public String getDefault(ConfigResource.Type type, String key) {
        ConfigDef configDef = configDefs.get(type);
        if (configDef == null) return null;
        ConfigDef.ConfigKey configKey = configDef.configKeys().get(key);
        if (configKey == null || !configKey.hasDefault()) {
            return null;
        }
        return ConfigDef.convertToString(configKey.defaultValue, configKey.type);
    }
Get the default value of the configuration key, or null if no default is specified.
metadata/src/main/java/org/apache/kafka/metadata/KafkaConfigSchema.java