public FileServiceBuilder entryCacheSpec(String entryCacheSpec) {
        requireNonNull(entryCacheSpec, "entryCacheSpec");
        checkState(canSetEntryCacheSpec,
                   "Cannot call entryCacheSpec() if called maxCacheEntries() already.");
        this.entryCacheSpec = validateEntryCacheSpec(entryCacheSpec);
        canSetMaxCacheEntries = false;
        return this;
    }
Sets the {@linkplain CaffeineSpec Caffeine specification string} of the cache that stores the content
of the {@link HttpFile}s read by the {@link FileService}.
If not set, {@link Flags#fileServiceCacheSpec()} is used by default.
core/src/main/java/com/linecorp/armeria/server/file/FileServiceBuilder.java