output: #fixed code
                                  public void handleReference(Reference<?> ref) {
  
          poolLock.lock();
          try {
  
              if (ref instanceof BasicPoolEntryRef) {
                  // check if the GCed pool entry was still in use
                  //@@@ find a way to detect this without lookup
                  //@@@ flag in the BasicPoolEntryRef, to be reset when freed?
                  final boolean lost = issuedConnections.remove(ref);
                  if (lost) {
                      final HttpRoute route =
                          ((BasicPoolEntryRef)ref).getRoute();
                      if (LOG.isDebugEnabled()) {
                          LOG.debug("Connection garbage collected. " + route);
                      }
                      handleLostEntry(route);
                  }
              }
  
          } finally {
              poolLock.unlock();
          }
      }
input: #vulnerable code 
                                  public void handleReference(Reference<?> ref) {
  
          poolLock.lock();
          try {
  
              if (ref instanceof BasicPoolEntryRef) {
                  // check if the GCed pool entry was still in use
                  //@@@ find a way to detect this without lookup
                  //@@@ flag in the BasicPoolEntryRef, to be reset when freed?
                  final boolean lost = issuedConnections.remove(ref);
                  if (lost) {
                      final HttpRoute route =
                          ((BasicPoolEntryRef)ref).getRoute();
                      if (LOG.isDebugEnabled()) {
                          LOG.debug("Connection garbage collected. " + route);
                      }
                      handleLostEntry(route);
                  }
              } else if (ref instanceof ConnMgrRef) {
                  if (LOG.isDebugEnabled()) {
                      LOG.debug("Connection manager garbage collected.");
                  }
                  shutdown();
              }
  
          } finally {
              poolLock.unlock();
          }
      } 
                              #location 23 
                              #vulnerability type THREAD_SAFETY_VIOLATION