public static Builder copy(final CacheConfig config) {
        Args.notNull(config, "Cache config");
        return new Builder()
            .setMaxObjectSize(config.getMaxObjectSize())
            .setMaxCacheEntries(config.getMaxCacheEntries())
            .setMaxUpdateRetries(config.getMaxUpdateRetries())
            .setHeuristicCachingEnabled(config.isHeuristicCachingEnabled())
            .setHeuristicCoefficient(config.getHeuristicCoefficient())
            .setHeuristicDefaultLifetime(config.getHeuristicDefaultLifetime())
            .setSharedCache(config.isSharedCache())
            .setAsynchronousWorkers(config.getAsynchronousWorkers())
            .setNeverCacheHTTP10ResponsesWithQueryString(config.isNeverCacheHTTP10ResponsesWithQuery())
            .setNeverCacheHTTP11ResponsesWithQueryString(config.isNeverCacheHTTP11ResponsesWithQuery())
            .setStaleIfErrorEnabled(config.isStaleIfErrorEnabled());
    }
Returns the maximum number of threads to allow for background
revalidations due to the {@code stale-while-revalidate} directive. A
value of 0 means background revalidations are disabled.
httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/CacheConfig.java