Scala案例类中init方法的java.lang.NoSuchMethodException

Question

我正在编写一个 Apache Flink 流应用程序，该应用程序反序列化从 Kafka 总线读取的数据（Avro 格式）（有关here 的更多详细信息）。数据被反序列化为 Scala 案例类。当我运行程序并收到来自 Kafka 的第一条消息时出现异常

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

Scala 案例类非常简单：

package org.myorg.quickstart


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

不确定为什么案例类需要“init”方法。如何做到这一点的例子？我应该使用案例类以外的其他数据结构吗？

Answer 1

Avro 序列化程序或更具体的SpecificData 要求目标类型具有默认构造函数（没有参数的构造函数）。否则 Avro 无法实例化目标类型的对象。

尝试通过添加默认构造函数

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