java.util.concurrent.ConcurrentLinkedQueue#isEmpty ( )源码实例Demo

/**
 * 统一添加订阅
 * @param list 订阅list
 * @param onInfo 订阅信息
 * @param symbol 订阅币对
 */
private <T> Okexv3WebSocketClient addInfo(ConcurrentLinkedQueue<WebSocketInfo<T>> list,
                                          WebSocketInfo<T> onInfo, String symbol) {
    if (!list.isEmpty()) {
        // 该币对已经有订阅, 加入订阅队列即可
        list.add(onInfo);
    } else { // 无订阅
        list.add(onInfo);
        // 取出这个币对的连接
        Okexv3WebSocketClient client = Okexv3WebSocketExchange.CLIENTS.get(symbol);
        if (!exist(client)) {
            // 该币对无连接，新建一个
            client = new Okexv3WebSocketClient(symbol, super.log);
            Okexv3WebSocketExchange.CLIENTS.put(symbol, client);
        }
        // 对这个连接订阅ticker
        return client;
    }
    return null;
}
 

private IOContext getIOContext() {
    IOContext ioContext = null;
    final ConcurrentLinkedQueue<IOContext> recyclerQueue = mRecyclers;
    if (recyclerQueue != null && !recyclerQueue.isEmpty()) {
        ioContext = recyclerQueue.poll();
    }

    if (ioContext == null) {
        ioContext = new IOContext();
    }

    ioContext.state = IOContext.IDLE;
    ioContext.ioRecord.reset();

    return ioContext;
}
 

@Test
public void throwsAnExceptionIfCalledFromMultipleThreads() throws InterruptedException {
	final NumGen numGen = new NumGen(123456);
	numGen.nextByte();

	final ConcurrentLinkedQueue<String> results = new ConcurrentLinkedQueue<>();

	new Thread(() -> {
		try {
			numGen.nextByte();
		} catch (RuntimeException e) {
			results.add(e.getMessage());
			return;
		}
		results.add("no exception");
	}).start();

	while (results.isEmpty())
		Thread.sleep(10);

	assertTrue(results.peek().startsWith("RanGen accessed from multiple threads"));
}
 

@Test
public void concurrentExecution() throws InterruptedException {
  int nThreads = Runtime.getRuntime().availableProcessors();
  int resultsPerThread = 10;
  ConcurrentLinkedQueue<String> errors = new ConcurrentLinkedQueue<>();
  CountDownLatch latch = new CountDownLatch(1);
  int expectedResultCount = nThreads * resultsPerThread;
  Listener listener = new Listener(errors, latch);
  Consumer<CalculationResults> wrapper =
      new ListenerWrapper(listener, expectedResultCount, ImmutableList.of(), ImmutableList.of());
  ExecutorService executor = Executors.newFixedThreadPool(nThreads);
  CalculationResult result = CalculationResult.of(0, 0, Result.failure(FailureReason.ERROR, "foo"));
  CalculationTarget target = new CalculationTarget() {};
  CalculationResults results = CalculationResults.of(target, ImmutableList.of(result));
  IntStream.range(0, expectedResultCount).forEach(i -> executor.submit(() -> wrapper.accept(results)));

  latch.await();
  executor.shutdown();

  if (!errors.isEmpty()) {
    String allErrors = errors.stream().collect(joining("\n"));
    fail(allErrors);
  }
}
 

/**
 * 合并commit 位置
 * ----------------------|    |--------------------|    |--------------------|
 * node1.isCommit = true | -> | node2.isCommit=true| -> |node3.isCommit=false| ...
 * ----------------------|    |--------------------|    |--------------------|
 * <p>
 * then the result remove 1node leave 2node... behind
 */
private void truncLogPosQueue(ConcurrentLinkedQueue<LogPosition> logPositions) {
    if (logPositions.isEmpty()) {
        LogUtils.warn.warn("no binlog position object in queue");
        return;
    }

    // 根据concurrent list 实现方式 一次size 相当于 直接遍历一遍 链表
    LogPosition curr = null;
    LogPosition pre = null;

    LinkedList<LogPosition> rms = new LinkedList<LogPosition>();
    Iterator<LogPosition> iterator = logPositions.iterator();

    while (iterator.hasNext()) {
        if (pre == null) {
            pre = iterator.next();
            continue;
        }
        curr = iterator.next();

        if (pre.isCommit() && curr.isCommit()) {
            rms.add(pre);
        }
        pre = curr;
    }

    removeQueueWithLock(logPositions, rms);

