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https://issues.apache.org/jira/browse/KAFKA-10036?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17119271#comment-17119271
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Alex Sun commented on KAFKA-10036:
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PR created: [https://github.com/apache/kafka/pull/8752]
> Improve error message if 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
> Assignee: Alex Sun
> Priority: Minor
> Labels: newbie
>
> Using the Processor API, users 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.
> We should improve the JavaDocs, too:
> [https://github.com/apache/kafka/blob/2.4/streams/src/main/java/org/apache/kafka/streams/processor/ProcessorSupplier.java#L34]
> (also for `Transformer` et al. – it seems to be too subtle what "new" means.
> Similarly for
> [https://github.com/apache/kafka/blob/2.4/streams/src/main/java/org/apache/kafka/streams/Topology.java]
> and
> [https://github.com/apache/kafka/blob/2.4/streams/src/main/java/org/apache/kafka/streams/kstream/KStream.java]
> (`process()`, `transform()` etc.)
> Furthermore, we should improve the docs: to explain the supplier pattern
> explicitly:
> [https://kafka.apache.org/25/documentation/streams/developer-guide/processor-api.html]
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