apache/flink · error · RuntimeException

Could not access the fields of the user defined class while…

Error message

Could not access the fields of the user defined class while checking for serializability.

What it means

Thrown by UserCodeObjectWrapper during the serializability check of the user-defined object: reflection could not read the object's fields (get/setAccessible failure or similar) while walking the object graph to find non-serializable fields. The original reflective failure is chained; the user class may be restricting field access.

Solutions

1. Address the cause reported by the error message: Could not access the fields of the user defined class while checking for serializability.
2. Verify the inputs, configuration values, and classpath/dependency setup related to this operation, then retry.

Example fix

Correct the condition described ("Could not access the fields of the user defined class while checking for serializability.") and rerun the job or command.

When it happens

Trigger: Triggered at runtime when the operation fails because: Could not access the fields of the user defined class while checking for serializability.

Common situations: Commonly caused by misconfiguration, missing dependencies or files, unsupported types or operations, or invalid user input leading to: Could not access the fields of the user defined class while checking for serializability.

AI-assisted analysis of apache/flink@2f3c205e92 (2026-08-14). Data as JSON: /api/errors/4e92a2cd1e3016a5. Report an issue: GitHub.

Appendix: source

Thrown at flink-core/src/main/java/org/apache/flink/api/common/operators/util/UserCodeObjectWrapper.java:114

                                "User-defined object "
                                        + userCodeObject
                                        + " ("
                                        + userCodeObject.getClass().getName()
                                        + ") contains non-serializable field "
                                        + f.getName()
                                        + " = "
                                        + f.get(current));
                    }
                }
                current = newCurrent;
            }
        } catch (NonSerializableUserCodeException e) {
            // forward those
            throw e;
        } catch (Exception e) {
            // should never happen, since we make the fields accessible.
            // anyways, do not swallow the exception, but report it
            throw new RuntimeException(
                    "Could not access the fields of the user defined class while checking for serializability.",
                    e);
        }
    }

    @Override
    public T getUserCodeObject(Class<? super T> superClass, ClassLoader cl) {
        return userCodeObject;
    }

    @Override
    public T getUserCodeObject() {
        return userCodeObject;
    }

    @Override
    public <A extends Annotation> A getUserCodeAnnotation(Class<A> annotationClass) {
        return userCodeObject.getClass().getAnnotation(annotationClass);

View on GitHub (pinned to 2f3c205e92)