apollo-biz/src/main/java/com/ctrip/framework/apollo/biz/message/ReleaseMessageScanner.java/ReleaseMessageScanner/ReleaseMessageScanner
class ReleaseMessageScanner:  public ReleaseMessageScanner() {
    listeners = Lists.newCopyOnWriteArrayList();
    executorService = Executors.newScheduledThreadPool(1, ApolloThreadFactory
        .create("ReleaseMessageScanner", true));
    missingReleaseMessages = Maps.newHashMap();
  }