https://beam.apache.org/releases/javadoc/2.71.0/org/apache/beam/runners/dataflow/internal/DataflowGroupByKey.Registrar.html
DataflowGroupByKey.Registrar (Apache Beam 2.71.0)
declaration: package: org.apache.beam.runners.dataflow.internal, class: DataflowGroupByKey, class: Registrar
apache beamregistrar