Shows using a thread-unsafe Function in a Spark Closure

SparkThreadUnsafe.java

public JavaDStream<Long> createLongStream(JavaDStream<Object> inputDStream) {
    return inputDStream.map(new ThreadUnsafeMapper());
  }

  private static class ThreadUnsafeMapper implements Function<Object, Long> {
    private static final long serialVersionUID = 1L;
    private long count = 0;

    // definitely not a thread-safe function
    @Override
    public Long call(Object ignored) throws Exception {
      return ++count;
    }
  }
/*
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Copyright (c) 2016 Matt Dailey

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 */

You certainly need to avoid stateful functions. They don't make sense in a distributed system as closures are serialized and sent to executors to be used. And at each executor, they may be used in parallel (for the number of CPUs assigned to the task at that executor).