如何用Java向kafka发送json数据 报错

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在网上都只看到一些Java生产STRING类型的消息。
按照Java的producer类来看，是可以自定义发送消息的类型，比如 producer.send(new KeyedMessage<String, HashMap<String , String>>(topic,message);
可是这样运行会报错，报错如下，请求高人解答：
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$$anonfun$serialize$1.apply(DefaultEventHandler.scala:130) at kafka.producer.async.DefaultEventHandler$$anonfun$serialize$1.apply(DefaultEventHandler.scala:125) at scala.collection.IndexedSeqOptimized$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)