OpenFeign/feign · error · IllegalArgumentException

Not supported type

Error message

Not supported type ${type}

What it means

JacksonIteratorDecoder decodes streaming Iterator<T> responses; actualIteratorTypeArgument requires the declared return type to be a ParameterizedType whose raw type is java.util.Iterator. Anything else throws IllegalArgumentException 'Not supported type'.

Solutions

1. Declare the method return type as Iterator<T> with an explicit type argument.
2. Use the regular JacksonDecoder for non-Iterator return types like List<T>.
3. Wire JacksonIteratorDecoder only for the streaming methods that need it.
4. If T itself is generic, ensure it resolves to a concrete class.

Example fix

// before
@GET Iterator getUsers(); // raw Iterator -> Not supported type
// after
@GET Iterator<User> getUsers();

static boolean isIteratorType(Type type) {
  return type instanceof ParameterizedType
      && java.util.Iterator.class.equals(((ParameterizedType) type).getRawType());
}
// verify each Feign method's generic return type before wiring JacksonIteratorDecoder

static boolean hasExplicitTypeArgument(java.lang.reflect.Method m) {
  return isIteratorType(m.getGenericReturnType());
}

try {
  return streamingClient.streamUsers();
} catch (feign.codec.DecodeException e) {
  if (e.getCause() instanceof IllegalArgumentException && e.getCause().getMessage().startsWith("Not supported type")) {
    throw new IllegalStateException("Method must declare Iterator<T>, not raw/other types", e);
  }
  throw e;
}

When it happens

Trigger: Declaring a Feign method with JacksonIteratorDecoder whose return type is Iterator (raw, no generic parameter), List<T>, or any non-Iterator ParameterizedType — e.g. returning raw Iterator without <T>.

Common situations: Forgetting the generic parameter (raw Iterator), accidentally wiring the iterator decoder for methods returning List<T>, copy-pasting decoder config across methods with different return types.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• Not an iterator type
• Not supported type
• Not an iterator type
• ${e.getMessage()}
• JAXB only supports decoding raw types. Found

AI-assisted analysis of OpenFeign/feign@e2a1e27560 (2026-09-10). Data as JSON: /api/errors/0d9871f7c3ea176d. Report an issue: GitHub.

Appendix: source

Thrown at jackson/src/main/java/feign/jackson/JacksonIteratorDecoder.java:97

      // Read the first byte to see if we have any data
      reader.mark(1);
      if (reader.read() == -1) {
        return null; // Eagerly returning null avoids "No content to map due to end-of-input"
      }
      reader.reset();
      return new JacksonIterator<Object>(
          actualIteratorTypeArgument(type), mapper, response, reader);
    } catch (RuntimeJsonMappingException e) {
      if (e.getCause() != null && e.getCause() instanceof IOException) {
        throw IOException.class.cast(e.getCause());
      }
      throw e;
    }
  }

  private static Type actualIteratorTypeArgument(Type type) {
    if (!(type instanceof ParameterizedType)) {
      throw new IllegalArgumentException("Not supported type " + type.toString());
    }
    ParameterizedType parameterizedType = (ParameterizedType) type;
    if (!Iterator.class.equals(parameterizedType.getRawType())) {
      throw new IllegalArgumentException(
          "Not an iterator type " + parameterizedType.getRawType().toString());
    }
    return ((ParameterizedType) type).getActualTypeArguments()[0];
  }

  public static JacksonIteratorDecoder create() {
    return create(Collections.<Module>emptyList());
  }

  public static JacksonIteratorDecoder create(Iterable<Module> modules) {
    return new JacksonIteratorDecoder(
        new ObjectMapper()
            .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
            .registerModules(modules));

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