apache/kafka · error · ConfigException

Configuration 'name' values must not be empty.

Error message

Configuration 'name' values must not be empty.

What it means

Thrown by ValidList.validateIndividualValues when iterating the elements of a list config and encountering an element that is an empty String. Unlike the top-level 'must not be empty' (which guards the whole list), this guards individual members: a list may be non-empty but still contain "" as one of its items, which Kafka treats as invalid.

Source

Thrown at clients/src/main/java/org/apache/kafka/common/config/ConfigDef.java:1087

                String validString = this.validString.validStrings.isEmpty() ? "any non-empty value" : this.validString.toString();
                throw new ConfigException("Configuration '" + name + "' must not be empty. Valid values include: " + validString);
            }

            if (Set.copyOf(values).size() != values.size()) {
                throw new ConfigException("Configuration '" + name + "' values must not be duplicated.");
            }

            validateIndividualValues(name, values);
        }

        private void validateIndividualValues(String name, List<Object> values) {
            boolean hasValidStrings = !validString.validStrings.isEmpty();

            for (Object value : values) {
                if (value instanceof String) {
                    String string = (String) value;
                    if (string.isEmpty()) {
                        throw new ConfigException("Configuration '" + name + "' values must not be empty.");
                    }
                    if (hasValidStrings) {
                        validString.ensureValid(name, value);
                    }
                }
            }
        }

        public String toString() {
            return !validString.validStrings.isEmpty() ? validString.toString() : "";
        }
    }

    public static class ValidString implements Validator {
        final List<String> validStrings;

        private ValidString(List<String> validStrings) {
            this.validStrings = validStrings;

View on GitHub (pinned to c31c9215e1)

Solutions

1. Remove empty tokens from the comma-separated value (fix 'a,,b' to 'a,b', drop leading/trailing commas).
2. Sanitize the raw string before assignment: value.replaceAll("(^,|,$|(?<=,),)", "").
3. If empty elements are intentional in your pipeline, filter them out before passing the list to ConfigDef.

Example fix

// before
props.put("my.list.config", "read,,write,");

// after
props.put("my.list.config", "read,write");

// Strip empty strings from list values before constructing the client:
for (Map.Entry<String, Object> e : new HashMap<>(props).entrySet()) {
    if (e.getValue() instanceof List<?>) {
        List<?> cleaned = ((List<?>) e.getValue()).stream()
            .filter(el -> !(el instanceof String) || !((String) el).isEmpty())
            .collect(Collectors.toList());
        props.put(e.getKey(), cleaned);
    }
}

null

try {
    def.parse(props);
} catch (ConfigException ce) {
    if (ce.getMessage().endsWith("values must not be empty.")) {
        stripEmptiesAndRetry(props, ce.getName());
    } else throw ce;
}

When it happens

Trigger: A ConfigKey using ValidList receives a list like ["a", "", "c"]. Common when the raw value is "a,,c" (consecutive/trailing comma) and the split logic yields an empty token. Triggered during ConfigDef.parse() inside the per-element loop in validateIndividualValues.

Common situations: Comma-separated configs with stray commas ("a,,b"), leading/trailing commas (",a,b" or "a,b,"), or whitespace-only entries that the splitter collapses to empty. Frequent when properties files are edited by hand or generated by templating systems.

Related errors

• Configuration 'name' must not be empty. Valid values include
• Configuration 'name' values must not be duplicated.
• String must be non-empty
• String may not be empty
• Configuration 'name' values must not be null.

AI-assisted analysis of apache/kafka@c31c9215e1 (2026-08-03). Data as JSON: /data/errors/82c137fffb56bb99.json. Report an issue: GitHub.