普通情况下，使用 实现apollo的监听器ConfigChangeListener，通过onChange方法来实时获取配置。

但是，如果对配置变化的时效性要求不高，只是想在用到配置的时候，使用新配置的话，可以不实现监听器，直接从environment里面获取配置。

apollo机制

图解

代码分析

还是通过老朋友refresh方法来层层揭晓吧。

首先，通过AbstractApplicationContext.refresh()中：

invokeBeanFactoryPostProcessors(beanFactory);

经历重重磨难，找到了PropertySourcesProcessor,并执行其postProcessBeanFactory方法，进入initializePropertySources()。

主要流程

1.创建Config

2.用CompositePropertySource包装Config，并塞入environment

private void initializePropertySources() {if (environment.getPropertySources().contains(PropertySourcesConstants.APOLLO_PROPERTY_SOURCE_NAME)) {// 已经初始化过apollo配置就直接返回return;}// 创建一个PropertySource，用于装入所有的配置CompositePropertySource composite = new CompositePropertySource(PropertySourcesConstants.APOLLO_PROPERTY_SOURCE_NAME);
......while (iterator.hasNext()) {int order = iterator.next();for (String namespace : NAMESPACE_NAMES.get(order)) {// 获取一个config,如果没有，则会创建一个config（后续分析，很重要哦）Config config = ConfigService.getConfig(namespace);// 用CompositePropertySource包装config为一个PropertySource，后续加入值environmentcomposite.addPropertySource(configPropertySourceFactory.getConfigPropertySource(namespace, config));}}// clean upNAMESPACE_NAMES.clear();// 如果存在ApolloBootstrapPropertySources，则要保证ApolloBootstrapPropertySources在第一个，并且将上述的composite设置在其后，否则就是将composite设置在第一个if (environment.getPropertySources().contains(PropertySourcesConstants.APOLLO_BOOTSTRAP_PROPERTY_SOURCE_NAME)) {ensureBootstrapPropertyPrecedence(environment);environment.getPropertySources().addAfter(PropertySourcesConstants.APOLLO_BOOTSTRAP_PROPERTY_SOURCE_NAME, composite);} else {// 将composite设置在第一个后，即便我们不适用listener方式，被通知配置的变更，也能直接从environment里面通过getProperty的方式直接获取配置environment.getPropertySources().addFirst(composite);}
}

那么，了解完两部曲，就来细细品品其中的机制吧。一个config，为啥就能让你实时知道apollo的变更呢？

创建RemoteConfigRepository

之前说的 ConfigService.getConfig(namespace) ，获取不到config，就会创建一个config的呀。此时创建的就是RemoteConfigRepository。看其构造方法，除了各种属性赋值后，还会调用三个方法，它们很关键哈。

public RemoteConfigRepository(String namespace) {m_namespace = namespace;m_configCache = new AtomicReference<>();m_configUtil = ApolloInjector.getInstance(ConfigUtil.class);m_httpUtil = ApolloInjector.getInstance(HttpUtil.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);gson = new Gson();// 首次同步apollothis.trySync();// 定时刷新配置（大部分情况返回304，定时刷新在于可以防止长轮询失败）this.schedulePeriodicRefresh();// 长轮询刷新配置（最主要的实时获取配置的途径）this.scheduleLongPollingRefresh();
}

首次同步apollo

@Override
protected synchronized void sync() {Transaction transaction = Tracer.newTransaction("Apollo.ConfigService", "syncRemoteConfig");try {ApolloConfig previous = m_configCache.get();// 根据你设定的apollo地址，appId，maxRetries等信息，发送get请求，获取当前apollo配置信息ApolloConfig current = loadApolloConfig();// 更新本地的apollo配置信息if (previous != current) {logger.debug("Remote Config refreshed!");m_configCache.set(current);// 刷新配置的任务可能会调用该方法，获取配置，并且通知客户端的监听器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();}
}

开启定时刷新配置

private void schedulePeriodicRefresh() {logger.debug("Schedule periodic refresh with interval: {} {}",m_configUtil.getRefreshInterval(), m_configUtil.getRefreshIntervalTimeUnit());m_executorService.scheduleAtFixedRate(new Runnable() {@Overridepublic void run() {Tracer.logEvent("Apollo.ConfigService", String.format("periodicRefresh: %s", m_namespace));logger.debug("refresh config for namespace: {}", m_namespace);// 和上述首次同步的trySync是同一个方法trySync();Tracer.logEvent("Apollo.Client.Version", Apollo.VERSION);}//时间间隔默认是5分钟}, m_configUtil.getRefreshInterval(), m_configUtil.getRefreshInterval(),m_configUtil.getRefreshIntervalTimeUnit());
}

