我尝试将JSON消息发送到Kafka，然后将它们插入Cassandra但是我收到以下错误：
java.lang.ClassCastException: java.util.HashMap cannot be cast to org.apache.kafka.connect.data.Struct
at com.datamountaineer.streamreactor.connect.schemas.ConverterUtil$class.convert(ConverterUtil.scala:59)
at com.datamountaineer.streamreactor.connect.cassandra.sink.CassandraJsonWriter.convert(CassandraJsonWriter.scala:40)
at com.datamountaineer.streamreactor.connect.cassandra.sink.CassandraJsonWriter.com$datamountaineer$streamreactor$connect$cassandra$sink$CassandraJsonWriter$$toJson(CassandraJsonWriter.scala:154)
at com.datamountaineer.streamreactor.connect.cassandra.sink.CassandraJsonWriter$$anonfun$3$$anonfun$apply$1.apply$mcV$sp(CassandraJsonWriter.scala:127)
at com.datamountaineer.streamreactor.connect.cassandra.sink.CassandraJsonWriter$$anonfun$3$$anonfun$apply$1.apply(CassandraJsonWriter.scala:125)
at com.datamountaineer.streamreactor.connect.cassandra.sink.CassandraJsonWriter$$anonfun$3$$anonfun$apply$1.apply(CassandraJsonWriter.scala:125)
at com.datamountaineer.streamreactor.connect.concurrent.ExecutorExtension$RunnableWrapper$$anon$1.run(ExecutorExtension.scala:30)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:748)
[2017-06-21 18:47:44,645] ERROR Encountered error java.util.HashMap cannot be cast to org.apache.kafka.connect.data.Struct (com.datamountaineer.streamreactor.connect.cassandra.sink.CassandraJsonWriter:46)
[2017-06-21 18:47:44,645] ERROR Task cassandra-connector-cassandra-Topic-1 threw an uncaught and unrecoverable exception (org.apache.kafka.connect.runtime.WorkerSinkTask:449)
java.lang.RuntimeException: java.lang.ClassCastException: java.util.HashMap cannot be cast to org.apache.kafka.connect.data.Struct
at com.datamountaineer.streamreactor.connect.errors.ThrowErrorPolicy.handle(ErrorPolicy.scala:58)
at com.datamountaineer.streamreactor.connect.errors.ErrorHandler$class.handleError(ErrorHandler.scala:67)
at com.datamountaineer.streamreactor.connect.errors.ErrorHandler$class.handleTry(ErrorHandler.scala:48)
at com.datamountaineer.streamreactor.connect.cassandra.sink.CassandraJsonWriter.handleTry(CassandraJsonWriter.scala:40)
at com.datamountaineer.streamreactor.connect.cassandra.sink.CassandraJsonWriter.insert(CassandraJsonWriter.scala:144)
at com.datamountaineer.streamreactor.connect.cassandra.sink.CassandraJsonWriter.write(CassandraJsonWriter.scala:104)
at com.datamountaineer.streamreactor.connect.cassandra.sink.CassandraSinkTask$$anonfun$put$2.apply(CassandraSinkTask.scala:72)
at com.datamountaineer.streamreactor.connect.cassandra.sink.CassandraSinkTask$$anonfun$put$2.apply(CassandraSinkTask.scala:72)
at scala.Option.foreach(Option.scala:257)
at com.datamountaineer.streamreactor.connect.cassandra.sink.CassandraSinkTask.put(CassandraSinkTask.scala:72)
at org.apache.kafka.connect.runtime.WorkerSinkTask.deliverMessages(WorkerSinkTask.java:429)
at org.apache.kafka.connect.runtime.WorkerSinkTask.poll(WorkerSinkTask.java:250)
at org.apache.kafka.connect.runtime.WorkerSinkTask.iteration(WorkerSinkTask.java:179)
at org.apache.kafka.connect.runtime.WorkerSinkTask.execute(WorkerSinkTask.java:148)
at org.apache.kafka.connect.runtime.WorkerTask.doRun(WorkerTask.java:139)
at org.apache.kafka.connect.runtime.WorkerTask.run(WorkerTask.java:182)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.ClassCastException: java.util.HashMap cannot be cast to org.apache.kafka.connect.data.Struct
at com.datamountaineer.streamreactor.connect.schemas.ConverterUtil$class.convert(ConverterUtil.scala:59)
at com.datamountaineer.streamreactor.connect.cassandra.sink.CassandraJsonWriter.convert(CassandraJsonWriter.scala:40)
at com.datamountaineer.streamreactor.connect.cassandra.sink.CassandraJsonWriter.com$datamountaineer$streamreactor$connect$cassandra$sink$CassandraJsonWriter$$toJson(CassandraJsonWriter.scala:154)
at com.datamountaineer.streamreactor.connect.cassandra.sink.CassandraJsonWriter$$anonfun$3$$anonfun$apply$1.apply$mcV$sp(CassandraJsonWriter.scala:127)
at com.datamountaineer.streamreactor.connect.cassandra.sink.CassandraJsonWriter$$anonfun$3$$anonfun$apply$1.apply(CassandraJsonWriter.scala:125)
at com.datamountaineer.streamreactor.connect.cassandra.sink.CassandraJsonWriter$$anonfun$3$$anonfun$apply$1.apply(CassandraJsonWriter.scala:125)
at com.datamountaineer.streamreactor.connect.concurrent.ExecutorExtension$RunnableWrapper$$anon$1.run(ExecutorExtension.scala:30)
... 3 more
我按照描述的方法描述了连接Kafka和MongoDB的方法：https://github.com/startappdev/kafka-connect-mongodb。我使用com.startapp.data.MongoSinkConnector发行版中的Cassandra com.datamountaineer.streamreactor.connect.cassandra.sink.CassandraSinkConnector连接器而不是连接器stream-reactor-release-0.2-3.0.0。 Kafka和Cassandra版本如下：
0.10.2.1 3.10 Kafka用于Confluent平台的开源版本，版本3.2.1.。
到目前为止，我发现了一个类似的问题：Kafka connect with Cassandra Connector但是这个问题没有得到答复，并且讨论了这个问题：https://github.com/datamountaineer/stream-reactor/issues/97。
我想知道是否可以使用这些设置通过Kafka向Cassandra发送JSON数据，或者Kafka和Cassandra之间是否还有其他可以执行此任务的连接器。据我所知，可以发送Avro消息，但我想知道是否也可以像使用MongoDB而不是Cassandra的方法一样发送JSON消息。