Google Guava's LoadingCache

    private LoadingCache<String, JWSVerifier> macVerifierCache;
    ...
    macVerifierCache = CacheBuilder.newBuilder()
        .concurrencyLevel(4)
        .maximumSize(1048000)
        .expireAfterAccess(10, TimeUnit.MINUTES)
        .build(new CacheLoader<String, JWSVerifier>() {
                public JWSVerifier load(String key) throws UnsupportedEncodingException {
                    byte[] keyBytes = key.getBytes(StandardCharsets.UTF_8);
                    // NB: this will throw if the string is not at least 16 chars long
                    return new MACVerifier(keyBytes);
                }
            }
            );