@Override
  public ListDecommissionedRegionServersResponse listDecommissionedRegionServers(
    RpcController controller, ListDecommissionedRegionServersRequest request)
    throws ServiceException {
    ListDecommissionedRegionServersResponse.Builder response =
      ListDecommissionedRegionServersResponse.newBuilder();
    try {
      server.checkInitialized();
      if (server.cpHost != null) {
        server.cpHost.preListDecommissionedRegionServers();
      }
      List<ServerName> servers = server.listDecommissionedRegionServers();
      response.addAllServerName((servers.stream().map(server -> ProtobufUtil.toServerName(server)))
        .collect(Collectors.toList()));
      if (server.cpHost != null) {
        server.cpHost.postListDecommissionedRegionServers();
      }
    } catch (IOException io) {
      throw new ServiceException(io);
    }

    return response.build();
  }
Determines if there is a coprocessor implementation in the provided argument which extends or
implements the provided {@code service}.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/MasterRpcServices.java