guava/src/com/google/common/base/Suppliers.java
public static <T> Supplier<T> synchronizedSupplier(Supplier<T> delegate) {
    return new ThreadSafeSupplier<T>(delegate);
  }
public static <T> Supplier<T> synchronizedSupplier(Supplier<T> delegate) {
    return new ThreadSafeSupplier<T>(delegate);
  }
Returns a supplier whose {@code get()} method synchronizes on {@code delegate} before calling
it, making it thread-safe.