Custom Nacos PropertySourceLocator in Spring Cloud Alibaba for Dynamic Config

Environment: Spring Boot 2.3.12.RELEASE, Spring Cloud Alibaba 2.2.5.RELEASE, Spring Cloud Hoxton.SR12.

The core technique is to create a custom PropertySourceLocator and register it via spring.factories.

Configuration is placed in the spring-cloud-context-xxx.jar file with the following entry:

org.springframework.cloud.bootstrap.BootstrapConfiguration=\
org.springframework.cloud.bootstrap.config.PropertySourceBootstrapConfiguration

Nacos loads configuration by default using three DataIds. The main class responsible for this is NacosPropertySourceLocator :

Automatic Configuration Core

@Configuration(proxyBeanMethods = false)
@ConditionalOnProperty(name = "spring.cloud.nacos.config.enabled", matchIfMissing = true)
public class NacosConfigBootstrapConfiguration {
    @Bean
    @ConditionalOnMissingBean
    public NacosConfigProperties nacosConfigProperties() {
        return new NacosConfigProperties();
    }
    @Bean
    @ConditionalOnMissingBean
    public NacosConfigManager nacosConfigManager(NacosConfigProperties nacosConfigProperties) {
        return new NacosConfigManager(nacosConfigProperties);
    }
    @Bean
    @ConditionalOnMissingBean
    public NacosPropertySourceLocator nacosPropertySourceLocator(NacosConfigManager nacosConfigManager) {
        return new NacosPropertySourceLocator(nacosConfigManager);
    }
}

org.springframework.cloud.bootstrap.BootstrapConfiguration=\
com.alibaba.cloud.nacos.NacosConfigBootstrapConfiguration

Custom Property Source

public class NacosPropertySourceLocator implements PropertySourceLocator {
    private NacosPropertySourceBuilder nacosPropertySourceBuilder;
    private NacosConfigProperties nacosConfigProperties;
    private NacosConfigManager nacosConfigManager;
    public NacosPropertySourceLocator(NacosConfigManager nacosConfigManager) {
        this.nacosConfigManager = nacosConfigManager;
        this.nacosConfigProperties = nacosConfigManager.getNacosConfigProperties();
    }
    public PropertySource<?> locate(Environment env) {
        nacosConfigProperties.setEnvironment(env);
        ConfigService configService = nacosConfigManager.getConfigService();
        long timeout = nacosConfigProperties.getTimeout();
        nacosPropertySourceBuilder = new NacosPropertySourceBuilder(configService, timeout);
        // load shared, extended, and application configurations
        CompositePropertySource composite = new CompositePropertySource(NACOS_PROPERTY_SOURCE_NAME);
        loadSharedConfiguration(composite);
        loadExtConfiguration(composite);
        loadApplicationConfiguration(composite, dataIdPrefix, nacosConfigProperties, env);
        return composite;
    }
}

Getting Config Service

public interface ConfigService {
    String getConfig(String dataId, String group, long timeoutMs) throws NacosException;
    String getConfigAndSignListener(String dataId, String group, long timeoutMs, Listener listener) throws NacosException;
    void addListener(String dataId, String group, Listener listener) throws NacosException;
    boolean publishConfig(String dataId, String group, String content) throws NacosException;
    boolean removeConfig(String dataId, String group) throws NacosException;
    void removeListener(String dataId, String group, Listener listener);
    String getServerStatus();
    void shutDown() throws NacosException;
}

NacosConfigManager

public class NacosConfigManager {
    private static ConfigService service = null;
    private NacosConfigProperties nacosConfigProperties;
    public NacosConfigManager(NacosConfigProperties nacosConfigProperties) {
        this.nacosConfigProperties = nacosConfigProperties;
        createConfigService(nacosConfigProperties);
    }
    static ConfigService createConfigService(NacosConfigProperties nacosConfigProperties) {
        if (Objects.isNull(service)) {
            synchronized (NacosConfigManager.class) {
                try {
                    if (Objects.isNull(service)) {
                        service = NacosFactory.createConfigService(nacosConfigProperties.assembleConfigServiceProperties());
                    }
                } catch (Throwable e) {
                    throw new NacosException(NacosException.CLIENT_INVALID_PARAM, e);
                }
            }
        }
        return service;
    }
}

Getting Application Configuration

public class NacosPropertySourceLocator implements PropertySourceLocator {
    // ...
    private void loadApplicationConfiguration(CompositePropertySource composite, String dataIdPrefix, NacosConfigProperties properties, Environment env) {
        String fileExtension = properties.getFileExtension();
        String nacosGroup = properties.getGroup();
        loadNacosDataIfPresent(composite, dataIdPrefix + "." + fileExtension, nacosGroup, fileExtension, true);
        for (String profile : env.getActiveProfiles()) {
            String dataId = dataIdPrefix + "-" + profile + "." + fileExtension;
            loadNacosDataIfPresent(composite, dataId, nacosGroup, fileExtension, true);
        }
    }
    // helper methods omitted for brevity
}

Finally, a ConfigService implementation ( NacosConfigService) retrieves remote configuration, falling back to local cache when possible, and continuously retries if the remote service is unavailable, ensuring the application can start only when configuration data is accessible.

Summary: Nacos Config first attempts to read from local cache; if unavailable, it fetches from the remote server, retrying as needed, and only proceeds when configuration data is successfully loaded.