WorkerConfig.java
 
public static final String INTERNAL_KEY_CONVERTER_CLASS_CONFIG = "internal.key.converter";
public static final String INTERNAL_KEY_CONVERTER_CLASS_DOC =
        "Converter class used to convert between Kafka Connect format and the serialized form that is written to Kafka." +
                " This controls the format of the keys in messages written to or read from Kafka, and since this is" +
                " independent of connectors it allows any connector to work with any serialization format." +
                " Examples of common formats include JSON and Avro." +
                " This setting controls the format used for internal bookkeeping data used by the framework, such as" +
                " configs and offsets, so users can typically use any functioning Converter implementation.";

public static final String INTERNAL_VALUE_CONVERTER_CLASS_CONFIG = "internal.value.converter";
public static final String INTERNAL_VALUE_CONVERTER_CLASS_DOC =
        "Converter class used to convert between Kafka Connect format and the serialized form that is written to Kafka." +
                " This controls the format of the values in messages written to or read from Kafka, and since this is" +
                " independent of connectors it allows any connector to work with any serialization format." +
                " Examples of common formats include JSON and Avro." +
                " This setting controls the format used for internal bookkeeping data used by the framework, such as" +
                " configs and offsets, so users can typically use any functioning Converter implementation.";
 
connect-standalone.properties
 
internal.key.converter=org.apache.kafka.connect.json.JsonConverter
internal.value.converter=org.apache.kafka.connect.json.JsonConverter
 
connect-distributed.properties
 
internal.key.converter=org.apache.kafka.connect.json.JsonConverter
internal.value.converter=org.apache.kafka.connect.json.JsonConverter