#vulnerable code 
                                HttpResponse callBackend(HttpHost target, HttpRequest request, HttpContext context)
            throws IOException {

        Date requestDate = getCurrentDate();

        log.trace("Calling the backend");
        HttpResponse backendResponse = backend.execute(target, request, context);
        backendResponse.addHeader("Via", generateViaHeader(backendResponse));
        return handleBackendResponse(target, request, requestDate, getCurrentDate(),
                backendResponse);

    } 
                            #location 8 
                            #vulnerability type INTERFACE_NOT_THREAD_SAFE
#fixed code
                                CachingHttpClient(
            HttpClient client,
            HttpCache cache,
            CacheConfig config) {
        super();
        if (client == null) {
            throw new IllegalArgumentException("HttpClient may not be null");
        }
        if (cache == null) {
            throw new IllegalArgumentException("HttpCache may not be null");
        }
        if (config == null) {
            throw new IllegalArgumentException("CacheConfig may not be null");
        }
        this.maxObjectSizeBytes = config.getMaxObjectSize();
        this.sharedCache = config.isSharedCache();
        this.backend = client;
        this.responseCache = cache;
        this.validityPolicy = new CacheValidityPolicy();
        this.responseCachingPolicy = new ResponseCachingPolicy(maxObjectSizeBytes, sharedCache);
        this.responseGenerator = new CachedHttpResponseGenerator(this.validityPolicy);
        this.cacheableRequestPolicy = new CacheableRequestPolicy();
        this.suitabilityChecker = new CachedResponseSuitabilityChecker(this.validityPolicy, config);
        this.conditionalRequestBuilder = new ConditionalRequestBuilder();

        this.responseCompliance = new ResponseProtocolCompliance();
        this.requestCompliance = new RequestProtocolCompliance();

        this.asynchRevalidator = makeAsynchronousValidator(config);
    }