OpenFeign/feign · error · EncodeException

is not a type supported by this encoder.

Error message

%s is not a type supported by this encoder.

What it means

JsonEncoder.encode() only serializes objects that are already org.json JSONObject or JSONArray instances; anything else throws an EncodeException saying the bodyType is not supported. Unlike Jackson/Gson encoders it performs no reflection-based data binding.

Solutions

1. Build the request body as an org.json JSONObject or JSONArray before encoding
2. Switch to JacksonEncoder or GsonEncoder for POJO/Map serialization
3. Write a custom Encoder that converts POJOs/Maps into JSONObject and delegates

Example fix

// before
api.createUser(user); // user is a POJO -> EncodeException
// after
api.createUser(new JSONObject(pojoToMap(user)));
// or: Feign.builder().encoder(new JacksonEncoder())

if (!(body instanceof JSONObject) && !(body instanceof JSONArray)) {
  throw new IllegalArgumentException("JsonEncoder requires JSONObject/JSONArray, got " + body.getClass());
}

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

try { encoder.encode(body, bodyType, template); }
catch (EncodeException e) { throw new IllegalStateException("Wrap body in JSONObject or use JacksonEncoder", e); }

When it happens

Trigger: Calling a POST/PUT Feign method whose body parameter is a POJO, Map, List, or String while using JsonEncoder as the encoder.

Common situations: Assuming JsonEncoder behaves like JacksonEncoder; sending request bodies after migrating encoders; passing a Map that was never wrapped in a JSONObject.

Understand the failure class

Background: json.Marshal / "failed to marshal" errors in Go: why "unsupported type" happens and how to fix it — this error's family across 22 libraries.

Related errors

• is not a type supported by this encoder.
• ${jsonException.getMessage()}
• This form encoder has no delegate encoder, so it can only…
• Output closing error
• Writing file's ' ' content error

AI-assisted analysis of OpenFeign/feign@e2a1e27560 (2026-09-10). Data as JSON: /api/errors/2039b544ede0bf3d. Report an issue: GitHub.

Appendix: source

Thrown at json/src/main/java/feign/json/JsonEncoder.java:65

 *                      .target(GitHub.class, "https://api.github.com");
 *
 *   JSONObject contributor = new JSONObject();
 *
 *   contributor.put("login", "radio-rogal");
 *   contributor.put("contributions", 0);
 *   github.create("openfeign", "feign", contributor);
 * </pre>
 */
public class JsonEncoder implements Encoder, PredicatedEncoder {

  @Override
  public void encode(Object object, Type bodyType, RequestTemplate template)
      throws EncodeException {
    if (object == null) return;
    if (object instanceof JSONArray || object instanceof JSONObject) {
      template.body(object.toString());
    } else {
      throw new EncodeException(format("%s is not a type supported by this encoder.", bodyType));
    }
  }

  @Override
  public boolean canEncode(Object object, Type bodyType, RequestTemplate template) {
    return Util.isJsonContentType(template);
  }
}

View on GitHub (pinned to e2a1e27560)