I recently upgraded to 2.35 and 2.36 using htmlunit as a crawling unit. This was not happening for 2.33. After crawling a lot of pages, I started to see tons of the following threads in the thread dump and eventually it eats up system resource and hangs the container (in docker).

"WebSocketClient@1404341633-126276" #126276 daemon prio=5 os_prio=0 tid=0x00007faba4228800 nid=0xbe7 runnable [0x00007fa1ec102000]
java.lang.Thread.State: RUNNABLE
at sun.nio.ch.EPollArrayWrapper.epollWait(Native Method)
at sun.nio.ch.EPollArrayWrapper.poll(EPollArrayWrapper.java:269)
at sun.nio.ch.EPollSelectorImpl.doSelect(EPollSelectorImpl.java:93)
at sun.nio.ch.SelectorImpl.lockAndDoSelect(SelectorImpl.java:86)
- locked <0x00007fba17c3b6f0> (a sun.nio.ch.Util$3)
- locked <0x00007fba17c3b6d8> (a java.util.Collections$UnmodifiableSet)
- locked <0x00007fba17c3b528> (a sun.nio.ch.EPollSelectorImpl)
at sun.nio.ch.SelectorImpl.select(SelectorImpl.java:97)
at sun.nio.ch.SelectorImpl.select(SelectorImpl.java:101)
at org.eclipse.jetty.io.ManagedSelector$SelectorProducer.select(ManagedSelector.java:464)
at org.eclipse.jetty.io.ManagedSelector$SelectorProducer.produce(ManagedSelector.java:401)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.produceTask(EatWhatYouKill.java:357)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.doProduce(EatWhatYouKill.java:181)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.tryProduce(EatWhatYouKill.java:168)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.produce(EatWhatYouKill.java:132)
at org.eclipse.jetty.io.ManagedSelector$$Lambda$71/1759199424.run(Unknown Source)
at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:786)
at org.eclipse.jetty.util.thread.QueuedThreadPool$2.run(QueuedThreadPool.java:743)
at java.lang.Thread.run(Thread.java:748)

"WebSocketClient@1404341633-126275" #126275 daemon prio=5 os_prio=0 tid=0x00007faba403b000 nid=0xbdb waiting on condition [0x00007fa28901c000]
java.lang.Thread.State: TIMED_WAITING (parking)
at sun.misc.Unsafe.park(Native Method)
- parking to wait for <0x00007fba17d68fd8> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
at java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:215)
at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.awaitNanos(AbstractQueuedSynchronizer.java:2078)
at org.eclipse.jetty.util.BlockingArrayQueue.poll(BlockingArrayQueue.java:392)
at org.eclipse.jetty.util.thread.QueuedThreadPool$2.run(QueuedThreadPool.java:727)
at java.lang.Thread.run(Thread.java:748)

The crawling code snippet looks like following. Because certain web access could get very slow, I create a future task for the crawling and "cancel" it.

.....
ExecutorService executor = Executors.newSingleThreadExecutor();
Future future = executor.submit(new HtmlunitCrawl(urlWithProto, timeout, useProxy));
.....
} finally {
future.cancel(true);
executor.shutdownNow();

The crawling code:

		java.util.logging.Logger.getLogger("com.gargoylesoftware.htmlunit").setLevel(Level.OFF);
		java.util.logging.Logger.getLogger("com.gargoylesoftware").setLevel(Level.OFF);
		java.util.logging.Logger.getLogger("org.apache.commons.httpclient").setLevel(Level.OFF);

		WebClient webClient = new WebClient(BROWSER_VERSION);
		webClient.getOptions().setTimeout(timeout*1000); 
		webClient.getOptions().setCssEnabled(false);
		webClient.getOptions().setPopupBlockerEnabled(true);
		webClient.getOptions().setThrowExceptionOnScriptError(false);
		webClient.getOptions().setRedirectEnabled(true);
		webClient.getOptions().setThrowExceptionOnFailingStatusCode(false);
		webClient.getOptions().setUseInsecureSSL(true);
		webClient.getOptions().setPrintContentOnFailingStatusCode(false);

		webClient.setJavaScriptTimeout(sJavascriptTimeout*1000);	    
		webClient.setAjaxController(new NicelyResynchronizingAjaxController());
		webClient.waitForBackgroundJavaScript(sJavascriptTimeout*1000);
		webClient.setScriptPreProcessor(new PandoraHtmlunitScriptPreprocessor());
		// webClient.setRefreshHandler(new ThreadedRefreshHandler());
		webClient.setRefreshHandler(new WaitingRefreshHandler(timeout));

.....

				PandoraWebConnection conn = new PandoraWebConnection(webClient);
				webClient.setWebConnection(conn);
				RedirectChain rc = new RedirectChain();
				rc.entryUrl = url;
				PandoraWebConnection.REDIRECT_TABLE.put(Thread.currentThread().getName(), rc);
				**aPage = webClient.getPage(urlWithProto);**
				Thread.sleep(sCrawlerWaitTime*1000);
				crawlResp = conn.getLastResponse();
				// System.out.println("Sleeping 6 seconds for page to fully load");
				// Thread.sleep(6000);
				PandoraHtmlunitScriptPreprocessor.RUNNING_SCRIPTS.remove(Thread.currentThread().getName());

		} finally {
			try {
				**webClient.close();**
			} catch (Exception ex) {
				ex.printStackTrace();
                            }
                   }