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  1. JDK
  2. JDK-8287580

(se) CancelledKeyException during channel registration



    • Bug
    • Resolution: Fixed
    • P4
    • 20
    • 11, 17
    • core-libs
    • None
    • b04


      Opening this issue on behalf of Oli Gillespie (see https://mail.openjdk.java.net/pipermail/nio-dev/2022-May/011331.html ).

      When registering a SocketChannel with a Selector for the first time,
      it's possible to get a CancelledKeyException even though this is the
      first register call.

      Exception in thread "main" java.nio.channels.CancelledKeyException
          at java.base/sun.nio.ch.SelectionKeyImpl.ensureValid(SelectionKeyImpl.java:75)
          at java.base/sun.nio.ch.SelectionKeyImpl.interestOps(SelectionKeyImpl.java:104)
          at java.base/sun.nio.ch.SelectorImpl.register(SelectorImpl.java:222)
          at java.base/java.nio.channels.spi.AbstractSelectableChannel.register(AbstractSelectableChannel.java:236)
          at java.base/java.nio.channels.SelectableChannel.register(SelectableChannel.java:260)
          at KeyCancelled.main(KeyCancelled.java:20)

      The javadoc states:

      @throws CancelledKeyException
          If this channel is currently registered with the given selector
            but the corresponding key has already been cancelled

      However in this case the channel is apparently _not_ registered, as shown by
      SocketChannel.isRegistered() returning false.

      This following sequence triggers this issue:

      1. Thread 1 starts SelectableChannel.register
      2. A new SelectionKey becomes visible via Selector.keys()
      3. Thread 2 iterates Selector.keys() and calls SelectorKey.cancel()
      4. Thread 1 (still in the register() invocation) finds that the key is
      cancelled and throws CancelledKeyException

      Below is a small reproducer which usually exhibits this issue:

      import java.nio.channels.SelectionKey;
      import java.nio.channels.Selector;
      import java.nio.channels.SocketChannel;

      public class KeyCancelled {
          public static void main(String[] args) throws Exception {
              Selector s = Selector.open();

              new Thread(() -> {
                  for (int i = 0; i < 100_000; i++) {

              for (int i = 0; i < 10_000; i++) {
                  SocketChannel c = s.provider().openSocketChannel();
                  // Sometimes this throws CancelledKeyException, because
      the key is cancelled by
                  // the other thread part-way through the register call.
                  c.register(s, SelectionKey.OP_READ);
                  // c.isRegistered() is false here after the exceptional case


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              bpb Brian Burkhalter
              simonis Volker Simonis
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