public T get() {
    // Based on guidance from https://en.wikipedia.org/wiki/Double-checked_locking#Usage_in_Java
    if (instance == null) {
      synchronized (this) {
        if (instance == null) {
          instance = supplier.get();
        }
      }
    }
    return instance;
  }
@return the single lazily-instantiated object.
ambry-utils/src/main/java/com/github/ambry/utils/Singleton.java