jar:4.1.79.Final]
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) ~[netty-transport-4.1.79.Final.jar:4.1.79.Final]
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) ~[netty-transport-4.1.79.Final.jar:4.1.79.Final]
	at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:919) ~[netty-transport-4.1.79.Final.jar:4.1.79.Final]
	at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:166) ~[netty-transport-4.1.79.Final.jar:4.1.79.Final]
	at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:722) ~[netty-transport-4.1.79.Final.jar:4.1.79.Final]
	at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:658) ~[netty-transport-4.1.79.Final.jar:4.1.79.Final]
	at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:584) ~[netty-transport-4.1.79.Final.jar:4.1.79.Final]
	at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:496) ~[netty-transport-4.1.79.Final.jar:4.1.79.Final]
	at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:997) ~[netty-common-4.1.79.Final.jar:4.1.79.Final]
	at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) ~[netty-common-4.1.79.Final.jar:4.1.79.Final]
	at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) ~[netty-common-4.1.79.Final.jar:4.1.79.Final]
	at java.base/java.lang.Thread.run(Thread.java:833) ~[na:na]

2022-08-28 18:09:08.842  WARN 87560 --- [-4 @coroutine#3] n.g.e.SimpleDataFetcherExceptionHandler  : Exception while fetching data (/allPosts/title) : object is not an instance of declaring class

[samuelAndalon]

Hello again, it would be very helpful if you can provide a query example and a very basic project setup with clear instructions for reproduce the issue.

judging by the error, looks like you are returning a type that does not match what your schema generator output.

[hantsy]

Thanks, I think it is caused by Flow Subscription Hooks, and tried to use Flow in a Query. See ##1531

When I change the Query to return list instead, it works well.