I'm running into this issue in a small spring boot / undertow service that handles websocket and plain old HTTP POST requests.

After running fine for about a week it eventually got the XNIO accept thread blocked on AbstractSelectableChannel.register (stack trace for the stuck thread below) and has remained stuck since.

I got what I believe was the same issue a couple weeks ago (but have no stack traces from that time), so this time around the application is running with the YourKit agent active, and I have a memory snapshot in the stuck state (the thread has been blocked for 24h+ at this point).

"XNIO-2 Accept" #28 prio=5 os_prio=0 tid=0x00007fd2737de000 nid=0x7e0f waiting for monitor entry [0x00007fd262999000]
java.lang.Thread.State: BLOCKED (on object monitor)
at sun.nio.ch.SelectorImpl.register(SelectorImpl.java:131)

• waiting to lock <0x00000005cd8325d8> (a java.util.Collections$UnmodifiableSet)
  at java.nio.channels.spi.AbstractSelectableChannel.register(AbstractSelectableChannel.java:212)
• locked <0x000000066e197560> (a java.lang.Object)
• locked <0x000000066e197550> (a java.lang.Object)
  at java.nio.channels.SelectableChannel.register(SelectableChannel.java:280)
  at org.xnio.nio.WorkerThread.registerChannel(WorkerThread.java:696)
  at org.xnio.nio.QueuedNioTcpServer.handleReady(QueuedNioTcpServer.java:465)
  at org.xnio.nio.QueuedNioTcpServerHandle.handleReady(QueuedNioTcpServerHandle.java:38)
  at org.xnio.nio.WorkerThread.run(WorkerThread.java:559)

The lock is held by another xnio worker thread:

"XNIO-2 I/O-7" #26 prio=5 os_prio=0 tid=0x00007fd27126c000 nid=0x7e0d runnable [0x00007fd262b9b000]
java.lang.Thread.State: RUNNABLE
at sun.nio.ch.EPollArrayWrapper.$$YJP$$epollWait(Native Method)
at sun.nio.ch.EPollArrayWrapper.epollWait(EPollArrayWrapper.java)
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 <0x00000005cd8325e8> (a sun.nio.ch.Util$2)
• locked <0x00000005cd8325d8> (a java.util.Collections$UnmodifiableSet)
• locked <0x00000005cd829370> (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.xnio.nio.WorkerThread.run(WorkerThread.java:509)