apollo在Springboot如何运行的

前言

配置中心我相信有一年开发经验的程序员都听过吧，有三年开发经验的程序100%都使用过配置中心配置吧。apollo做为常用的配置中心，你知道它的原理吗？你知道它是怎么集成到springboot的吗？ 本篇文章带你了解其原理，让你也能够自定义组件。

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一、apollo是如何完成初始化的,拉取配置的呢

1. apollo的初始化阶段

Springboot环境准备阶段，发布ApplicationEnvironmentPreparedEvent 事件，EnvironmentPostProcessorApplicationListener 监听到事件之后 执行 postProcessEnvironment方法。apollo 中 的初始化类 ApolloApplicationContextInitializer 就实现EnvironmentPostProcessor接口。 ApolloApplicationContextInitializer

  public void postProcessEnvironment(ConfigurableEnvironment configurableEnvironment, SpringApplication springApplication) {

    // should always initialize system properties like app.id in the first place
    initializeSystemProperty(configurableEnvironment);

    Boolean eagerLoadEnabled = configurableEnvironment.getProperty(PropertySourcesConstants.APOLLO_BOOTSTRAP_EAGER_LOAD_ENABLED, Boolean.class, false);

    //EnvironmentPostProcessor should not be triggered if you don't want Apollo Loading before Logging System Initialization
    if (!eagerLoadEnabled) {
      return;
    }

    Boolean bootstrapEnabled = configurableEnvironment.getProperty(PropertySourcesConstants.APOLLO_BOOTSTRAP_ENABLED, Boolean.class, false);

    if (bootstrapEnabled) {
      DeferredLogger.enable();
      //重点实现逻辑的方法
      initialize(configurableEnvironment);
    }

  }

2.初始化RemoteConfigRepository

经过initialize()方法层层调用最后进入到RemoteConfigRepository init

  public RemoteConfigRepository(String namespace) {
    m_namespace = namespace;
    m_configCache = new AtomicReference<>();
    m_configUtil = ApolloInjector.getInstance(ConfigUtil.class);
    m_httpClient = ApolloInjector.getInstance(HttpClient.class);
    m_serviceLocator = ApolloInjector.getInstance(ConfigServiceLocator.class);
    remoteConfigLongPollService = ApolloInjector.getInstance(RemoteConfigLongPollService.class);
    m_longPollServiceDto = new AtomicReference<>();
    m_remoteMessages = new AtomicReference<>();
    m_loadConfigRateLimiter = RateLimiter.create(m_configUtil.getLoadConfigQPS());
    m_configNeedForceRefresh = new AtomicBoolean(true);
    m_loadConfigFailSchedulePolicy = new ExponentialSchedulePolicy(m_configUtil.getOnErrorRetryInterval(),
        m_configUtil.getOnErrorRetryInterval() * 8);
    //下面三个方法就是拉去配置的核心代码了  
    //同步远程配置 
    this.trySync();
    //固定频率刷新、内部实现也是调用trySync()
    this.schedulePeriodicRefresh();
    //轮询访问远程配置中心，每次访问直到超时或者apollo 配置发生变化
    this.scheduleLongPollingRefresh();
  }

3.同步远程配置

AbstractConfigRepository.trySync()同步远程配置方法，方法内部主要实现逻辑在sync()中

protected synchronized void sync() {
    Transaction transaction = Tracer.newTransaction("Apollo.ConfigService", "syncRemoteConfig");

    try {
      //m_configCache 
      ApolloConfig previous = m_configCache.get();
      //加载 远程 apollo config
      ApolloConfig current = loadApolloConfig();

      //reference equals means HTTP 304
      if (previous != current) {
        logger.debug("Remote Config refreshed!");
        m_configCache.set(current);
     	//getConfig 方法 将apollo配置放进 properties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();
    }
  }

private volatile AtomicReference<ApolloConfig> m_configCache; 使用了volatile 修饰，原子引用类型进行封装引用，这就是看源码的意义吧

4.发布ConfigFileChangeEvent事件

sync() 方法中的 调用fireRepositoryChange(),最后会调用到AbstractConfigFile.fireConfigChange()方法

  private void fireConfigChange(final ConfigFileChangeEvent changeEvent) {
    for (final ConfigFileChangeListener listener : m_listeners) {
      m_executorService.submit(new Runnable() {
        @Override
        public void run() {
          String listenerName = listener.getClass().getName();
          Transaction transaction = Tracer.newTransaction("Apollo.ConfigFileChangeListener", listenerName);
          try {
          	//listner 是一个PropertySourcesProcessor 中的一个lambda 类，
          	//ConfigChangeListener configChangeEventPublisher = changeEvent ->
        //applicationEventPublisher.publishEvent(new ApolloConfigChangeEvent(changeEvent));
            listener.onChange(changeEvent);
            transaction.setStatus(Transaction.SUCCESS);
          } catch (Throwable ex) {
            transaction.setStatus(ex);
            Tracer.logError(ex);
            logger.error("Failed to invoke config file change listener {}", listenerName, ex);
          } finally {
            transaction.complete();
          }
        }
      });
    }
  }

因此我们可以监听ApolloConfigChangeEvent事件来监听apollo配置是否发生产变化 eg：

public class ListenerApollo implements ConfigChangeListener, ApplicationListener<ApolloConfigChangeEvent> {
  @Override
  public void onChange(ConfigChangeEvent configChangeEvent) {
    //核心线程配置发生变化、重新set
    ConfigChange coreSize = configChangeEvent.getChange("threadPool.corePoolSize");
    if( coreSize!= null){
      ThreadPoolExecutor executor = SpringUtil.getBean(ThreadPoolExecutor.class);
      executor.setCorePoolSize(Integer.valueOf(coreSize.getNewValue()));
    }
  }

  @Override
  public void onApplicationEvent(ApolloConfigChangeEvent event) {
    this.onChange(event.getConfigChangeEvent());
  }
}

总结

分析apollo在Springboot启动的哪个环节被集成初始化的，以及跟了同步远程配置的逻辑，初始化时同步一次，定时同步，长轮询监听配置是否发生变化，发生变化又通知程序进行同步。并且同步后也会发布一个ApolloConfigChangeEvent事件,放开发者去监听配置变化。