Netty源码分析NioEventLoop执行select操作入口

protected void run() {
    for (;;) {
        try {
            switch (selectStrategy.calculateStrategy(selectNowSupplier, hasTasks())) {
                case SelectStrategy.CONTINUE:
                    continue;
                case SelectStrategy.SELECT:
                    //轮询io事件(1)
                    select(wakenUp.getAndSet(false));
                    if (wakenUp.get()) {
                        selector.wakeup();
                    }
                default:
            }
            cancelledKeys = 0;
            needsToSelectAgain = false;
            //默认是50
            final int ioRatio = this.ioRatio; 
            if (ioRatio == 100) {
                try {
                    processSelectedKeys();
                } finally {
                    runAllTasks();
                }
            } else {
                //记录下开始时间
                final long ioStartTime = System.nanoTime();
                try {
                    //处理轮询到的key(2)
                    processSelectedKeys();
                } finally {
                    //计算耗时
                    final long ioTime = System.nanoTime() - ioStartTime;
                    //执行task(3)
                    runAllTasks(ioTime * (100 - ioRatio) / ioRatio);
                }
            }
        } catch (Throwable t) {
            handleLoopException(t);
        }
        //代码省略
    }
}

private void select(boolean oldWakenUp) throws IOException {
    Selector selector = this.selector;
    try {
        int selectCnt = 0;
        //当前系统的纳秒数
        long currentTimeNanos = System.nanoTime();
        //截止时间=当前时间+队列第一个任务剩余执行时间
        long selectDeadLineNanos = currentTimeNanos + delayNanos(currentTimeNanos);
        for (;;) {
            //阻塞时间(毫秒)=(截止时间-当前时间+0.5毫秒)
            long timeoutMillis = (selectDeadLineNanos - currentTimeNanos + 500000L) / 1000000L;
            if (timeoutMillis <= 0) {
                if (selectCnt == 0) {
                    selector.selectNow();
                    selectCnt = 1;
                }
                break;
            }
            if (hasTasks() && wakenUp.compareAndSet(false, true)) {
                selector.selectNow();
                selectCnt = 1;
                break;
            }
            //进行阻塞式的select操作
            int selectedKeys = selector.select(timeoutMillis);
            //轮询次数
            selectCnt ++;
            //如果轮询到一个事件(selectedKeys != 0), 或者当前select操作需要唤醒(oldWakenUp), 
            //或者在执行select操作时已经被外部线程唤醒(wakenUp.get()), 
            //或者任务队列已经有任务(hasTask), 或者定时任务队列中有任务(hasScheduledTasks())
            if (selectedKeys != 0 || oldWakenUp || wakenUp.get() || hasTasks() || hasScheduledTasks()) {
                break;
            }
            //省略
            //记录下当前时间
            long time = System.nanoTime();
            //当前时间-开始时间>=超时时间(条件成立, 执行过一次select操作, 条件不成立, 有可能发生空轮询)
            if (time - TimeUnit.MILLISECONDS.toNanos(timeoutMillis) >= currentTimeNanos) {
                //代表已经进行了一次阻塞式select操作, 操作次数重置为1
                selectCnt = 1;
            } else if (SELECTOR_AUTO_REBUILD_THRESHOLD > 0 && selectCnt >= SELECTOR_AUTO_REBUILD_THRESHOLD) {
                //省略日志代码
                //如果空轮询的次数大于一个阈值(512), 解决空轮询的bug
                rebuildSelector();
                selector = this.selector;
                selector.selectNow();
                selectCnt = 1;
                break;
            }
            currentTimeNanos = time;
        }
        //代码省略
    } catch (CancelledKeyException e) {
        //省略
    }
}

public void rebuildSelector() {
    //是否是由其他线程发起的
    if (!inEventLoop()) {
        //如果是其他线程发起的, 将rebuildSelector()封装成任务队列, 由NioEventLoop进行调用
        execute(new Runnable() {
            @Override
            public void run() {
                rebuildSelector();
            }
        });
        return;
    }
    final Selector oldSelector = selector;
    final Selector newSelector;
    if (oldSelector == null) {
        return;
    }
    try {
        //重新创建一个select
        newSelector = openSelector();
    } catch (Exception e) {
        logger.warn("Failed to create a new Selector.", e);
        return;
    }
    int nChannels = 0;
    for (;;) {
        try {
            //拿到旧select中所有的key
            for (SelectionKey key: oldSelector.keys()) {
                Object a = key.attachment();
                try {
                    Object a = key.attachment();
                    //代码省略

                    //获取key注册的事件
                    int interestOps = key.interestOps();
                    //将key注册的事件取消
                    key.cancel();
                    //注册到重新创建的新的selector中
                    SelectionKey newKey = key.channel().register(newSelector, interestOps, a);
                    //如果channel是NioChannel
                    if (a instanceof AbstractNioChannel) {
                        //重新赋值
                        ((AbstractNioChannel) a).selectionKey = newKey;
                    }
                    nChannels ++;
                } catch (Exception e) {
                    //代码省略
                }
            }
        } catch (ConcurrentModificationException e) {
            continue;
        }
        break;
    }
    selector = newSelector;
    //代码省略
}