java.lang.NoSuchMethodException for init method in Scala case class

问题

I am writing an Apache Flink streaming application that deserializes data (Avro format) read off a Kafka bus (more details on here). The data is being deserialized into a Scala case class. I am getting an exception when i run the program and it received the first message from Kafka

Exception in thread "main" org.apache.flink.runtime.client.JobExecutionException: java.lang.RuntimeException: java.lang.NoSuchMethodException: org.myorg.quickstart.DeviceData.<init>()
    at org.apache.flink.runtime.minicluster.MiniCluster.executeJobBlocking(MiniCluster.java:625)
    at org.apache.flink.streaming.api.environment.LocalStreamEnvironment.execute(LocalStreamEnvironment.java:121)
    at org.apache.flink.streaming.api.scala.StreamExecutionEnvironment.execute(StreamExecutionEnvironment.scala:654)
    at org.myorg.quickstart.StreamingKafkaClient$.main(StreamingKafkaClient.scala:26)
    at org.myorg.quickstart.StreamingKafkaClient.main(StreamingKafkaClient.scala)
Caused by: java.lang.RuntimeException: java.lang.NoSuchMethodException: org.myorg.quickstart.DeviceData.<init>()
    at org.apache.avro.specific.SpecificData.newInstance(SpecificData.java:353)
    at org.apache.avro.specific.SpecificData.newRecord(SpecificData.java:369)
    at org.apache.avro.reflect.ReflectData.newRecord(ReflectData.java:901)
    at org.apache.avro.generic.GenericDatumReader.readRecord(GenericDatumReader.java:212)
    at org.apache.avro.generic.GenericDatumReader.readWithoutConversion(GenericDatumReader.java:175)
    at org.apache.avro.generic.GenericDatumReader.read(GenericDatumReader.java:153)
    at org.apache.avro.generic.GenericDatumReader.read(GenericDatumReader.java:145)
    at org.myorg.quickstart.AvroDeserializationSchema.deserialize(AvroDeserializationSchema.scala:20)
    at org.apache.flink.streaming.util.serialization.KeyedDeserializationSchemaWrapper.deserialize(KeyedDeserializationSchemaWrapper.java:44)
    at org.apache.flink.streaming.connectors.kafka.internal.Kafka09Fetcher.runFetchLoop(Kafka09Fetcher.java:142)
    at org.apache.flink.streaming.connectors.kafka.FlinkKafkaConsumerBase.run(FlinkKafkaConsumerBase.java:738)
    at org.apache.flink.streaming.api.operators.StreamSource.run(StreamSource.java:87)
    at org.apache.flink.streaming.api.operators.StreamSource.run(StreamSource.java:56)
    at org.apache.flink.streaming.runtime.tasks.SourceStreamTask.run(SourceStreamTask.java:99)
    at org.apache.flink.streaming.runtime.tasks.StreamTask.invoke(StreamTask.java:306)
    at org.apache.flink.runtime.taskmanager.Task.run(Task.java:703)
    at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.NoSuchMethodException: org.myorg.quickstart.DeviceData.<init>()
    at java.lang.Class.getConstructor0(Class.java:3082)
    at java.lang.Class.getDeclaredConstructor(Class.java:2178)
    at org.apache.avro.specific.SpecificData.newInstance(SpecificData.java:347)
    ... 16 more

Process finished with exit code 1

The Scala case class is very simple:

package org.myorg.quickstart


/** Case class to hold the Device data. */
case class DeviceData(deviceId: String,
                    sw_version: String,
                    timestamp: String,
                    reading: Double
                   ) 

Not sure why an "init" method is needed for the case class. An examples of how to do this? Should i be using a different data structure other than the case class?

回答1:

The Avro serializer or more specifically the SpecificData requires the target type to have a default constructor (constructor with no arguments). Otherwise Avro cannot instantiate an object of the target type.

Try to add a default constructor via

case class DeviceData(
    deviceId: String,
    sw_version: String,
    timestamp: String,
    reading: Double) {
  def this() = this("default", "default", "default", 0)
} 

来源：https://stackoverflow.com/questions/51129809/java-lang-nosuchmethodexception-for-init-method-in-scala-case-class