    // 轻易不要开work日志
    debugLogPosition(logPositions);
}
 

/**
 * 尝试从过期未应答队列中获取分区段进行消费
 *
 * @param consumer 消费者
 * @return 过期未应答的分区段
 */
private PartitionSegment pollPartitionSegment(Consumer consumer, List<Short> partitionList) {
    for (Short partition : partitionList) {
        ConsumePartition consumePartition = new ConsumePartition(consumer.getTopic(), consumer.getApp(), partition);
        ConcurrentLinkedQueue<PartitionSegment> partitionSegmentQueue = expireQueueMap.get(consumePartition);
        if (partitionSegmentQueue != null && !partitionSegmentQueue.isEmpty()) {
            return partitionSegmentQueue.poll();
        }
    }
    // 获取不到，则返回空
    return null;
}
 

/**
 * Call to initiate work
 */
synchronized public void initiateWork()
{
    ConcurrentLinkedQueue<EmojiWorker> taskList = getTaskList();
    if (!taskList.isEmpty())
    {
        currentWorker = taskList.poll();
        setLocation(getParent().getLocation().x + (getParent().getWidth() - getWidth()) / 2, getParent().getLocation().y + (getParent().getHeight() - getHeight()) / 2);
        cancelButton.setEnabled(true);
        update(currentWorker.getInitialReport());
        currentWorker.execute();
    }
}
 

private static void fillHashtableWithUnlinked(BuildingJobDataProvider provider, String startJob, Hashtable<String, SimpleBuildingJob> converted) {
	ConcurrentLinkedQueue<String> toBuild = new ConcurrentLinkedQueue<>();
	toBuild.offer(startJob);
	while (!toBuild.isEmpty()) {
		String currentName = toBuild.poll();
		if (!converted.containsKey(currentName)) {
			SimpleBuildingJob job = createUnlinkedJob(provider, toBuild, currentName);

			converted.put(currentName, job);
		}
	}
}
 

@SuppressWarnings("unchecked")
private void setupInputChannels() throws IOException, InterruptedException {

	for (int i = 0; i < numInputChannels; i++) {
		final int channelIndex = i;
		final RecordSerializer<SerializationDelegate<Object>> recordSerializer = new SpanningRecordSerializer<SerializationDelegate<Object>>();
		final SerializationDelegate<Object> delegate = (SerializationDelegate<Object>) (SerializationDelegate<?>)
			new SerializationDelegate<StreamElement>(new StreamElementSerializer<T>(serializer));

		inputQueues[channelIndex] = new ConcurrentLinkedQueue<InputValue<Object>>();
		inputChannels[channelIndex] = new TestInputChannel(inputGate, i);

		final BufferAndAvailabilityProvider answer = () -> {
			ConcurrentLinkedQueue<InputValue<Object>> inputQueue = inputQueues[channelIndex];
			InputValue<Object> input;
			boolean moreAvailable;
			synchronized (inputQueue) {
				input = inputQueue.poll();
				moreAvailable = !inputQueue.isEmpty();
			}
			if (input != null && input.isStreamEnd()) {
				inputChannels[channelIndex].setReleased();
				return Optional.of(new BufferAndAvailability(EventSerializer.toBuffer(EndOfPartitionEvent.INSTANCE), moreAvailable, 0));
			} else if (input != null && input.isStreamRecord()) {
				Object inputElement = input.getStreamRecord();

				delegate.setInstance(inputElement);
				recordSerializer.serializeRecord(delegate);
				BufferBuilder bufferBuilder = createBufferBuilder(bufferSize);
				recordSerializer.copyToBufferBuilder(bufferBuilder);
				bufferBuilder.finish();

				// Call getCurrentBuffer to ensure size is set
				return Optional.of(new BufferAndAvailability(buildSingleBuffer(bufferBuilder), moreAvailable, 0));
			} else if (input != null && input.isEvent()) {
				AbstractEvent event = input.getEvent();
				return Optional.of(new BufferAndAvailability(EventSerializer.toBuffer(event), moreAvailable, 0));
			} else {
				return Optional.empty();
			}
		};

		inputChannels[channelIndex].addBufferAndAvailability(answer);

		inputGate.setInputChannel(new IntermediateResultPartitionID(),
			inputChannels[channelIndex]);
	}
}
 

