Minor Description
https://github.com/apache/kafka/pull/7309
If a processor causes a class cast exception, atm you get a bit of a cryptic error if you're not used to them, and without a context sensitive suggestion as to what could be wrong. Often these can be cause by missconfigured Serdes (defaults).
As an example of the improvement over the case exception:
{{ org.apache.kafka.streams.errors.StreamsException: Exception caught in process. taskId=0_0, processor=KSTREAM-SOURCE-0000000000, topic=streams-plaintext-input, partition=0, offset=0, stacktrace=org.apache.kafka.streams.errors.StreamsException: ClassCastException invoking Processor. Do the Processor's input types match the deserialized types? Check the Serde setup and change the default Serdes in StreamConfig or provide correct Serdes via method parameters. Deserialized input types are: key: [B, value: [B}}
{{ {{ at org.apache.kafka.streams.processor.internals.ProcessorNode.process(ProcessorNode.java:123)}}}}
{{ {{ at org.apache.kafka.streams.processor.internals.ProcessorContextImpl.forward(ProcessorContextImpl.java:201)}}}}
{{ {{ at org.apache.kafka.streams.processor.internals.ProcessorContextImpl.forward(ProcessorContextImpl.java:180)}}}}
{{ {{ at org.apache.kafka.streams.processor.internals.ProcessorContextImpl.forward(ProcessorContextImpl.java:133)}}}}
{{ {{ at org.apache.kafka.streams.processor.internals.SourceNode.process(SourceNode.java:87)}}}}
{{ {{ at org.apache.kafka.streams.processor.internals.StreamTask.process(StreamTask.java:366)}}}}
{{ {{ at org.apache.kafka.streams.TopologyTestDriver.pipeInput(TopologyTestDriver.java:419)}}}}
{{ {{ at org.apache.kafka.streams.processor.internals.ProcessorNodeTest.testTopologyLevelClassCastException(ProcessorNodeTest.java:176)}}}}
{{ {{ 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.runners.ParentRunner$3.evaluate(ParentRunner.java:305)}}}}
{{ {{ at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)}}}}
{{ {{ at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:365)}}}}
{{ {{ 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:330)}}}}
{{ {{ at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:78)}}}}
{{ {{ at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:328)}}}}
{{ {{ at org.junit.runners.ParentRunner.access$100(ParentRunner.java:65)}}}}
{{ {{ at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:292)}}}}
{{ {{ at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:305)}}}}
{{ {{ at org.junit.runners.ParentRunner.run(ParentRunner.java:412)}}}}
{{ {{ at org.junit.runner.JUnitCore.run(JUnitCore.java:137)}}}}
{{ {{ at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)}}}}
{{ {{ at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)}}}}
{{ {{ at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)}}}}
{{ {{ at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)}}}}
{{ {{ Caused by: java.lang.ClassCastException: class [B cannot be cast to class java.lang.String ([B and java.lang.String are in module java.base of loader 'bootstrap')}}}}
{{ {{ at org.apache.kafka.streams.kstream.internals.AbstractStream.lambda$withKey$1(AbstractStream.java:103)}}}}
{{ {{ at org.apache.kafka.streams.kstream.internals.KStreamFlatMapValues$KStreamFlatMapValuesProcessor.process(KStreamFlatMapValues.java:40)}}}}
{{ {{ at org.apache.kafka.streams.processor.internals.ProcessorNode.process(ProcessorNode.java:119)}}}}
{{ {
}}}
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