apollo-client/src/main/java/com/ctrip/framework/apollo/spring/property/SpringValueRegistry.java/SpringValueRegistry/scanAndClean
class SpringValueRegistry:  private void scanAndClean() {
    Iterator<Multimap<String, SpringValue>> iterator = registry.values().iterator();
    while (!Thread.currentThread().isInterrupted() && iterator.hasNext()) {
      Multimap<String, SpringValue> springValues = iterator.next();
      Iterator<Entry<String, SpringValue>> springValueIterator = springValues.entries().iterator();
      while (springValueIterator.hasNext()) {
        Entry<String, SpringValue> springValue = springValueIterator.next();
        if (!springValue.getValue().isTargetBeanValid()) {
          // clear unused spring values
          springValueIterator.remove();
        }
      }
    }
  }