code: public void setFilter(String filter) throws IllegalArgumentException {
          if (StringUtils.isBlank(filter)) {
              this.filter = ClassPathFilter.TRUE;
          } else {
              this.filter = new Parser().parse(filter);
          }
      }
docs: Set the class filter. Only classes that pass the filter will be included.
  By default the filter accepts all classes. Passing a null or null length
  string will result in all classes passing the test.
  
  @param filter
  The string representation of the filter. May be null.
  @throws IllegalArgumentException if the filter can not be parsed.