Spring远程加载配置的实现方法详解

        <dependency>
            <groupId>com.ctrip.framework.apollo</groupId>
            <artifactId>apollo-client</artifactId>
            <version>2.0.1</version>
        </dependency>
        <dependency>
            <groupId>com.alibaba.cloud</groupId>
            <artifactId>spring-cloud-starter-alibaba-nacos-config</artifactId>
            <version>2021.1</version>
        </dependency>

public class PropertySourcesProcessor implements BeanFactoryPostProcessor, EnvironmentAware,
    ApplicationEventPublisherAware, PriorityOrdered {
	// aware回调接口设置
	private ConfigurableEnvironment environment;
	@Override
  	public void setEnvironment(Environment environment) {
    	//it is safe enough to cast as all known environment is derived from ConfigurableEnvironment
    	this.environment = (ConfigurableEnvironment) environment;
  	}
	@Override
  	public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
  		// 获取配置
    	this.configUtil = ApolloInjector.getInstance(ConfigUtil.class);
    	// 从远程获取PropertySource
    	initializePropertySources();
    	// 为每个ConfigPropertySource注册ConfigChangeEvent监听器
    	// 监听器监听到ConfigChangeEvent后publish一个ApolloConfigChangeEvent
    	// 等于将apollo自定义的ConfigChangeEvent事件机制转化为了spring的ApolloConfigChangeEvent事件
    	initializeAutoUpdatePropertiesFeature(beanFactory);
  	}
	private void initializePropertySources() {
		// 聚合类，该类也是一个PropertySource，代理了一堆PropertySource
		// 该类中有一个 Set<PropertySource<?>> 字段
		CompositePropertySource composite = new ...;
		...
		// 从 远程 或 本地缓存 获取配置
		Config config = ConfigService.getConfig(namespace);
		// 适配Config到PropertySource，并加入聚合类		
		composite.addPropertySource(configPropertySourceFactory.getConfigPropertySource(namespace, config));
		// 添加到ConfigurableEnvironment
		environment.getPropertySources().addFirst(composite);
	}
 	private void initializeAutoUpdatePropertiesFeature(ConfigurableListableBeanFactory beanFactory) {
    	if (!AUTO_UPDATE_INITIALIZED_BEAN_FACTORIES.add(beanFactory)) {
      		return;
    	}
		// 定义监听器，监听器监听到ConfigChangeEvent后发布ApolloConfigChangeEvent
    	ConfigChangeListener configChangeEventPublisher = changeEvent ->
        	applicationEventPublisher.publishEvent(new ApolloConfigChangeEvent(changeEvent));
		// 注册监听器到每个PropertySource
    	List<ConfigPropertySource> configPropertySources = configPropertySourceFactory.getAllConfigPropertySources();
    	for (ConfigPropertySource configPropertySource : configPropertySources) {
      		configPropertySource.addChangeListener(configChangeEventPublisher);
   		}
  	}
	...
}

public class RemoteConfigRepository extends AbstractConfigRepository implements ConfigRepository{
  public RemoteConfigRepository(String namespace) {
  	...
  	// 定时拉取
	this.schedulePeriodicRefresh();
	// 长轮询
	this.scheduleLongPollingRefresh();
	...
  }
  private void schedulePeriodicRefresh() {
    // 定时线程池
    m_executorService.scheduleAtFixedRate(
        new Runnable() {
          @Override
          public void run() {
          	// 调用父抽象类trySync()
          	// trySync()调用模版方法sync()
            trySync();
          }
        }, m_configUtil.getRefreshInterval(), m_configUtil.getRefreshInterval(),
        m_configUtil.getRefreshIntervalTimeUnit());
  }
  @Override
  protected synchronized void sync() {
  	// 事务
    Transaction transaction = Tracer.newTransaction("Apollo.ConfigService", "syncRemoteConfig");
    try {
      ApolloConfig previous = m_configCache.get();
      // http远程拉取配置
      ApolloConfig current = loadApolloConfig();
      // reference equals means HTTP 304
      if (previous != current) {
        logger.debug("Remote Config refreshed!");
        // 设置缓存
        m_configCache.set(current);
        // 发布事件，该方法在父抽象类中
        this.fireRepositoryChange(m_namespace, this.getConfig());
      }

