apache/flink · error · InvalidFieldReferenceException

Invalid format of POJO field expression "{}".

Error message

Invalid format of POJO field expression "{}".

What it means

Thrown by PojoTypeInfo.getTypeAt(String) when the fieldExpression does not match the POJO nested-field regex and is not a wildcard character. The expression is syntactically malformed for a POJO field reference. This is an InvalidFieldReferenceException.

Source

Thrown at flink-core/src/main/java/org/apache/flink/api/java/typeutils/PojoTypeInfo.java:251

                                + fieldType
                                + ".");
            }
        }
    }

    @SuppressWarnings("unchecked")
    @Override
    @PublicEvolving
    public <X> TypeInformation<X> getTypeAt(String fieldExpression) {

        Matcher matcher = PATTERN_NESTED_FIELDS.matcher(fieldExpression);
        if (!matcher.matches()) {
            if (fieldExpression.startsWith(ExpressionKeys.SELECT_ALL_CHAR)
                    || fieldExpression.startsWith(ExpressionKeys.SELECT_ALL_CHAR_SCALA)) {
                throw new InvalidFieldReferenceException(
                        "Wildcard expressions are not allowed here.");
            } else {
                throw new InvalidFieldReferenceException(
                        "Invalid format of POJO field expression \"" + fieldExpression + "\".");
            }
        }

        String field = matcher.group(1);
        // get field
        int fieldPos = -1;
        TypeInformation<?> fieldType = null;
        for (int i = 0; i < fields.length; i++) {
            if (fields[i].getField().getName().equals(field)) {
                fieldPos = i;
                fieldType = fields[i].getTypeInformation();
                break;
            }
        }
        if (fieldPos == -1) {
            throw new InvalidFieldReferenceException(
                    "Unable to find field \"" + field + "\" in type " + this + ".");

View on GitHub (pinned to 2f3c205e92)

Solutions

1. Use a valid POJO field-name identifier (letter/underscore/'$' start, alphanumeric/underscore/'$' body).
2. For nested access use dot-separated valid identifiers: "outerField.innerField".

Example fix

// before
pojoTypeInfo.getTypeAt("f0"); // f0 is tuple syntax, not POJO
// after
pojoTypeInfo.getTypeAt("fieldName");

private static final Pattern POJO_NESTED =
    Pattern.compile("([\\p{L}_$][\\p{L}\\p{Digit}_$]*)(\\\.(.+))?");

void validateGetTypeAtExpr(String expr) {
    if (!POJO_NESTED.matcher(expr).matches()) {
        throw new IllegalArgumentException("Malformed field expression: " + expr);
    }
}

When it happens

Trigger: Calling pojoTypeInfo.getTypeAt("123"), getTypeAt(""), getTypeAt("field-name"), or getTypeAt("a."). The PATTERN_NESTED_FIELDS matcher fails and the expression does not start with '*' or '_'.

Common situations: Direct programmatic use of getTypeAt with a non-identifier string, or passing a tuple-style key ('f0') to a POJO type's getTypeAt. Occasionally reached when ExpressionKeys strips a trailing wildcard and leaves an empty or malformed remainder.

Related errors

• Invalid POJO field reference "{}".
• Unable to find field "{}" in type {}.
• Wildcard expressions are not allowed here.
• Invalid format of Row field expression "{}".
• Nested field expression "{}" not possible on atomic type {}.

AI-assisted analysis of apache/flink@2f3c205e92 (2026-08-14). Data as JSON: /api/errors/57e64cf68f93e8c7. Report an issue: GitHub.