Periodic NPE In Kafka Streams Processor Context

问题

Using kafka-streams 0.10.0.0, I am periodically seeing a null pointer exception in the StreamTask when forwarding a message. It varies between 10% to 50% of the invocations. The NPE occurs in this method:

public <K, V> void forward(K key, V value) {
    ProcessorNode thisNode = currNode;
    try {
        for (ProcessorNode childNode : (List<ProcessorNode<K, V>>) thisNode.children()) {
            currNode = childNode;
            childNode.process(key, value);
        }
    } finally {
        currNode = thisNode;
    }
}

It seems that in some cases, the thisNode field is null. Any idea what might be causing this ? The stack trace is below.

[ERROR] 2016-08-21 14:50:39.288 [StreamThread-1] StreamedMetricMeter - Forwarding failed
java.lang.NullPointerException
    at org.apache.kafka.streams.processor.internals.StreamTask.forward(StreamTask.java:336) ~[kafka-streams-0.10.0.0.jar:?]
    at org.apache.kafka.streams.processor.internals.ProcessorContextImpl.forward(ProcessorContextImpl.java:187) ~[kafka-streams-0.10.0.0.jar:?]
    at com.heliosapm.streams.metrics.processors.AbstractStreamedMetricProcessor.forward(AbstractStreamedMetricProcessor.java:552) [classes/:?]
    at com.heliosapm.streams.metrics.processors.impl.StreamedMetricMeter.doProcess(StreamedMetricMeter.java:89) [classes/:?]
    at com.heliosapm.streams.metrics.processors.impl.StreamedMetricMeter.doProcess(StreamedMetricMeter.java:1) [classes/:?]
    at com.heliosapm.streams.metrics.processors.AbstractStreamedMetricProcessor.process(AbstractStreamedMetricProcessor.java:166) [classes/:?]
    at org.apache.kafka.streams.processor.internals.ProcessorNode.process(ProcessorNode.java:68) [kafka-streams-0.10.0.0.jar:?]
    at org.apache.kafka.streams.processor.internals.StreamTask.forward(StreamTask.java:338) [kafka-streams-0.10.0.0.jar:?]
    at org.apache.kafka.streams.processor.internals.ProcessorContextImpl.forward(ProcessorContextImpl.java:187) [kafka-streams-0.10.0.0.jar:?]
    at org.apache.kafka.streams.processor.internals.SourceNode.process(SourceNode.java:64) [kafka-streams-0.10.0.0.jar:?]
    at org.apache.kafka.streams.processor.internals.StreamTask.process(StreamTask.java:174) [kafka-streams-0.10.0.0.jar:?]
    at org.apache.kafka.streams.processor.internals.StreamThread.runLoop(StreamThread.java:320) [kafka-streams-0.10.0.0.jar:?]
    at org.apache.kafka.streams.processor.internals.StreamThread.run(StreamThread.java:218) [kafka-streams-0.10.0.0.jar:?]

回答1:

The problem was that my ProcessorSuppliers were returning the same instance of the Processor for every call to get. In turn, the Kafka Streams engine was attempting to create multiple processor instances, which I have no doubt created a multi-threaded dumpster fire. Note to the similarly unwary.... ProcessorSupplier.get() should return a new instance of a processor on each call.

来源：https://stackoverflow.com/questions/39067846/periodic-npe-in-kafka-streams-processor-context