1. 先来看看Netty源码 #NioEventLoop.run()

    @Override
    protected void run() {
        int selectCnt = 0;//此处记录空循环次数
        for (;;) { 
                //省略无关代码
                selectCnt++;//这里是不是都+1  
//这里做ranTasks和strategy判断，如果两个都不符合条件，就进入空循环判断              
                if (ranTasks || strategy > 0) {
                    if (selectCnt > MIN_PREMATURE_SELECTOR_RETURNS && logger.isDebugEnabled()) {
                        logger.debug("Selector.select() returned prematurely {} times in a row for Selector {}.",
                                selectCnt - 1, selector);
                    }
                    selectCnt = 0;//正常返回,这里清零
                } else if (unexpectedSelectorWakeup(selectCnt)) { // 这里做空循环判断，看看是否为无意唤醒
                    selectCnt = 0;//意外唤醒,这里清零
                }
    }

2. 再看#NioEventLoop.unexpectedSelectorWakeup(selectCnt)

    // returns true if selectCnt should be reset
    private boolean unexpectedSelectorWakeup(int selectCnt) {
        if (Thread.interrupted()) {
            // Thread was interrupted so reset selected keys and break so we not run into a busy loop.
            // As this is most likely a bug in the handler of the user or it's client library we will
            // also log it.
            //
            // See https://github.com/netty/netty/issues/2426
            if (logger.isDebugEnabled()) {
                logger.debug("Selector.select() returned prematurely because " +
                        "Thread.currentThread().interrupt() was called. Use " +
                        "NioEventLoop.shutdownGracefully() to shutdown the NioEventLoop.");
            }
            return true;
        }
        //SELECTOR_AUTO_REBUILD_THRESHOLD可以通过properties修改，默认512，selectCnt超过阈值就进行重建
        if (SELECTOR_AUTO_REBUILD_THRESHOLD > 0 &&
                selectCnt >= SELECTOR_AUTO_REBUILD_THRESHOLD) {
            // The selector returned prematurely many times in a row.
            // Rebuild the selector to work around the problem.
            logger.warn("Selector.select() returned prematurely {} times in a row; rebuilding Selector {}.",
                    selectCnt, selector);
            rebuildSelector();
            return true;
        }
        return false;
    }

3. 我们直接看#NioEventLoop.rebuildSelector0()

      private void rebuildSelector0() {
        final Selector oldSelector = selector;//把这个NioEventLoop绑定的selector存起来
        final SelectorTuple newSelectorTuple;

        if (oldSelector == null) {
            return;
        }

        try {
            newSelectorTuple = openSelector();//直接创建一个新的selector
        } catch (Exception e) {
            logger.warn("Failed to create a new Selector.", e);
            return;
        }

        // Register all channels to the new Selector.
        int nChannels = 0;
        for (SelectionKey key: oldSelector.keys()) {
            Object a = key.attachment();//拿到Channel
            try {
                if (!key.isValid() || key.channel().keyFor(newSelectorTuple.unwrappedSelector) != null) {
                    continue;
                }

                int interestOps = key.interestOps();
                key.cancel();//取消注册
                //重新注册channel到新selector
                SelectionKey newKey = key.channel().register(newSelectorTuple.unwrappedSelector, interestOps, a);
                if (a instanceof AbstractNioChannel) {
                    // Update SelectionKey
                    ((AbstractNioChannel) aselectionKey = newKey;
                }
                nChannels ++;
            } catch (Exception e) {
                logger.warn("Failed to re-register a Channel to the new Selector.", e);
                if (a instanceof AbstractNioChannel) {
                    AbstractNioChannel ch = (AbstractNioChannel) a;
                    ch.unsafe().close(ch.unsafe().voidPromise());
                } else {
                    @SuppressWarnings("unchecked")
                    NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
                    invokeChannelUnregistered(task, key, e);
                }
            }
        }

        selector = newSelectorTuple.selector;
        unwrappedSelector = newSelectorTuple.unwrappedSelector;

        try {
            // time to close the old selector as everything else is registered to the new one
            //把老的selector关闭
            oldSelector.close();
        } catch (Throwable t) {
            if (logger.isWarnEnabled()) {
                logger.warn("Failed to close the old Selector.", t);
            }
        }

        if (logger.isInfoEnabled()) {
            logger.info("Migrated " + nChannels + " channel(s) to the new Selector.");
        }
    }