@SuppressWarnings("unchecked")
private void setupInputChannels() throws IOException, InterruptedException {

	for (int i = 0; i < numInputChannels; i++) {
		final int channelIndex = i;
		final RecordSerializer<SerializationDelegate<Object>> recordSerializer = new SpanningRecordSerializer<SerializationDelegate<Object>>();
		final SerializationDelegate<Object> delegate = (SerializationDelegate<Object>) (SerializationDelegate<?>)
			new SerializationDelegate<StreamElement>(new StreamElementSerializer<T>(serializer));

		inputQueues[channelIndex] = new ConcurrentLinkedQueue<InputValue<Object>>();
		inputChannels[channelIndex] = new TestInputChannel(inputGate, i);

		final BufferAndAvailabilityProvider answer = () -> {
			ConcurrentLinkedQueue<InputValue<Object>> inputQueue = inputQueues[channelIndex];
			InputValue<Object> input;
			boolean moreAvailable;
			synchronized (inputQueue) {
				input = inputQueue.poll();
				moreAvailable = !inputQueue.isEmpty();
			}
			if (input != null && input.isStreamEnd()) {
				inputChannels[channelIndex].setReleased();
				return Optional.of(new BufferAndAvailability(EventSerializer.toBuffer(EndOfPartitionEvent.INSTANCE), moreAvailable, 0));
			} else if (input != null && input.isStreamRecord()) {
				Object inputElement = input.getStreamRecord();

				delegate.setInstance(inputElement);
				recordSerializer.serializeRecord(delegate);
				BufferBuilder bufferBuilder = createBufferBuilder(bufferSize);
				recordSerializer.copyToBufferBuilder(bufferBuilder);
				bufferBuilder.finish();

				// Call getCurrentBuffer to ensure size is set
				return Optional.of(new BufferAndAvailability(buildSingleBuffer(bufferBuilder), moreAvailable, 0));
			} else if (input != null && input.isEvent()) {
				AbstractEvent event = input.getEvent();
				if (event instanceof EndOfPartitionEvent) {
					inputChannels[channelIndex].setReleased();
				}

				return Optional.of(new BufferAndAvailability(EventSerializer.toBuffer(event), moreAvailable, 0));
			} else {
				return Optional.empty();
			}
		};

		inputChannels[channelIndex].addBufferAndAvailability(answer);

		inputGate.setInputChannel(new IntermediateResultPartitionID(),
			inputChannels[channelIndex]);
	}
}
 

public CoreDataStore(CrailConfiguration conf) throws Exception {
	CrailConstants.updateConstants(conf);
	CrailConstants.printConf();
	CrailConstants.verify();

	this.bufferCache = BufferCache.createInstance(CrailConstants.CACHE_IMPL);
	this.statistics = new CrailStatistics();

	//Datanodes
	StringTokenizer tokenizer = new StringTokenizer(CrailConstants.STORAGE_TYPES, ",");
	LinkedList<StorageClient> dataNodeClients = new LinkedList<StorageClient>();
	while (tokenizer.hasMoreTokens()){
		String name = tokenizer.nextToken();
		StorageClient dataNode = StorageClient.createInstance(name);
		dataNode.init(statistics, bufferCache, conf, null);
		dataNode.printConf(LOG);
		dataNodeClients.add(dataNode);
	}
	this.datanodeEndpointCache = new EndpointCache(fsId, dataNodeClients);

	//Namenode
	InetSocketAddress nnAddr = CrailUtils.getNameNodeAddress();
	this.rpcClient = RpcClient.createInstance(CrailConstants.NAMENODE_RPC_TYPE);
	rpcClient.init(conf, null);
	rpcClient.printConf(LOG);
	ConcurrentLinkedQueue<InetSocketAddress> namenodeList = CrailUtils.getNameNodeList();
	ConcurrentLinkedQueue<RpcConnection> connectionList = new ConcurrentLinkedQueue<RpcConnection>();
	while(!namenodeList.isEmpty()){
		InetSocketAddress address = namenodeList.poll();
		RpcConnection connection = rpcClient.connect(address);
		connectionList.add(connection);
	}
	if (connectionList.size() == 1){
		this.rpcConnection = connectionList.poll();
	} else {
		this.rpcConnection = new RpcDispatcher(connectionList);
	}
	LOG.info("connected to namenode(s) " + rpcConnection);

