@VisibleForTesting @Nullable TimeUnit accessExpirationTimeUnit;
  @VisibleForTesting long refreshDuration;
  @VisibleForTesting @Nullable TimeUnit refreshTimeUnit;

  /** Specification; used for toParseableString(). */
  private final String specification;

  private CacheBuilderSpec(String specification) {
    this.specification = specification;
  }

  /**
   * Creates a CacheBuilderSpec from a string.

  @VisibleForTesting @Nullable TimeUnit accessExpirationTimeUnit;
  @VisibleForTesting long refreshDuration;
  @VisibleForTesting @Nullable TimeUnit refreshTimeUnit;

  /** Specification; used for toParseableString(). */
  private final String specification;

  private CacheBuilderSpec(String specification) {
    this.specification = specification;
  }

  /**
   * Creates a CacheBuilderSpec from a string.