apache/kafka · error · IllegalArgumentException

String must be UPPERCASE.

Error message

String must be UPPERCASE.

What it means

Thrown by RaftVoterEndpoint.requireNonNullAllCapsNonEmpty when the listener string contains lowercase characters (input.toUpperCase(Locale.ROOT) != input). Kafka listener names are conventionally uppercase identifiers (CONTROLLER, BROKER, etc.) and the voter endpoint enforces this to prevent silent mismatches against broker listener definitions.

Source

Thrown at clients/src/main/java/org/apache/kafka/clients/admin/RaftVoterEndpoint.java:46

@InterfaceStability.Stable
@InterfaceAudience.Public
public class RaftVoterEndpoint {
    private final String listener;
    private final String host;
    private final int port;

    private static String requireNonNullAllCapsNonEmpty(String input) {
        if (input == null) {
            throw new IllegalArgumentException("Null argument not allowed.");
        }
        if (!input.trim().equals(input)) {
            throw new IllegalArgumentException("Leading or trailing whitespace is not allowed.");
        }
        if (input.isEmpty()) {
            throw new IllegalArgumentException("Empty string is not allowed.");
        }
        if (!input.toUpperCase(Locale.ROOT).equals(input)) {
            throw new IllegalArgumentException("String must be UPPERCASE.");
        }
        return input;
    }

    /**
     * Create an endpoint for a metadata quorum voter.
     *
     * @param listener          The human-readable name for this endpoint. For example, CONTROLLER.
     * @param host              The DNS hostname for this endpoint.
     * @param port              The network port for this endpoint.
     */
    public RaftVoterEndpoint(
        String listener,
        String host,
        int port
    ) {
        this.listener = requireNonNullAllCapsNonEmpty(listener);
        this.host = Objects.requireNonNull(host);

View on GitHub (pinned to 996fb4585a)

Solutions

1. Use the canonical uppercase listener name from your broker config (e.g. "CONTROLLER").
2. Uppercase the value programmatically before passing: `listener.toUpperCase(Locale.ROOT)` (but only if you're certain of the intended name).
3. Align the source config so listener names are written uppercase consistently.

Example fix

// before
new RaftVoterEndpoint("controller", host, port);

// after
new RaftVoterEndpoint("CONTROLLER", host, port);

String upper = listener == null ? null : listener.toUpperCase(Locale.ROOT);
new RaftVoterEndpoint(upper, host, port); // only if you are sure of the canonical name

static boolean isUpperCaseIdentifier(String s) {
    return s != null && !s.isEmpty() && s.trim().equals(s) && s.toUpperCase(Locale.ROOT).equals(s);
}

try {
    new RaftVoterEndpoint(listener, host, port);
} catch (IllegalArgumentException e) {
    log.error("Listener '{}' rejected: {}", listener, e.getMessage());
}

When it happens

Trigger: Constructing `new RaftVoterEndpoint("controller", host, port)` or any mixed-case/lowercase value such as "Controller". Triggered when code reads a listener name from a case-insensitive source.

Common situations: Lowercasing the value for storage and forgetting to re-uppercase; reading from a YAML config where lowercase was used stylistically; copying a listener name from a log line that was lowercased for display.

Related errors

• Null argument not allowed.
• Leading or trailing whitespace is not allowed.
• Empty string is not allowed.
• Cannot create a new partition reassignment without any repli
• Invalid empty members has been provided

AI-assisted analysis of apache/kafka@996fb4585a (2026-08-11). Data as JSON: /api/errors/9c940ed2ba2b2631. Report an issue: GitHub.