apollo-client/src/main/java/com/ctrip/framework/apollo/spring/property/SpringValueRegistry.java/SpringValueRegistry/initialize
class SpringValueRegistry:  private void initialize() {
    Executors.newSingleThreadScheduledExecutor(ApolloThreadFactory.create("SpringValueRegistry", true)).scheduleAtFixedRate(
        new Runnable() {
          @Override
          public void run() {
            try {
              scanAndClean();
            } catch (Throwable ex) {
              logger.error(ex.getMessage(), ex);
            }
          }
        }, CLEAN_INTERVAL_IN_SECONDS, CLEAN_INTERVAL_IN_SECONDS, TimeUnit.SECONDS);
  }