apache/hadoop · error · IllegalArgumentException

{} parameters are required but {} arguments are provided

Error message

{} parameters are required but {} arguments are provided

What it means

ReflectionUtils.newInstance(theClass, conf, argTypes, values) constructs an object via a reflected constructor, but first enforces argTypes.length == values.length; a mismatch throws IllegalArgumentException("N parameters are required but M arguments are provided") before any reflection happens. It is a pure caller-side arity bug — not a classpath or access problem.

Source

Thrown at hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/util/ReflectionUtils.java:143

  public static <T> T newInstance(Class<T> theClass, Configuration conf) {
    return newInstance(theClass, conf, EMPTY_ARRAY);
  }

  /** Create an object for the given class and initialize it from conf
   *
   * @param theClass class of which an object is created
   * @param conf Configuration
   * @param argTypes the types of the arguments
   * @param values the values of the arguments
   * @param <T> Generics Type.
   * @return a new object
   */
  @SuppressWarnings("unchecked")
  public static <T> T newInstance(Class<T> theClass, Configuration conf,
      Class<?>[] argTypes, Object ... values) {
    T result;
    if (argTypes.length != values.length) {
      throw new IllegalArgumentException(argTypes.length
          + " parameters are required but "
          + values.length
          + " arguments are provided");
    }
    try {
      Constructor<T> meth = (Constructor<T>) CONSTRUCTOR_CACHE.get(theClass);
      if (meth == null) {
        meth = theClass.getDeclaredConstructor(argTypes);
        meth.setAccessible(true);
        CONSTRUCTOR_CACHE.put(theClass, meth);
      }
      result = meth.newInstance(values);
    } catch (Exception e) {
      throw new RuntimeException(e);
    }
    setConf(result, conf);
    return result;
  }

View on GitHub (pinned to 2add963021)

Solutions

1. Assert argTypes.length == values.length before the call, or derive both from one source
2. Use the two-argument newInstance(theClass, conf) when the constructor takes no arguments
3. Check the traceback line to see which construction site dropped or added an element

Example fix

// before
Object plugin = ReflectionUtils.newInstance(cls, conf,
    new Class<?>[] {String.class, int.class}, pathOnly);

// after
if (argTypes.length != values.length) {
  throw new IllegalArgumentException("arity mismatch");
}
Object plugin = ReflectionUtils.newInstance(cls, conf, argTypes, values);

if (argTypes.length != values.length) {
  throw new IllegalArgumentException(argTypes.length + " params vs " + values.length + " args");
}
T obj = ReflectionUtils.newInstance(cls, conf, argTypes, values);

When it happens

Trigger: Passing argTypes = {String.class, int.class} with only one varargs value; building values from a parsed list whose size is computed independently of the fixed types array; the varargs Object... parameter silently accepting the wrong count at compile time.

Common situations: Glue code instantiating pluggable classes (filesystems, schedulers, codecs) from config where the types are fixed but values come from parsing; refactors that add a parameter to one array but not the other.

Related errors

• Illegal initial capacity: " + initCapacity
• Param class [{0}] does not have default constructor
• Illegal option {}
• Not enough arguments: expected {} but got {}
• Too many arguments: expected {} but got {}

AI-assisted analysis of apache/hadoop@2add963021 (2026-08-22). Data as JSON: /api/errors/a0cb8b22c9b16fa9. Report an issue: GitHub.