开启长轮询刷新配置

public boolean submit(String namespace, RemoteConfigRepository remoteConfigRepository) {boolean added = m_longPollNamespaces.put(namespace, remoteConfigRepository);m_notifications.putIfAbsent(namespace, INIT_NOTIFICATION_ID);if (!m_longPollStarted.get()) {// 开启拉取  startLongPolling();}return added;
}private void doLongPollingRefresh(String appId, String cluster, String dataCenter) {final Random random = new Random();ServiceDTO lastServiceDto = null;while (!m_longPollingStopped.get() && !Thread.currentThread().isInterrupted()) {if (!m_longPollRateLimiter.tryAcquire(5, TimeUnit.SECONDS)) {// 等待5秒try {TimeUnit.SECONDS.sleep(5);} catch (InterruptedException e) {}}// 获取各种信息，发起请求，看apollo中配置有无变更......// 配置发生变化，则主动获取新配置，且调用ConfigChangeListener的实现类的onChange方法    if (response.getStatusCode() == 200 && response.getBody() != null) {updateNotifications(response.getBody());updateRemoteNotifications(response.getBody());transaction.addData("Result", response.getBody().toString());notify(lastServiceDto, response.getBody());}......} finally {transaction.complete();}}
}

notify方法会最终进入fireRepositoryChange

fireRepositoryChange

该方法是之前介绍的sync()方法里面提到的，两类刷新配置的任务都会执行此方法，来通知各个监听器，有哪些配置变更。
1.获取新配置

2.统计各种配置的变更

3.通知各个监听器

protected void fireRepositoryChange(String namespace, Properties newProperties) {// 遍历所有的监听器for (RepositoryChangeListener listener : m_listeners) {try {// 把最新的配置信息传递给监听器listener.onRepositoryChange(namespace, newProperties);} catch (Throwable ex) {Tracer.logError(ex);logger.error("Failed to invoke repository change listener {}", listener.getClass(), ex);}}
}@Override
public synchronized void onRepositoryChange(String namespace, Properties newProperties) {if (newProperties.equals(m_configProperties.get())) {return;}ConfigSourceType sourceType = m_configRepository.getSourceType();Properties newConfigProperties = new Properties();newConfigProperties.putAll(newProperties);// 整理配置信息，确定配置的变更类型Map<String, ConfigChange> actualChanges = updateAndCalcConfigChanges(newConfigProperties, sourceType);//check double checked resultif (actualChanges.isEmpty()) {return;}// 调用各个ConfigChangeListener实现类的onChange方法，发送配置信息this.fireConfigChange(new ConfigChangeEvent(m_namespace, actualChanges));Tracer.logEvent("Apollo.Client.ConfigChanges", m_namespace);
}private Map<String, ConfigChange> updateAndCalcConfigChanges(Properties newConfigProperties,ConfigSourceType sourceType) {// 初次统计变化的配置  List<ConfigChange> configChanges =calcPropertyChanges(m_namespace, m_configProperties.get(), newConfigProperties);ImmutableMap.Builder<String, ConfigChange> actualChanges =new ImmutableMap.Builder<>();/** === Double check since DefaultConfig has multiple config sources ==== **///1. 为配置设置旧值for (ConfigChange change : configChanges) {change.setOldValue(this.getProperty(change.getPropertyName(), change.getOldValue()));}//2. 更新 m_configPropertiesupdateConfig(newConfigProperties, sourceType);clearConfigCache();//3. 遍历所有的新配置，最后确认各个配置的type（ADDED/MODIFIED/DELETED）for (ConfigChange change : configChanges) {change.setNewValue(this.getProperty(change.getPropertyName(), change.getNewValue()));switch (change.getChangeType()) {case ADDED:if (Objects.equals(change.getOldValue(), change.getNewValue())) {break;}if (change.getOldValue() != null) {change.setChangeType(PropertyChangeType.MODIFIED);}actualChanges.put(change.getPropertyName(), change);break;case MODIFIED:if (!Objects.equals(change.getOldValue(), change.getNewValue())) {actualChanges.put(change.getPropertyName(), change);}break;case DELETED:if (Objects.equals(change.getOldValue(), change.getNewValue())) {break;}if (change.getNewValue() != null) {change.setChangeType(PropertyChangeType.MODIFIED);}actualChanges.put(change.getPropertyName(), change);break;default://do nothingbreak;}}return actualChanges.build();
}