code: public String getStringValue(DataBinder binder) {
      // binder can't get a non existent param
      if (this.name == null || this.name.equals("")) {
        return null;
      }
  
      String value = (String) binder.getLocal(this.name);
  
      if (value == null && this.required) {
        throw new IllegalArgumentException("Parameter " + name + " is required");
      }
  
      return value;
    }
docs: Get the string value from binder.