languagetool-org/languagetool · error · RuntimeException

Constructor of filter class '${className}' must not have arg

Error message

Constructor of filter class '${className}' must not have arguments: ${constructor}

What it means

Reflection-based setup check in RegexRuleFilterCreator.getFilter: the filter class has a single constructor, but that constructor declares parameters, while the creator can only instantiate filters with a no-argument constructor. The input at fault is the filter class referenced from the <filter> element of the rule.

Source

Thrown at languagetool-core/src/main/java/org/languagetool/rules/patterns/RegexRuleFilterCreator.java:45

 * @since 5.2
 */
public class RegexRuleFilterCreator {

  /**
   * @param className fully qualified class Name of a class implementing {@link RegexRuleFilter}
   */
  public RegexRuleFilter getFilter(String className) {
    try {
      Class<?> aClass = JLanguageTool.getClassBroker().forName(className);
      Constructor<?>[] constructors = aClass.getConstructors();
      if (constructors.length != 1) {
        throw new RuntimeException("Constructor of filter class '"
                + className + "' must have exactly one constructor, but it has " + constructors.length);
      }
      Constructor<?> constructor = constructors[0];
      try {
        if (constructor.getParameterTypes().length != 0) {
          throw new RuntimeException("Constructor of filter class '" + className + "' must not have arguments: " + constructor);
        }
        Object filter = constructor.newInstance();
        if (filter instanceof RegexRuleFilter) {
          return (RegexRuleFilter) filter;
        } else {
          throw new RuntimeException("Filter class '" + className + "' must implement interface " + RegexRuleFilter.class.getSimpleName());
        }
      } catch (Exception e) {
        throw new RuntimeException("Could not create filter class using constructor " + constructor, e);
      }
    } catch (ClassNotFoundException e) {
      throw new RuntimeException("Could not find filter class: '"
              + className + "' - make sure to use a fully qualified class name like 'org.languagetool.rules.MyFilter'");
    }
  }
 
}

View on GitHub (pinned to 2e990059ce)

Solutions

1. Make the filter class's constructor take no parameters
2. Obtain any configuration the filter needs from the filter arguments map passed to acceptRuleMatch instead of constructor parameters

When it happens

Trigger: Thrown at languagetool-core/src/main/java/org/languagetool/rules/patterns/RegexRuleFilterCreator.java:45 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of languagetool-org/languagetool@2e990059ce (2026-09-06). Data as JSON: /api/errors/b7013c49d882187a. Report an issue: GitHub.