OpenFeign/feign · error · IOException

is not a type supported by this decoder.

Error message

${type.getTypeName()} is not a type supported by this decoder.

What it means

JsonEncoder/JsonDecoder's convert() method (used to coerce objects into org.json structures) throws an IOException naming the type when the object is neither a JSONObject, JSONArray, nor Map. It signals an incompatibility between the Java object being converted and the org.json model.

Solutions

Convert the object to a Map before passing it to convert()
Wrap collections manually into JSONArray/JSONObject first
Use a full data-binding encoder/decoder (Jackson/Gson) for POJOs

Example fix

// before
Object out = decoder.convert(myPojo, MyPojo.class); // IOException
// after
Map<String, Object> map = pojoToMap(myPojo);
Object out = decoder.convert(map, Map.class);

Defensive patterns

Strategy: type-guard

Validate before calling

if (!(src instanceof Map) && !(src instanceof JSONObject) && !(src instanceof JSONArray)) {
  throw new IllegalArgumentException("convert() needs Map or org.json type, got " + src.getClass());
}

Type guard

boolean convertible(Object o){ return o instanceof Map || o instanceof JSONObject || o instanceof JSONArray; }

Try / catch

try { return convert(obj, type); }
catch (IOException e) { throw new IllegalArgumentException("Unsupported source: " + obj.getClass(), e); }

Prevention

Only feed Maps and org.json types into JsonDecoder conversion helpers
Serialize POJOs with a dedicated mapper first

When it happens

Trigger: Calling convert() with a plain POJO, primitive wrapper, List, or other non-Map/non-org.json object as the source type.

Common situations: Attempting to reuse JsonDecoder/JsonEncoder internals for arbitrary object conversion; feeding a Collection or custom bean where a Map or org.json type is required.

Understand the failure class

Background: "is not a compatible type" / "cannot merge" errors: when a value's type doesn't match what the library requires — this error's family across 65 libraries.

Related errors

AI-assisted analysis of OpenFeign/feign@e2a1e27560 (2026-09-10). Data as JSON: /api/errors/759521421ad9fc5f. Report an issue: GitHub.

Appendix: source

Thrown at json/src/main/java/feign/json/JsonDecoder.java:116

          format("%s is not a type supported by this decoder.", type),
          response.request());
  }

  @Override
  public Object convert(Object object, Type type) throws IOException {
    if (type instanceof Class) {
      Class<?> cls = (Class<?>) type;
      if (cls == JSONObject.class && object instanceof Map) {
        return new JSONObject((Map<?, ?>) object);
      }
      if (cls == String.class) {
        return object.toString();
      }
    }
    if (object instanceof Map) {
      return new JSONObject((Map<?, ?>) object);
    }
    throw new IOException(type.getTypeName() + " is not a type supported by this decoder.");
  }

  @Override
  public boolean canDecode(Response response, Type type) {
    return Util.isJsonContentType(response);
  }
}

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