private ConfigEntry resolveEffectiveTopicConfig(ConfigDef.ConfigKey configKey,
            Map<String, ?> staticNodeConfig,
            Map<String, ?> dynamicClusterConfigs,
            Map<String, ?> dynamicNodeConfigs,
            Map<String, ?> dynamicTopicConfigs) {
        if (dynamicTopicConfigs.containsKey(configKey.name)) {
            return toConfigEntry(configKey,
                dynamicTopicConfigs.get(configKey.name),
                ConfigSource.DYNAMIC_TOPIC_CONFIG, Function.identity());
        }
        List<ConfigSynonym> synonyms = logConfigSynonyms.getOrDefault(configKey.name, emptyList());
        for (ConfigSynonym synonym : synonyms) {
            if (dynamicNodeConfigs.containsKey(synonym.name())) {
                return toConfigEntry(configKey, dynamicNodeConfigs.get(synonym.name()),
                    ConfigSource.DYNAMIC_BROKER_CONFIG, synonym.converter());
            }
        }
        for (ConfigSynonym synonym : synonyms) {
            if (dynamicClusterConfigs.containsKey(synonym.name())) {
                return toConfigEntry(configKey, dynamicClusterConfigs.get(synonym.name()),
                    ConfigSource.DYNAMIC_DEFAULT_BROKER_CONFIG, synonym.converter());
            }
        }
        for (ConfigSynonym synonym : synonyms) {
            if (staticNodeConfig.containsKey(synonym.name())) {
                return toConfigEntry(configKey, staticNodeConfig.get(synonym.name()),
                    ConfigSource.STATIC_BROKER_CONFIG, synonym.converter());
            }
        }
        return toConfigEntry(configKey, configKey.hasDefault() ? configKey.defaultValue : null,
            ConfigSource.DEFAULT_CONFIG, Function.identity());
    }
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