output: #fixed code
                                  @Override
      public void execute(Tuple input) {
  
          // triggered by the arrival of a tuple
          // be it a tick or normal one
          flushQueues();
  
          if (isTickTuple(input)) {
              _collector.ack(input);
              return;
          }
  
          CountMetric metric = metricGauge.scope("activethreads");
          metric.getValueAndReset();
          metric.incrBy(this.activeThreads.get());
  
          metric = metricGauge.scope("in queues");
          metric.getValueAndReset();
          metric.incrBy(this.fetchQueues.inQueues.get());
  
          metric = metricGauge.scope("queues");
          metric.getValueAndReset();
          metric.incrBy(this.fetchQueues.queues.size());
  
          LOG.info("[Fetcher #" + taskIndex + "] Threads : "
                  + this.activeThreads.get() + "\tqueues : "
                  + this.fetchQueues.queues.size() + "\tin_queues : "
                  + this.fetchQueues.inQueues.get());
  
          String url = input.getStringByField("url");
          // check whether this tuple has a url field
          if (url == null) {
              LOG.info("[Fetcher #" + taskIndex
                      + "] Missing url field for tuple " + input);
              // ignore silently
              _collector.ack(input);
              return;
          }
  
          fetchQueues.addFetchItem(input);
      }
input: #vulnerable code 
                                  @Override
      public void execute(Tuple input) {
  
          // main thread in charge of acking and failing
          // see
          // https://github.com/nathanmarz/storm/wiki/Troubleshooting#nullpointerexception-from-deep-inside-storm
  
          int acked = 0;
          int failed = 0;
          int emitted = 0;
  
          // emit with or without anchors
          // before acking
          synchronized (emitQueue) {
              for (Object[] toemit : this.emitQueue) {
                  String streamID = (String) toemit[0];
                  Tuple anchor = (Tuple) toemit[1];
                  Values vals = (Values) toemit[2];
                  if (anchor == null)
                      _collector.emit(streamID, vals);
                  else
                      _collector.emit(streamID, Arrays.asList(anchor), vals);
              }
              emitted = emitQueue.size();
              emitQueue.clear();
          }
  
          // have a tick tuple to make sure we don't get starved
          synchronized (ackQueue) {
              for (Tuple toack : this.ackQueue) {
                  _collector.ack(toack);
              }
              acked = ackQueue.size();
              ackQueue.clear();
          }
  
          synchronized (failQueue) {
              for (Tuple toack : this.failQueue) {
                  _collector.fail(toack);
              }
              failed = failQueue.size();
              failQueue.clear();
          }
  
          if (acked + failed + emitted > 0)
              LOG.info("[Fetcher #" + taskIndex + "] Acked : " + acked
                      + "\tFailed : " + failed + "\tEmitted : " + emitted);
  
          if (isTickTuple(input)) {
              _collector.ack(input);
              return;
          }
  
          CountMetric metric = metricGauge.scope("activethreads");
          metric.getValueAndReset();
          metric.incrBy(this.activeThreads.get());
  
          metric = metricGauge.scope("in queues");
          metric.getValueAndReset();
          metric.incrBy(this.fetchQueues.inQueues.get());
  
          metric = metricGauge.scope("queues");
          metric.getValueAndReset();
          metric.incrBy(this.fetchQueues.queues.size());
  
          LOG.info("[Fetcher #" + taskIndex + "] Threads : "
                  + this.activeThreads.get() + "\tqueues : "
                  + this.fetchQueues.queues.size() + "\tin_queues : "
                  + this.fetchQueues.inQueues.get());
  
          String url = input.getStringByField("url");
          // check whether this tuple has a url field
          if (url == null) {
              LOG.info("[Fetcher #" + taskIndex
                      + "] Missing url field for tuple " + input);
              // ignore silently
              _collector.ack(input);
              return;
          }
  
          fetchQueues.addFetchItem(input);
      } 
                              #location 50 
                              #vulnerability type THREAD_SAFETY_VIOLATION