@Nullable
    protected final String generateEntityTag(HttpFileAttributes attrs) {
        requireNonNull(attrs, "attrs");
        return entityTagFunction != null ? entityTagFunction.apply(pathOrUri(), attrs) : null;
    }
Generates an entity tag of the file with the given attributes using the {@code entityTagFunction}
which was specified with the constructor.

@return the entity tag or {@code null} if {@code entityTagFunction} is {@code null}.
core/src/main/java/com/linecorp/armeria/server/file/AbstractHttpFile.java