Matthias J. Sax created KAFKA-10036:
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Summary: Improve error message is user violates `Supplier` pattern
Key: KAFKA-10036
URL: https://issues.apache.org/jira/browse/KAFKA-10036
Project: Kafka
Issue Type: Improvement
Components: streams
Reporter: Matthias J. Sax
Using the Processor API, uses need to pass in a `ProcessorSupplier` that needs
to return new `Processor` instance each time `get()` is called.
Users violate this rule on a regular basis and return the same instance on
`get()`. This mistake leads to a (not very informative) `NullPointerException`
during runtime. (Cf:
[https://stackoverflow.com/questions/61790984/kafka-stream-forward-method-throwing-nullpointerexception-because-processornode/61978396)]
We could improve the error message by checking if `currentNode()` returns
`null`
([https://github.com/apache/kafka/blob/2.4/streams/src/main/java/org/apache/kafka/streams/processor/internals/ProcessorContextImpl.java#L183)]
and throw an informative error message for this case.
Furthermore, we could do a "sanity" check within `KafkaStreams` constructor
before we start the process threads: we get all `Suppliers` for the `Topology`
and call `get()` two times on each supplier to compare if the returned object
references are different – if they are the same, we throw an informative error
message.
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