Plugin fails to load on Kafka Connect 4.1.0 due to NoSuchMethodError in JsonConverter (Utils.mkSet)

[drprolegend]

Description

After upgrading Kafka Connect to version 4.1.0 and the Elasticsearch sink connector to 15.0.0, the Kafka Connect worker fails to load during plugin scanning.

---

Environment

• Kafka Connect version: 4.1.0
• Connector: io.confluent:kafka-connect-elasticsearch:15.0.0
• Platform: Kubernetes (Connect container with mounted plugin directory)

---

Kafka Connect configuration snippet

apiVersion: kafka.strimzi.io/v1beta2
kind: KafkaConnect
metadata:
  name: my-cluster
  annotations:
    strimzi.io/use-connector-resources: "true"
spec:
  version: 4.1.0
  replicas: 1
  bootstrapServers: my-cluster-kafka-bootstrap:9092
  config:
    config.providers: env
    config.providers.env.class: org.apache.kafka.common.config.provider.EnvVarConfigProvider
    group.id: connect-cluster
    offset.storage.topic: connect-cluster-offsets
    config.storage.topic: connect-cluster-configs
    status.storage.topic: connect-cluster-status
    config.storage.replication.factor: -1
    offset.storage.replication.factor: -1
    status.storage.replication.factor: -1
  build:
    output:
      type: docker
      image: ttl.sh/my-cluster-connect-4.1.0:24h
    plugins:
      - name: kafka-connect-file
        artifacts:
          - type: maven
            group: org.mongodb.kafka
            artifact: mongo-kafka-connect
            version: 2.0.1
      - name: kafka-connect-elasticsearch-file
        artifacts:
          - type: maven
            repository: https://packages.confluent.io/maven
            group: io.confluent
            artifact: kafka-connect-elasticsearch
            version: 15.0.0

---

Observed behavior

When Kafka Connect starts, it fails to load the Elasticsearch connector plugin due to a missing method in
org.apache.kafka.common.utils.Utils:

2025-10-08 13:41:18 INFO  [main] PluginScanner:75 - Loading plugin from: /opt/kafka/plugins/kafka-connect-elasticsearch-file
2025-10-08 13:41:19 ERROR [main] PluginScanner:138 - Failed to discover Converter in /opt/kafka/plugins/kafka-connect-elasticsearch-file: Plugin class has a dependency which is missing or invalid
java.util.ServiceConfigurationError: org.apache.kafka.connect.storage.Converter: Provider org.apache.kafka.connect.json.JsonConverter could not be instantiated
	at java.base/java.util.ServiceLoader.fail(ServiceLoader.java:586) ~[?:?]
	at java.base/java.util.ServiceLoader$ProviderImpl.newInstance(ServiceLoader.java:813) ~[?:?]
	at java.base/java.util.ServiceLoader$ProviderImpl.get(ServiceLoader.java:729) ~[?:?]
	at java.base/java.util.ServiceLoader$3.next(ServiceLoader.java:1403) ~[?:?]
	at org.apache.kafka.connect.runtime.isolation.PluginScanner.handleLinkageError(PluginScanner.java:176) ~[connect-runtime-4.1.0.jar:?]
	at org.apache.kafka.connect.runtime.isolation.PluginScanner.getServiceLoaderPluginDesc(PluginScanner.java:136) [connect-runtime-4.1.0.jar:?]
	at org.apache.kafka.connect.runtime.isolation.ServiceLoaderScanner.scanPlugins(ServiceLoaderScanner.java:61) [connect-runtime-4.1.0.jar:?]
	at org.apache.kafka.connect.runtime.isolation.PluginScanner.scanUrlsAndAddPlugins(PluginScanner.java:79) [connect-runtime-4.1.0.jar:?]
	at org.apache.kafka.connect.runtime.isolation.PluginScanner.discoverPlugins(PluginScanner.java:67) [connect-runtime-4.1.0.jar:?]
	at org.apache.kafka.connect.runtime.isolation.Plugins.initLoaders(Plugins.java:88) [connect-runtime-4.1.0.jar:?]
	at org.apache.kafka.connect.runtime.isolation.Plugins.<init>(Plugins.java:80) [connect-runtime-4.1.0.jar:?]
	at org.apache.kafka.connect.runtime.isolation.Plugins.<init>(Plugins.java:69) [connect-runtime-4.1.0.jar:?]
	at org.apache.kafka.connect.cli.AbstractConnectCli.startConnect(AbstractConnectCli.java:121) [connect-runtime-4.1.0.jar:?]
	at org.apache.kafka.connect.cli.AbstractConnectCli.run(AbstractConnectCli.java:95) [connect-runtime-4.1.0.jar:?]
	at org.apache.kafka.connect.cli.ConnectDistributed.main(ConnectDistributed.java:112) [connect-runtime-4.1.0.jar:?]
Caused by: java.lang.NoSuchMethodError: 'java.util.Set org.apache.kafka.common.utils.Utils.mkSet(java.lang.Object[])'
	at org.apache.kafka.connect.json.JsonConverter.<init>(JsonConverter.java:251) ~[connect-json-7.9.0-ccs.jar:?]
	at org.apache.kafka.connect.json.JsonConverter.<init>(JsonConverter.java:239) ~[connect-json-7.9.0-ccs.jar:?]
	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[?:?]
	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:77) ~[?:?]
	at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[?:?]
	at java.base/java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:500) ~[?:?]
	at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:481) ~[?:?]
	at java.base/java.util.ServiceLoader$ProviderImpl.newInstance(ServiceLoader.java:789) ~[?:?]
	... 13 more

I am looking for suggestions as to how to fix this. Any help would be appreciated. Thank you!

[JKrehling]

You get this from old libraries that are being loaded on your kafka connect classpath.
I hit this with a different connector from another project.
apache/camel-kafka-connector#1734

Make sure your plugin dir doesn't have any old kafka-connect libs lying around from dependencies or other stuff.