关于 Netty NioEventLoop 的一些疑惑

Question

Netty version 4.1.1.7 Final. I wonder what the if is for and why you must select#selectNow before working on the task.

// io.netty.channel.nio.NioEventLoop#select

      private void select(boolean oldWakenUp) throws IOException {
             // ...
             for (;;) {
          long timeoutMillis = (selectDeadLineNanos - currentTimeNanos + 500000L) / 1000000L;
                if (timeoutMillis <= 0) {
                    if (selectCnt == 0) {
                        selector.selectNow();
                        selectCnt = 1;
                    }
                    break;
                }

           // ...
      }

}

Answer 1

If this line is reached it is already decided that the select operation should not wait for some Channel / SelectionKey to become ready or wait for a timeout.

But for the further operations on the selector it is still important to know which Channel s are ready to be processed and therefore update the SelectionKey s.

And this is what Selector.selectNow() does:

Selects a set of keys whose corresponding channels are ready for I/O operations.

This method performs a non-blocking selection operation. If no channels have become selectable since the previous selection operation then this method immediately returns zero.