public final class NioEventLoop extends SingleThreadEventLoop {

    /**
     * 作为 NIO框架 的 Reactor线程，NioEventLoop 需要处理 网络I/O读写事件，因此它必
     * 须聚合一个多路复用器对象 Selector
     */
    private Selector selector;
    // 通过 provider.open() 从操作系统底层获取 Selector实例
    private final SelectorProvider provider;

	/**
	 * 轮询 事件就绪的channel，进行 IO事件处理
	 */
    private void processSelectedKeys() {
        if (selectedKeys != null) {
            processSelectedKeysOptimized();
        } else {
            processSelectedKeysPlain(selector.selectedKeys());
        }
    }

    private void processSelectedKeysPlain(Set<SelectionKey> selectedKeys) {
        // check if the set is empty and if so just return to not create garbage by
        // creating a new Iterator every time even if there is nothing to process.
        // See https://github.com/netty/netty/issues/597
        if (selectedKeys.isEmpty()) {
            return;
        }

		// 这些代码在 nio编程中应该很熟悉咯
        Iterator<SelectionKey> i = selectedKeys.iterator();
        for (;;) {
            final SelectionKey k = i.next();
            final Object a = k.attachment();
            i.remove();

            if (a instanceof AbstractNioChannel) {
                processSelectedKey(k, (AbstractNioChannel) a);
            } else {
                @SuppressWarnings("unchecked")
                NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
                processSelectedKey(k, task);
            }

            if (!i.hasNext()) {
                break;
            }

            if (needsToSelectAgain) {
                selectAgain();
                selectedKeys = selector.selectedKeys();

                // Create the iterator again to avoid ConcurrentModificationException
                if (selectedKeys.isEmpty()) {
                    break;
                } else {
                    i = selectedKeys.iterator();
                }
            }
        }
    }

	/**
	 * 轮询 事件就绪的channel，进行 IO事件处理
	 */
    private void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
    	// 获取 channel 的内部辅助类 Unsafe，通过 Unsafe 进行IO事件处理
        final AbstractNioChannel.NioUnsafe unsafe = ch.unsafe();
        if (!k.isValid()) {
            final EventLoop eventLoop;
            try {
            	// 获取要处理 channel 所绑定的 eventLoop线程，如果绑定的不是当前的 IO线程的事件，就不处理
                eventLoop = ch.eventLoop();
            } catch (Throwable ignored) {
                // If the channel implementation throws an exception because there is no event loop, we ignore this
                // because we are only trying to determine if ch is registered to this event loop and thus has authority
                // to close ch.
                return;
            }
            // Only close ch if ch is still registered to this EventLoop. ch could have deregistered from the event loop
            // and thus the SelectionKey could be cancelled as part of the deregistration process, but the channel is
            // still healthy and should not be closed.
            // See https://github.com/netty/netty/issues/5125
            if (eventLoop != this || eventLoop == null) {
                return;
            }
            // close the channel if the key is not valid anymore
            unsafe.close(unsafe.voidPromise());
            return;
        }

        try {
            int readyOps = k.readyOps();
            // We first need to call finishConnect() before try to trigger a read(...) or write(...) as otherwise
            // the NIO JDK channel implementation may throw a NotYetConnectedException.
            if ((readyOps & SelectionKey.OP_CONNECT) != 0) {
                // remove OP_CONNECT as otherwise Selector.select(..) will always return without blocking
                // See https://github.com/netty/netty/issues/924
                int ops = k.interestOps();
                ops &= ~SelectionKey.OP_CONNECT;
                k.interestOps(ops);

                unsafe.finishConnect();
            }

            // 处理写事件
            if ((readyOps & SelectionKey.OP_WRITE) != 0) {
                // Call forceFlush which will also take care of clear the OP_WRITE once there is nothing left to write
                ch.unsafe().forceFlush();
            }

            // 处理读事件
            if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
                unsafe.read();
            }
        } catch (CancelledKeyException ignored) {
            unsafe.close(unsafe.voidPromise());
        }
    }
}