public Map<String, ConfigEntry> resolveEffectiveTopicConfigs(
            Map<String, ?> staticNodeConfig,
            Map<String, ?> dynamicClusterConfigs,
            Map<String, ?> dynamicNodeConfigs,
            Map<String, ?> dynamicTopicConfigs) {
        ConfigDef configDef = configDefs.getOrDefault(ConfigResource.Type.TOPIC, EMPTY_CONFIG_DEF);
        HashMap<String, ConfigEntry> effectiveConfigs = new HashMap<>();
        for (ConfigDef.ConfigKey configKey : configDef.configKeys().values()) {
            ConfigEntry entry = resolveEffectiveTopicConfig(configKey, staticNodeConfig,
                dynamicClusterConfigs, dynamicNodeConfigs, dynamicTopicConfigs);
            effectiveConfigs.put(entry.name(), entry);
        }
        return effectiveConfigs;
    }
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