	//Client
	this.fsId = fsCount.getAndIncrement();
	this.localClass = CrailUtils.getLocationClass();
	this.blockCache = new BlockCache();
	this.nextBlockCache = new NextBlockCache();
	this.openInputStreams = new ConcurrentHashMap<Long, CoreInputStream>();
	this.openOutputStreams = new ConcurrentHashMap<Long, CoreOutputStream>();
	this.streamCounter = new AtomicLong(0);
	this.isOpen = true;
	this.bufferCheckpoint = new BufferCheckpoint();
	this.locationMap = new ConcurrentHashMap<String, String>();
	CrailUtils.parseMap(CrailConstants.LOCATION_MAP, locationMap);

	this.ioStatsIn = new CoreIOStatistics("core/input");
	statistics.addProvider(ioStatsIn);
	this.ioStatsOut = new CoreIOStatistics("core/output");
	statistics.addProvider(ioStatsOut);
	this.streamStats = new CoreStreamStatistics();
	statistics.addProvider(streamStats);
	statistics.addProvider(bufferCache);
	statistics.addProvider(datanodeEndpointCache);
}
 

public CoreFileSystem(CrailConfiguration conf) throws Exception {
	CrailConstants.updateConstants(conf);
	CrailConstants.printConf();
	CrailConstants.verify();	
	
	//Datanodes
	StringTokenizer tokenizer = new StringTokenizer(CrailConstants.STORAGE_TYPES, ",");
	LinkedList<StorageClient> dataNodeClients = new LinkedList<StorageClient>(); 
	while (tokenizer.hasMoreTokens()){
		String name = tokenizer.nextToken();
		StorageClient dataNode = StorageClient.createInstance(name);
		dataNode.init(conf, null);
		dataNode.printConf(LOG);
		dataNodeClients.add(dataNode);
	}
	this.datanodeEndpointCache = new EndpointCache(fsId, dataNodeClients);
	
	//Namenode
	InetSocketAddress nnAddr = CrailUtils.getNameNodeAddress();
	this.rpcClient = RpcClient.createInstance(CrailConstants.NAMENODE_RPC_TYPE);
	rpcClient.init(conf, null);
	rpcClient.printConf(LOG);	
	ConcurrentLinkedQueue<InetSocketAddress> namenodeList = CrailUtils.getNameNodeList();
	ConcurrentLinkedQueue<RpcConnection> connectionList = new ConcurrentLinkedQueue<RpcConnection>();
	while(!namenodeList.isEmpty()){
		InetSocketAddress address = namenodeList.poll();
		RpcConnection connection = rpcClient.connect(address);
		connectionList.add(connection);
	}
	if (connectionList.size() == 1){
		this.rpcConnection = connectionList.poll();
	} else {
		this.rpcConnection = new RpcDispatcher(connectionList);
	}
	LOG.info("connected to namenode(s) " + rpcConnection);		
	
	//Client
	this.fsId = fsCount.getAndIncrement();
	this.localClass = CrailUtils.getLocationClass();
	this.bufferCache = BufferCache.createInstance(CrailConstants.CACHE_IMPL);
	this.blockCache = new BlockCache();
	this.nextBlockCache = new NextBlockCache();
	this.openInputStreams = new ConcurrentHashMap<Long, CoreInputStream>();
	this.openOutputStreams = new ConcurrentHashMap<Long, CoreOutputStream>();
	this.streamCounter = new AtomicLong(0);
	this.isOpen = true;
	this.bufferCheckpoint = new BufferCheckpoint();
	this.locationMap = new ConcurrentHashMap<String, String>();
	CrailUtils.parseMap(CrailConstants.LOCATION_MAP, locationMap);
	
	this.statistics = new CrailStatistics();
	this.ioStatsIn = new CoreIOStatistics("core/input");
	statistics.addProvider(ioStatsIn);
	this.ioStatsOut = new CoreIOStatistics("core/output");
	statistics.addProvider(ioStatsOut);
	this.streamStats = new CoreStreamStatistics();
	statistics.addProvider(streamStats);
	statistics.addProvider(bufferCache);
	statistics.addProvider(datanodeEndpointCache);
}
 

@Test
public void testParserDbDataThreadSafety() throws Throwable {
    final int numThreads = 500;
    final String uaQuery = "Mozilla/5.0 (Windows NT 10.0; WOW64; rv:40.0) Gecko/20100101 Firefox/40.0";

    final CyclicBarrier gate = new CyclicBarrier(numThreads);
    final ConcurrentLinkedQueue<Throwable> failures = new ConcurrentLinkedQueue<>();

