scala - org.apache.spark.SparkException: Task not serializable

Question

This is a working code example:

JavaPairDStream<String, String> messages = KafkaUtils.createStream(javaStreamingContext, zkQuorum, group, topicMap);
messages.print();
JavaDStream<String> lines = messages.map(new Function<Tuple2<String, String>, String>() {
    @Override
    public String call(Tuple2<String, String> tuple2) {
        return tuple2._2();
    }
});

I get the below error:

ERROR:
org.apache.spark.SparkException: Task not serializable
    at org.apache.spark.util.ClosureCleaner$.ensureSerializable(ClosureCleaner.scala:166)
    at org.apache.spark.util.ClosureCleaner$.clean(ClosureCleaner.scala:158)
    at org.apache.spark.SparkContext.clean(SparkContext.scala:1435)
    at org.apache.spark.streaming.dstream.DStream.map(DStream.scala:438)
    at org.apache.spark.streaming.api.java.JavaDStreamLike$class.map(JavaDStreamLike.scala:140)
    at org.apache.spark.streaming.api.java.JavaPairDStream.map(JavaPairDStream.scala:46)

Answer 1

Since you're defining your map function using an anonymous inner class, the containing class must also be Serializable. Define your map function as a separate class or make it a static inner class. From the Java documentation (http://docs.oracle.com/javase/8/docs/platform/serialization/spec/serial-arch.html):

Note - Serialization of inner classes (i.e., nested classes that are not static member classes), including local and anonymous classes, is strongly discouraged for several reasons. Because inner classes declared in non-static contexts contain implicit non-transient references to enclosing class instances, serializing such an inner class instance will result in serialization of its associated outer class instance as well.