[fix][connector] KCA: use reflection to get pulsar-client impl classes

[nicoloboschi]

Motivation

With the latest changes to the KCA module (#16098) now you get this error if the message is batched:

java.lang.ClassCastException: class org.apache.pulsar.client.impl.BatchMessageIdImpl cannot be cast to class org.apache.pulsar.client.impl.MessageIdImpl (org.apache.pulsar.client.impl.BatchMessageIdImpl is in unnamed module of loader java.net.URLClassLoader @2ff4acd0; org.apache.pulsar.client.impl.MessageIdImpl is in unnamed module of loader org.apache.pulsar.common.nar.NarClassLoader @6ff415ad)
	at org.apache.pulsar.io.kafka.connect.KafkaConnectSink.getMessageOffset(KafkaConnectSink.java:327) ~[pulsar-io-kafka-connect-adaptor-?.jar]
	at org.apache.pulsar.io.kafka.connect.KafkaConnectSink.toSinkRecord(KafkaConnectSink.java:408) ~[pulsar-io-kafka-connect-adaptor-?.jar:]
	at org.apache.pulsar.io.kafka.connect.KafkaConnectSink.write(KafkaConnectSink.java:121) [pulsar-io-kafka-connect-adaptor-?.jar:]
	at org.apache.pulsar.functions.instance.JavaInstanceRunnable.sendOutputMessage(JavaInstanceRunnable.java:368) [com.datastax.oss-pulsar-functions-instance-?-SNAPSHOT.jar:?-SNAPSHOT]
	at org.apache.pulsar.functions.instance.JavaInstanceRunnable.handleResult(JavaInstanceRunnable.java:351) [com.datastax.oss-pulsar-functions-instance-?-SNAPSHOT.jar:?-SNAPSHOT]
	at org.apache.pulsar.functions.instance.JavaInstanceRunnable.run(JavaInstanceRunnable.java:300) [com.datastax.oss-pulsar-functions-instance-?-SNAPSHOT.jar:?-SNAPSHOT]

The root problem is that connectors should not bundle the Pulsar client impl because it's provided at runtime by the Java Functions framework.

Modifications

• Use reflection to get the custom sequenceId

• doc-not-needed
  (Please explain why)

[eolivelli]

Lgtm

[BewareMyPower]

Hi Nico, I met a similar error (see https://github.com/BewareMyPower/pulsar-sink-demo) when I'm trying the connector.

because it's provided at runtime by the Java Functions framework.

Could you explain a bit more? You mean the Pulsar client impl JAR is in the classpath and the connector is loaded by the Java Functions framework so the connector cannot access classes in Pulsar broker's classpath?

BTW, if a connector cannot access methods from modules like pulsar-client-original directly, it would be a bad experience for Pulsar connector developers.