    Thread[] threads = new Thread[numThreads];
    for (int i = 0; i < numThreads; i++) {
        threads[i] = new Thread(new Runnable() {
            @Override
            public void run() {
                UdgerParser threadParser = new UdgerParser(parserDbData);
                try {
                    gate.await();
                    for (int j = 0; j < 100; j++) {
                        UdgerUaResult qr = threadParser.parseUa(uaQuery);
                        assertEquals(qr.getUa(), "Firefox 40.0");
                        assertEquals(qr.getOs(), "Windows 10");
                        assertEquals(qr.getUaFamily(), "Firefox");
                    }
                } catch (Throwable t) {
                    failures.add(t);
                }
            }
        });
        threads[i].start();
    }

    for (int i = 0; i < numThreads; i++) {
        threads[i].join();
    }

    if (!failures.isEmpty()) {
        for (Throwable throwable : failures) {
            throwable.printStackTrace();
        }

        fail("Parsing threads failed, see printed exceptions");
    }
}
 

@SuppressWarnings("unchecked")
private void setupInputChannels() {

	for (int i = 0; i < numInputChannels; i++) {
		final int channelIndex = i;
		final RecordSerializer<SerializationDelegate<Object>> recordSerializer = new SpanningRecordSerializer<SerializationDelegate<Object>>();
		final SerializationDelegate<Object> delegate = (SerializationDelegate<Object>) (SerializationDelegate<?>)
			new SerializationDelegate<>(new StreamElementSerializer<T>(serializer));

		inputQueues[channelIndex] = new ConcurrentLinkedQueue<>();
		inputChannels[channelIndex] = new TestInputChannel(inputGate, i);

		final BufferAndAvailabilityProvider answer = () -> {
			ConcurrentLinkedQueue<InputValue<Object>> inputQueue = inputQueues[channelIndex];
			InputValue<Object> input;
			boolean moreAvailable;
			synchronized (inputQueue) {
				input = inputQueue.poll();
				moreAvailable = !inputQueue.isEmpty();
			}
			if (input != null && input.isStreamEnd()) {
				inputChannels[channelIndex].setReleased();
				return Optional.of(new BufferAndAvailability(EventSerializer.toBuffer(EndOfPartitionEvent.INSTANCE), moreAvailable, 0));
			} else if (input != null && input.isStreamRecord()) {
				Object inputElement = input.getStreamRecord();

				delegate.setInstance(inputElement);
				recordSerializer.serializeRecord(delegate);
				BufferBuilder bufferBuilder = createBufferBuilder(bufferSize);
				BufferConsumer bufferConsumer = bufferBuilder.createBufferConsumer();
				recordSerializer.copyToBufferBuilder(bufferBuilder);
				bufferBuilder.finish();

				// Call getCurrentBuffer to ensure size is set
				return Optional.of(new BufferAndAvailability(bufferConsumer.build(), moreAvailable, 0));
			} else if (input != null && input.isEvent()) {
				AbstractEvent event = input.getEvent();
				if (event instanceof EndOfPartitionEvent) {
					inputChannels[channelIndex].setReleased();
				}

				return Optional.of(new BufferAndAvailability(EventSerializer.toBuffer(event), moreAvailable, 0));
			} else {
				return Optional.empty();
			}
		};

		inputChannels[channelIndex].addBufferAndAvailability(answer);
	}
	inputGate.setInputChannels(inputChannels);
}
 

public static void testCacheMultiThreaded(final BlockCache toBeTested,
    final int blockSize, final int numThreads, final int numQueries,
    final double passingScore) throws Exception {

  Configuration conf = new Configuration();
  MultithreadedTestUtil.TestContext ctx = new MultithreadedTestUtil.TestContext(
      conf);

  final AtomicInteger totalQueries = new AtomicInteger();
  final ConcurrentLinkedQueue<HFileBlockPair> blocksToTest = new ConcurrentLinkedQueue<>();
  final AtomicInteger hits = new AtomicInteger();
  final AtomicInteger miss = new AtomicInteger();