      if (current != null) {
        Tracer.logEvent(String.format("Apollo.Client.Configs.%s", current.getNamespaceName()),
            current.getReleaseKey());
      }

      transaction.setStatus(Transaction.SUCCESS);
    } catch (Throwable ex) {
      transaction.setStatus(ex);
      throw ex;
    } finally {
      transaction.complete();
    }
    ...
  }

    <dependency>
      <groupId>org.springframework.cloud</groupId>
      <artifactId>spring-cloud-context</artifactId>
      <version>...</version>
      <scope>compile</scope>
    </dependency>

public interface PropertySourceLocator {
	// 实现类实现该方法
	PropertySource<?> locate(Environment environment);
	default Collection<PropertySource<?>> locateCollection(Environment environment) {
		return locateCollection(this, environment);
	}
	static Collection<PropertySource<?>> locateCollection(PropertySourceLocator locator, Environment environment) {
		// 调用实现类
		PropertySource<?> propertySource = locator.locate(environment);
		if (propertySource == null) {
			return Collections.emptyList();
		}
		// 如果该PropertySource是代理了list的CompositePropertySource，提取全部
		if (CompositePropertySource.class.isInstance(propertySource)) {
			Collection<PropertySource<?>> sources = ((CompositePropertySource) propertySource).getPropertySources();
			List<PropertySource<?>> filteredSources = new ArrayList<>();
			for (PropertySource<?> p : sources) {
				if (p != null) {
					filteredSources.add(p);
				}
			}
			return filteredSources;
		}
		else {
			return Arrays.asList(propertySource);
		}
	}
}

@Configuration(proxyBeanMethods = false)
@EnableConfigurationProperties(PropertySourceBootstrapProperties.class)
public class PropertySourceBootstrapConfiguration
		implements ApplicationContextInitializer<ConfigurableApplicationContext>, Ordered {
	@Autowired(required = false)
	private List<PropertySourceLocator> propertySourceLocators = new ArrayList<>();
	public void setPropertySourceLocators(Collection<PropertySourceLocator> propertySourceLocators) {
		this.propertySourceLocators = new ArrayList<>(propertySourceLocators);
	}
	@Override
	public void initialize(ConfigurableApplicationContext applicationContext) {
		List<PropertySource<?>> composite = new ArrayList<>();
		// 排序
		AnnotationAwareOrderComparator.sort(this.propertySourceLocators);
		boolean empty = true;
		// applicationContext由回调接口提供
		ConfigurableEnvironment environment = applicationContext.getEnvironment();
		for (PropertySourceLocator locator : this.propertySourceLocators) {
			// 调用PropertySourceLocator
			Collection<PropertySource<?>> source = locator.locateCollection(environment);
			...
			for (PropertySource<?> p : source) {
				// 是否代理了PropertySource的list做分类
				if (p instanceof EnumerablePropertySource) {
					EnumerablePropertySource<?> enumerable = (EnumerablePropertySource<?>) p;
					sourceList.add(new BootstrapPropertySource<>(enumerable));
				}
				else {
					sourceList.add(new SimpleBootstrapPropertySource(p));
				}
			}
			composite.addAll(sourceList);
			empty = false;
		}
		if (!empty) {
			// 获取 ConfigurableEnvironment中的MutablePropertySources
			MutablePropertySources propertySources = environment.getPropertySources();
			...
			// 执行插入到ConfigurableEnvironment的MutablePropertySources
			insertPropertySources(propertySources, composite);
			...
		}
	}
}