@Override
  public void register(MessageType messageType,
      Map<KnownVersion, Command> versionToNewCommand) {
    if (!registerCommand(messageType, versionToNewCommand, modifiableRegisteredCommands)) {
      throw new InternalGemFireError(String.format("Message %s was not registered.", messageType));
    }
  }
Gets legacy singleton instance.

@deprecated Efforts should be made to get and instance from the cache or other object.

@return legacy singleton instance. Instance is not immutable.
geode-core/src/main/java/org/apache/geode/internal/cache/tier/sockets/CommandInitializer.java