  HFileBlockPair[] blocks = generateHFileBlocks(numQueries, blockSize);
  blocksToTest.addAll(Arrays.asList(blocks));

  for (int i = 0; i < numThreads; i++) {
    TestThread t = new MultithreadedTestUtil.RepeatingTestThread(ctx) {
      @Override
      public void doAnAction() throws Exception {
        if (!blocksToTest.isEmpty()) {
          HFileBlockPair ourBlock = blocksToTest.poll();
          // if we run out of blocks to test, then we should stop the tests.
          if (ourBlock == null) {
            ctx.setStopFlag(true);
            return;
          }
          toBeTested.cacheBlock(ourBlock.blockName, ourBlock.block);
          Cacheable retrievedBlock = toBeTested.getBlock(ourBlock.blockName,
              false, false, true);
          if (retrievedBlock != null) {
            assertEquals(ourBlock.block, retrievedBlock);
            toBeTested.evictBlock(ourBlock.blockName);
            hits.incrementAndGet();
            assertNull(toBeTested.getBlock(ourBlock.blockName, false, false, true));
          } else {
            miss.incrementAndGet();
          }
          totalQueries.incrementAndGet();
        }
      }
    };
    t.setDaemon(true);
    ctx.addThread(t);
  }
  ctx.startThreads();
  while (!blocksToTest.isEmpty() && ctx.shouldRun()) {
    Thread.sleep(10);
  }
  ctx.stop();
  if (hits.get() / ((double) hits.get() + (double) miss.get()) < passingScore) {
    fail("Too many nulls returned. Hits: " + hits.get() + " Misses: "
        + miss.get());
  }
}
 

@Override
public <R> void batchCallback(List<? extends Row> actions, Object[] results, Callback<R> callback)
    throws IOException, InterruptedException {
  ConcurrentLinkedQueue<ThrowableWithExtraContext> errors = new ConcurrentLinkedQueue<>();
  CountDownLatch latch = new CountDownLatch(actions.size());
  AsyncTableRegionLocator locator = conn.getRegionLocator(getName());
  List<CompletableFuture<R>> futures = table.<R> batch(actions);
  for (int i = 0, n = futures.size(); i < n; i++) {
    final int index = i;
    FutureUtils.addListener(futures.get(i), (r, e) -> {
      if (e != null) {
        errors.add(new ThrowableWithExtraContext(e, EnvironmentEdgeManager.currentTime(),
          "Error when processing " + actions.get(index)));
        if (!ArrayUtils.isEmpty(results)) {
          results[index] = e;
        }
        latch.countDown();
      } else {
        if (!ArrayUtils.isEmpty(results)) {
          results[index] = r;
        }
        FutureUtils.addListener(locator.getRegionLocation(actions.get(index).getRow()),
          (l, le) -> {
            if (le != null) {
              errors.add(new ThrowableWithExtraContext(le, EnvironmentEdgeManager.currentTime(),
                "Error when finding the region for row " +
                  Bytes.toStringBinary(actions.get(index).getRow())));
            } else {
              callback.update(l.getRegion().getRegionName(), actions.get(index).getRow(), r);
            }
            latch.countDown();
          });
      }
    });
  }
  latch.await();
  if (!errors.isEmpty()) {
    throw new RetriesExhaustedException(errors.size(),
      errors.stream().collect(Collectors.toList()));
  }
}
 

private void cancelAllJobsInQueue(ConcurrentLinkedQueue<Job> jobQueue) {
  while (!jobQueue.isEmpty()) {
    Objects.requireNonNull(jobQueue.poll()).cancel(Objects.requireNonNull(hardFailure));
  }
}
 

public MessageExt getMessage(String topic) {

        MessageExt result = null;

        ConcurrentLinkedQueue<MessageExt> messages= map.get(topic);

        if (messages!=null && !messages.isEmpty()) {
            result = messages.poll();
        }

        return result;
    }
 

/**
 * Check if this emitter has listeners for the specified event.
 *
 * @param event an event name.
 * @return a reference to this object.
 */
public boolean hasListeners(String event) {
    ConcurrentLinkedQueue<Listener> callbacks = this.callbacks.get(event);
    return callbacks != null && !callbacks.isEmpty();
}
 

/**
 * Check if this emitter has listeners for the specified event.
 *
 * @param event an event name.
 * @return a reference to this object.
 */
public boolean hasListeners(String event) {
    ConcurrentLinkedQueue<Listener> callbacks = this.callbacks.get(event);
    return callbacks != null && !callbacks.isEmpty();
}