docs: public V get(K key, Callable<V> callable) {
          Future<V> future = concurrentMap.get(key);
          if (future == null) {
              FutureTask<V> futureTask = new FutureTask<V>(callable);
              future = concurrentMap.putIfAbsent(key, futureTask);
              if (future == null) {
                  future = futureTask;
                  futureTask.run();
              }
          }
          try {
              // 此时阻塞
              return future.get();
          } catch (Exception e) {
              concurrentMap.remove(key);
              return null;
          }
      }