apache/kafka · error · ConfigException

Expected value to be a double, but it was a value.getClass()

Error message

Expected value to be a double, but it was a value.getClass().getName()

What it means

Thrown by ConfigDef.parseType for Type.DOUBLE when the supplied value is neither a Number nor a String. Doubles in Kafka configs (e.g. metrics.sample.window.ms is LONG, but broker metric/recording-level and some custom configs use DOUBLE) must be coerced from Number/String; anything else (Boolean, List, Class, Map) cannot be converted and is rejected with the actual class reported.

Source

Thrown at clients/src/main/java/org/apache/kafka/common/config/ConfigDef.java:767

                    } else {
                        throw new ConfigException(name, value, "Expected value to be a 16-bit integer (short), but it was a " + value.getClass().getName());
                    }
                case LONG:
                    if (value instanceof Integer)
                        return ((Integer) value).longValue();
                    if (value instanceof Long)
                        return value;
                    else if (value instanceof String)
                        return Long.parseLong(trimmed);
                    else
                        throw new ConfigException(name, value, "Expected value to be a 64-bit integer (long), but it was a " + value.getClass().getName());
                case DOUBLE:
                    if (value instanceof Number)
                        return ((Number) value).doubleValue();
                    else if (value instanceof String)
                        return Double.parseDouble(trimmed);
                    else
                        throw new ConfigException(name, value, "Expected value to be a double, but it was a " + value.getClass().getName());
                case LIST:
                    if (value instanceof List)
                        return value;
                    else if (value instanceof String)
                        if (trimmed.isEmpty())
                            return List.of();
                        else
                            return Arrays.asList(COMMA_WITH_WHITESPACE.split(trimmed, -1));
                    else
                        throw new ConfigException(name, value, "Expected a comma separated list.");
                case CLASS:
                    if (value instanceof Class)
                        return value;
                    else if (value instanceof String) {
                        return Utils.loadClass(trimmed, Object.class);
                    } else
                        throw new ConfigException(name, value, "Expected a Class instance or class name.");
                default:

View on GitHub (pinned to c31c9215e1)

Solutions

1. Provide the value as a numeric literal (Double, Integer, Long, etc.) or a numeric String like "0.5".
2. If reading from external config, ensure the loader does not coerce the cell to a non-numeric type (quote it as a string if unsure).
3. Check the ConfigDef entry for the key: confirm its declared Type is really DOUBLE and you are not confusing it with a STRING enum.

Example fix

// before
props.put("my.ratio", true); // Boolean -> ConfigException

// after
props.put("my.ratio", 0.5);  // Double
props.put("my.ratio", "0.5");

Object v = rawValue;
if (!(v instanceof Number) && !(v instanceof String)) {
    throw new IllegalArgumentException("config '" + key + "' must be a Number or numeric string");
}
if (v instanceof String) {
    Double.parseDouble(((String) v).trim());
}

public static boolean isDoubleValue(Object v) {
    if (v instanceof Number) return true;
    if (v instanceof String) {
        try { Double.parseDouble(((String) v).trim()); return true; }
        catch (NumberFormatException e) { return false; }
    }
    return false;
}

try {
    configDef.parse(configs);
} catch (ConfigException e) {
    if (e.getMessage().contains("Expected value to be a double")) {
        // coerce the value to a Number and retry
    } else throw e;
}

When it happens

Trigger: Constructing a client or calling ConfigDef.validate with a DOUBLE-typed config key set to a Boolean, List, Map, Class, or a custom object that is not a java.lang.Number and not a String.

Common situations: A config framework that returns typed objects (e.g. HOCON returning a Boolean for an unquoted true/false) feeding a DOUBLE key; YAML aliases resolving to nested maps; programmatic configs that mistakenly put a comma-separated list into a numeric key; copy-paste of a value from a STRING-typed setting into a DOUBLE-typed one.

Related errors

• Expected value to be a 64-bit integer (long), but it was a v
• Expected a comma separated list.
• Expected a Class instance or class name.
• Invalid url in bootstrap.servers: {url}
• Not a number of type type

AI-assisted analysis of apache/kafka@c31c9215e1 (2026-08-03). Data as JSON: /data/errors/b5654fa79b5ba116.json. Report an issue: GitHub.