在網路上都只看到一些Java生產STRING類型的消息。
依照Java的producer類別來看，是可以自訂發送訊息的類型，例如producer.send(new KeyedMessage<String, HashMap<String , String>>(topic,message);
可是這樣運行會報錯，報錯如下，請求高人解答：
Exception in thread "Thread-4" java.lang.ClassCastException: java.util.HashMap cannot be cast to java.lang.String

at kafka.serializer.StringEncoder.toBytes(Encoder.scala:46)
at kafka.producer.async.DefaultEventHandler$$anonfun$serialize.apply(DefaultEventHandler.scala:130)
at kafka.producer.async.DefaultEventHandler$$anonfun$serialize.apply(DefaultEventHandler.scala:125)
at scala.collection.IndexedSeqOptimized$class.foreach(IndexedSeqOptimized.scala:33)
at scala.collection.mutable.WrappedArray.foreach(WrappedArray.scala:34)
at kafka.producer.async.DefaultEventHandler.serialize(DefaultEventHandler.scala:125)
at kafka.producer.async.DefaultEventHandler.handle(DefaultEventHandler.scala:52)
at kafka.producer.Producer.send(Producer.scala:77)
at kafka.javaapi.producer.Producer.send(Producer.scala:33)
at com.Model.Producer.kafkaProducer.run(kafkaProducer.java:35)