title: Flink with Confluent Kafka schema registry
body: <p>I am trying to write to Confluent kafka with schema registry from Flink using FlinkKafkaProducer10. Below error is produced. 
  I created custom schema serializer see ConfluentAvroSerializationSchema class. Code compiles but produces runtime error. Complete code example that produces the error is on <a href="https://github.com/dmiljkovic/test-flink-schema-registry" rel="nofollow noreferrer">https://github.com/dmiljkovic/test-flink-schema-registry</a>. All resources are mocked, kafka cluster and schema registry. Code is actually test case.</p>
  
  <pre><code>import io.confluent.kafka.serializers.KafkaAvroSerializer;
  import org.apache.flink.streaming.util.serialization.SerializationSchema;
  
  public class ConfluentAvroSerializationSchema&lt;T&gt; implements SerializationSchema&lt;T&gt; {
  private static final long serialVersionUID = 1L;
  private final String topic;
  private final KafkaAvroSerializer avroSerializer;
  
  public ConfluentAvroSerializationSchema(String topic, KafkaAvroSerializer avroSerializer) {
      this.topic =topic;
      this.avroSerializer = avroSerializer;
  }
  
  @Override
  public byte[] serialize(T obj) {
      return avroSerializer.serialize(topic, obj);
  }
  }
  
      //serialize avro
      KafkaAvroSerializer avroSerializer = new KafkaAvroSerializer(schemaRegistry);
      avroSerializer.configure(new HashMap(registryProp), false);
      ConfluentAvroSerializationSchema ser =
              new ConfluentAvroSerializationSchema&lt;TestRecordAvro&gt;(topic, avroSerializer);
  
      //write to kafka
      FlinkKafkaProducer010.writeToKafkaWithTimestamps(ds, topic, ser, flinkProp);
  </code></pre>
  
  <hr>
  
  <pre><code>org.apache.flink.api.common.InvalidProgramException: Object org.apache.flink.streaming.util.serialization.KeyedSerializationSchemaWrapper@6e28bb87 is not serializable
  
  at org.apache.flink.api.java.ClosureCleaner.ensureSerializable(ClosureCleaner.java:109)
  at org.apache.flink.streaming.connectors.kafka.FlinkKafkaProducerBase.&lt;init&gt;(FlinkKafkaProducerBase.java:145)
  at org.apache.flink.streaming.connectors.kafka.FlinkKafkaProducer09.&lt;init&gt;(FlinkKafkaProducer09.java:130)
  at org.apache.flink.streaming.connectors.kafka.FlinkKafkaProducer010.&lt;init&gt;(FlinkKafkaProducer010.java:227)
  at org.apache.flink.streaming.connectors.kafka.FlinkKafkaProducer010.writeToKafkaWithTimestamps(FlinkKafkaProducer010.java:137)
  at org.apache.flink.streaming.connectors.kafka.FlinkKafkaProducer010.writeToKafkaWithTimestamps(FlinkKafkaProducer010.java:115)
  at com.acme.kafka_avro_util.TestProducer.testAvroConsumer(TestProducer.java:59)
  at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
  at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  at java.lang.reflect.Method.invoke(Method.java:498)
  at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
  at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
  at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
  at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
  at org.junit.rules.TestWatcher$1.evaluate(TestWatcher.java:55)
  at org.junit.rules.RunRules.evaluate(RunRules.java:20)
  at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
  at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
  at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
  at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
  at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
  at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
  at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
  at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
  at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
  at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
  at org.junit.rules.ExternalResource$1.evaluate(ExternalResource.java:48)
  at org.junit.rules.RunRules.evaluate(RunRules.java:20)
  at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
  at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
  at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
  at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
  at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
  at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
  Caused by: java.io.NotSerializableException: io.confluent.kafka.serializers.KafkaAvroSerializer
  at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1184)
  at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1548)
  at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1509)
  at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
  at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
  at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1548)
  at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1509)
  at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
  at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
  at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)
  at org.apache.flink.util.InstantiationUtil.serializeObject(InstantiationUtil.java:315)
  at org.apache.flink.api.java.ClosureCleaner.ensureSerializable(ClosureCleaner.java:107)
  </code></pre>