eclipse-vertx/vert.x · error · DecodeException

Mapping ${toValueType.getName()} is not available without J

Error message

Mapping ${toValueType.getName()}  is not available without Jackson Databind on the classpath

What it means

JacksonCodec's non-Databind variant cannot map JSON values onto arbitrary Java classes; it only produces basic JSON types (Map, List, String, Number, Boolean, null). Mapping to a user class requires Jackson Databind on the classpath, so calling fromValue(Object, Class) without it always throws this DecodeException.

Source

Thrown at vertx-core/src/main/java/io/vertx/core/json/jackson/JacksonCodec.java:147

  @Override
  public <T> T fromBuffer(Buffer json, Class<T> clazz) throws DecodeException {
    return fromParser(createParser(json), clazz);
  }

  @Override
  public <T> T fromStream(InputStream in, Class<T> clazz) throws DecodeException {
    try {
      JsonParser parser = factory.createParser(in);
      parser.disable(JsonParser.Feature.AUTO_CLOSE_SOURCE);
      return fromParser(parser, clazz);
    } catch (IOException e) {
      throw new DecodeException("Failed to decode:" + e.getMessage(), e);
    }
  }

  @Override
  public <T> T fromValue(Object json, Class<T> toValueType) {
    throw new DecodeException("Mapping " + toValueType.getName() + "  is not available without Jackson Databind on the classpath");
  }

  @Override
  public String toString(Object object, boolean pretty) throws EncodeException {
    BufferRecycler br = factory._getBufferRecycler();
    try (SegmentedStringWriter sw = new SegmentedStringWriter(br)) {
      JsonGenerator generator = createGenerator(sw, pretty);
      encodeJson(object, generator);
      generator.close();
      return sw.getAndClear();
    } catch (IOException e) {
      throw new EncodeException(e.getMessage(), e);
    } finally {
      br.releaseToPool();
    }
  }

  @Override

View on GitHub (pinned to fb308bd8c3)

Solutions

1. Add io.vertx:vertx-codec-jackson (or jackson-databind) to the classpath
2. Use fromValue(json, Map.class) or parse to Map/List instead of a POJO
3. Manually map the resulting Map<String,Object> onto your class
4. Verify no dependency exclusion/shading removed jackson-databind

Example fix

// before
MyConfig cfg = JacksonCodec.fromValue(json, MyConfig.class); // no databind
// after
Map<String, Object> map = JacksonCodec.fromValue(json, Map.class);
// or add dependency: io.vertx:vertx-codec-jackson with jackson-databind

// Java
static boolean databindAvailable() {
  try {
    Class.forName("com.fasterxml.jackson.databind.ObjectMapper");
    return true;
  } catch (ClassNotFoundException e) { return false;
  }
}

static boolean canMapPojo(Class<?> target) {
  return databindAvailable() && !Map.class.isAssignableFrom(target) && !List.class.isAssignableFrom(target);
}

try {
  return JacksonCodec.fromValue(json, MyConfig.class);
} catch (DecodeException e) {
  if (e.getMessage().contains("not available without Jackson Databind")) {
    return manualMap(JacksonCodec.fromValue(json, Map.class)); // fallback
  }
  throw e;
}

When it happens

Trigger: Calling JacksonCodec.fromValue(json, SomeClass.class) (or Json.decodeValue targeting a POJO) while jackson-databind is absent from the classpath.

Common situations: Using vertx-core without the jackson-databind dependency and trying to decode JSON into a POJO; a build change (shade/exclude) dropped databind; using the wrong codec class in a custom SPI after removing Databind.

Understand the failure class

Background: "X is not installed. Please install it with pip install Y": missing optional dependency errors — ImportError/ValueError raised when a library's optional extra was never installed — this error's family across 22 libraries.

Related errors

• Failed to decode:${e.getMessage()}
• Failed to decode:
• Unexpected trailing token
• Expecting the current parser token to be the start of an obj
• Expecting the current parser token to be the start of an arr

AI-assisted analysis of eclipse-vertx/vert.x@fb308bd8c3 (2026-09-06). Data as JSON: /api/errors/6b76eb1d6f479832. Report an issue: GitHub.