OpenFeign/feign · error · UnsupportedOperationException

JAXB only supports encoding raw types. Found

Error message

JAXB only supports encoding raw types. Found ${bodyType}

What it means

JAXBEncoder.encode only supports request body types that are plain Classes (JAXB-mappable raw types). If the declared body type is a ParameterizedType such as List<Foo> or Map<String,Foo>, it throws UnsupportedOperationException because JAXB cannot marshal generic types without a root wrapper element.

Solutions

Encode a single @XmlRootElement-annotated root object instead of a generic collection.
Introduce a wrapper DTO containing the collection as an @XmlElement field and post that.
Use a JSON encoder module for generic types and reserve JAXB for raw annotated types.

Example fix

// before
@POST void send(List<Item> items);
// after
@XmlRootElement
class ItemList { @XmlElement(name="item") List<Item> items; }
@POST void send(ItemList items);

Defensive patterns

Strategy: type-guard

Validate before calling

// before encoding
if (!(bodyType instanceof Class)) {
  throw new IllegalArgumentException("JAXB encoder requires a raw Class body type, got: " + bodyType);
}

Type guard

boolean isRawType(java.lang.reflect.Type t) {
  return t instanceof Class;
}

Try / catch

try {
  encoder.encode(obj, type, template);
} catch (UnsupportedOperationException e) {
  // switch to a wrapper @XmlRootElement DTO or a JSON encoder
}

Prevention

Only declare raw, JAXB-annotated classes as request bodies when using JAXB.
Wrap collections in an @XmlRootElement DTO.
Document that the JAXB module does not handle generic types.

When it happens

Trigger: Declaring a Feign method whose request body is a generic collection or other ParameterizedType while using JAXBEncoder, then invoking encode (via Feign's customEncoder path).

Common situations: Trying to POST a List<MyDto> with JAXB encoding; developers expect automatic collection handling like JSON encoders provide.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of OpenFeign/feign@e2a1e27560 (2026-09-10). Data as JSON: /api/errors/1b91a98d3853206b. Report an issue: GitHub.

Appendix: source

Thrown at jaxb-jakarta/src/main/java/feign/jaxb/JAXBEncoder.java:58

 *     .encoder(new JAXBEncoder(jaxbFactory))
 *     .target(MyApi.class, "http://api");
 * </pre>
 *
 * <p>The JAXBContextFactory should be reused across requests as it caches the created JAXB
 * contexts.
 */
public class JAXBEncoder implements Encoder, PredicatedEncoder {

  private final JAXBContextFactory jaxbContextFactory;

  public JAXBEncoder(JAXBContextFactory jaxbContextFactory) {
    this.jaxbContextFactory = jaxbContextFactory;
  }

  @Override
  public void encode(Object object, Type bodyType, RequestTemplate template) {
    if (!(bodyType instanceof Class)) {
      throw new UnsupportedOperationException(
          "JAXB only supports encoding raw types. Found " + bodyType);
    }
    try {
      Marshaller marshaller = jaxbContextFactory.createMarshaller((Class<?>) bodyType);
      StringWriter stringWriter = new StringWriter();
      marshaller.marshal(object, stringWriter);
      template.body(stringWriter.toString());
    } catch (JAXBException e) {
      throw new EncodeException(e.toString(), e);
    }
  }

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

View on GitHub (pinned to e2a1e27560)