public class MyServlet extends HttpServlet {

    public Cache cache;

    public void init() {
        cache = new Cache();
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) {

        cache.putAsync(...);

    }

    public void destroy() {
        if (cache != null)
            cache.close();
    }
}

public class Cache {
    public ExecutorService executor = Executors.newFixedThreadPool(20);

    public CompletableFuture<Boolean> putAsync(String idInCache, Map<String, Object> parameters) {

        CompletableFuture<Boolean> future = CompletableFuture.supplyAsync(() -> put(idInCache, parameters), executor);
        return future;
    }

    public void close() {
        executor.shutdowns();
        try {
            executor.awaitTermination(3000, TimeUnit.MILLISECONDS);
        } catch (InterruptedException e) {
            loggerCache.log(Level.SEVERE, e.getMessage());
        }
    }
}