apache/iceberg · error · RuntimeException

Could not read object

Error message

Could not read object 

What it means

deserializeFromBytes throws a plain RuntimeException with this (trailing-space) message when ObjectInputStream.readObject cannot resolve a class on the classpath (ClassNotFoundException). It means the serialized stream is structurally valid but references a class that is not available to the deserializing JVM.

Solutions

1. Add the jar/module containing the serialized class to the deserializing classpath
2. Confirm the class's package and serialVersionUID match on both sides (fix serialVersionUID or re-serialize)
3. Regenerate the payload with the current code instead of deserializing legacy bytes
4. Read the chained ClassNotFoundException cause to identify the exact missing class name

Example fix

// before
T obj = SerializationUtil.deserializeFromBytes(storedBytes); // CNFE on legacy class
// after
MyDto dto = readJson(storedBytes); // prefer a stable format
T obj = convert(dto);

try {
  Class.forName(payloadClassName, false, getClass().getClassLoader());
} catch (ClassNotFoundException e) {
  throw new IllegalStateException("payload class missing from classpath: " + payloadClassName);
}

try {
  T obj = SerializationUtil.deserializeFromBytes(bytes);
} catch (RuntimeException e) {
  if (e.getCause() instanceof ClassNotFoundException) {
    throw new IllegalStateException("missing class: " + e.getCause().getMessage(), e);
  }
  throw e;
}

When it happens

Trigger: Deserializing bytes whose payload class was written by a different classloader/application, the class was removed or renamed across versions, or the deserializing process lacks the module/jar containing the serialized type.

Common situations: Shipping payloads between services where only one has the concrete class; upgrading Iceberg or application code so an old stored serialized object references a deleted class; fat-jar shading changing package names.

Related errors

• Could not deserialize the WriteResult object
• Could not deserialize the WriteResult object
• Could not deserialize the WriteResult object
• Failed to deserialize the split.
• Failed to deserialize the split.

AI-assisted analysis of apache/iceberg@86d9c8fc54 (2026-09-12). Data as JSON: /api/errors/be2ecc95237c1f1f. Report an issue: GitHub.

Appendix: source

Thrown at core/src/main/java/org/apache/iceberg/util/SerializationUtil.java:84

      return baos.toByteArray();
    } catch (IOException e) {
      throw new UncheckedIOException("Failed to serialize object", e);
    }
  }

  @SuppressWarnings({"DangerousJavaDeserialization", "unchecked"})
  public static <T> T deserializeFromBytes(byte[] bytes) {
    if (bytes == null) {
      return null;
    }

    try (ByteArrayInputStream bais = new ByteArrayInputStream(bytes);
        ObjectInputStream ois = new ObjectInputStream(bais)) {
      return (T) ois.readObject();
    } catch (IOException e) {
      throw new UncheckedIOException("Failed to deserialize object", e);
    } catch (ClassNotFoundException e) {
      throw new RuntimeException("Could not read object ", e);
    }
  }

  public static String serializeToBase64(Object obj) {
    byte[] bytes = serializeToBytes(obj);
    return new String(Base64.getMimeEncoder().encode(bytes), StandardCharsets.UTF_8);
  }

  public static <T> T deserializeFromBase64(String base64) {
    if (base64 == null) {
      return null;
    }
    byte[] bytes = Base64.getMimeDecoder().decode(base64.getBytes(StandardCharsets.UTF_8));
    return deserializeFromBytes(bytes);
  }
}

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