code: @Override
      public void init(NamedList args) {
  
          Object regex = args.remove(PARAM_REGEX);
          if (null == regex) {
              throw new SolrException(ErrorCode.SERVER_ERROR, "Missing required init parameter: " + PARAM_REGEX);
          }
          try {
              m_regex = Pattern.compile(regex.toString());
          } catch (PatternSyntaxException e) {
              throw new SolrException(ErrorCode.SERVER_ERROR, "Invalid regex: " + regex, e);
          }
  
          Object replacement = args.remove(PARAM_REPLACEMENT);
          if (null == replacement) {
              throw new SolrException(ErrorCode.SERVER_ERROR, "Missing required init parameter: " + PARAM_REPLACEMENT);
          }
          m_replacement = replacement.toString();
  
          Object source = args.remove(PARAM_SOURCE);
          if (null == source) {
              throw new SolrException(ErrorCode.SERVER_ERROR, "Missing required init parameter: " + PARAM_SOURCE);
          }
          m_source = source.toString();
  
          Object target = args.remove(PARAM_TARGET);
          if (null == target) {
              throw new SolrException(ErrorCode.SERVER_ERROR, "Missing required init parameter: " + PARAM_TARGET);
          }
          m_target = target.toString();
  
      }
docs: Read the parameters on initialization.
  
  @see org.apache.solr.update.processor.UpdateRequestProcessorFactory#init(org.apache.solr.common.util.NamedList)