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前言

本文是手撕netty源码系列的开篇文章，会先介绍一下netty对NIO关键代码的封装位置，主要介绍 NioEventLoopGroup 对象的创建过程，看看new一个对象可以做哪些事情。

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一、NIO 与 netty

平时使用NIO的主要步骤：

/*创建选择器的实例*/
Selector selector = Selector.open();
/*创建ServerSocketChannel的实例*/
ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();/*设置通道为非阻塞模式*/
serverSocketChannel.configureBlocking(false);
/*绑定端口*/
serverSocketChannel.socket().bind(new InetSocketAddress(port));
/*注册事件，表示关心客户端连接*/
serverSocketChannel.register(selector,SelectionKey.OP_ACCEPT);
while(true){/*获取当前有哪些事件*/selector.select(1000);/*获取事件的集合*/Set<SelectionKey> selectionKeys = selector.selectedKeys();Iterator<SelectionKey> iterator = selectionKeys.iterator();while(iterator.hasNext()){SelectionKey key = iterator.next();/*我们必须首先将处理过的 SelectionKey 从选定的键集合中删除。如果我们没有删除处理过的键，那么它仍然会在主集合中以一个激活的键出现，这会导致我们尝试再次处理它。*/iterator.remove();handleInput(key);}
}/*处理事件的发生*/
private void handleInput(SelectionKey key) throws IOException {if(key.isValid()){/*处理新接入的客户端的请求*/if(key.isAcceptable()){/*获取关心当前事件的Channel*/ServerSocketChannel ssc= (ServerSocketChannel) key.channel();/*接受连接*/SocketChannel sc = ssc.accept();System.out.println("==========建立连接=========");sc.configureBlocking(false);/*关注读事件*/sc.register(selector,SelectionKey.OP_READ);}/*处理对端的发送的数据*/if(key.isReadable()){SocketChannel sc = (SocketChannel) key.channel();/*创建ByteBuffer，开辟一个缓冲区*/ByteBuffer buffer = ByteBuffer.allocate(1024);/*从通道里读取数据，然后写入buffer*/int readBytes = sc.read(buffer);if(readBytes>0){/*将缓冲区当前的limit设置为position,position=0，用于后续对缓冲区的读取操作*/buffer.flip();/*根据缓冲区可读字节数创建字节数组*/byte[] bytes = new byte[buffer.remaining()];/*将缓冲区可读字节数组复制到新建的数组中*/buffer.get(bytes);String message = new String(bytes,"UTF-8");System.out.println("服务器收到消息："+message);/*处理数据*/String result = Const.response(message);、、、、、}else if(readBytes<0){/*取消特定的注册关系*/key.cancel();/*关闭通道*/sc.close();}}、、、、}}

平时使用netty的主要步骤：

// 配置服务端的NIO线程组
EventLoopGroup bossGroup = new NioEventLoopGroup();
EventLoopGroup workerGroup = new NioEventLoopGroup();
ServerBootstrap b = new ServerBootstrap();
b.group(bossGroup, workerGroup).channel(NioServerSocketChannel.class).option(ChannelOption.SO_BACKLOG, 1024).childHandler(new ServerInit());// 绑定端口，同步等待成功
b.bind(NettyConstant.SERVER_PORT).sync();

那么，netty 对 NIO 的封装具体体现在哪里呢？先揭晓答案，后续一点点细嚼慢咽

1. 创建选择器的实例
   io/netty/channel/nio/NioEventLoop.java

NioEventLoop(NioEventLoopGroup parent, Executor executor, SelectorProvider selectorProvider,SelectStrategy strategy, RejectedExecutionHandler rejectedExecutionHandler,EventLoopTaskQueueFactory taskQueueFactory, EventLoopTaskQueueFactory tailTaskQueueFactory) {super(parent, executor, false, newTaskQueue(taskQueueFactory), newTaskQueue(tailTaskQueueFactory),rejectedExecutionHandler);this.provider = ObjectUtil.checkNotNull(selectorProvider, "selectorProvider");this.selectStrategy = ObjectUtil.checkNotNull(strategy, "selectStrategy");final SelectorTuple selectorTuple = openSelector();this.selector = selectorTuple.selector;this.unwrappedSelector = selectorTuple.unwrappedSelector;
}

2. 创建ServerSocketChannel的实例
   io/netty/channel/socket/nio/NioServerSocketChannel.java

private static ServerSocketChannel newChannel(SelectorProvider provider, InternetProtocolFamily family) {try {ServerSocketChannel channel =SelectorProviderUtil.newChannel(OPEN_SERVER_SOCKET_CHANNEL_WITH_FAMILY, provider, family);return channel == null ? provider.openServerSocketChannel() : channel;} catch (IOException e) {throw new ChannelException("Failed to open a socket.", e);}
}

3. 设置通道为非阻塞模式
   io/netty/channel/nio/AbstractNioChannel.java

protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {super(parent);this.ch = ch;this.readInterestOp = readInterestOp;try {ch.configureBlocking(false);} catch (IOException e) {try {ch.close();} catch (IOException e2) {logger.warn("Failed to close a partially initialized socket.", e2);}throw new ChannelException("Failed to enter non-blocking mode.", e);}}

4. 绑定端口
   io/netty/bootstrap/AbstractBootstrap.java

private static void doBind0(final ChannelFuture regFuture, final Channel channel,final SocketAddress localAddress, final ChannelPromise promise) {// This method is invoked before channelRegistered() is triggered.  Give user handlers a chance to set up// the pipeline in its channelRegistered() implementation.channel.eventLoop().execute(new Runnable() {@Overridepublic void run() {if (regFuture.isSuccess()) {channel.bind(localAddress, promise).addListener(ChannelFutureListener.CLOSE_ON_FAILURE);} else {promise.setFailure(regFuture.cause());}}});}

5. 注册事件，表示关心客户端连接
   io/netty/channel/nio/AbstractNioChannel.java

protected void doRegister() throws Exception {boolean selected = false;for (;;) {try {selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);return;} catch (CancelledKeyException e) {if (!selected) {// Force the Selector to select now as the "canceled" SelectionKey may still be// cached and not removed because no Select.select(..) operation was called yet.eventLoop().selectNow();selected = true;} else {// We forced a select operation on the selector before but the SelectionKey is still cached// for whatever reason. JDK bug ?throw e;}}}}

6. 获取当前事件的集合
7. 处理事件
   io/netty/channel/nio/NioEventLoop.java

private int select(long deadlineNanos) throws IOException {if (deadlineNanos == NONE) {return selector.select();}// Timeout will only be 0 if deadline is within 5 microsecslong timeoutMillis = deadlineToDelayNanos(deadlineNanos + 995000L) / 1000000L;return timeoutMillis <= 0 ? selector.selectNow() : selector.select(timeoutMillis);
}private void processSelectedKeys() {if (selectedKeys != null) {processSelectedKeysOptimized();} else {processSelectedKeysPlain(selector.selectedKeys());}
}

其实，netty 也不难对吧
学习netty，主要学习它的设计思想和对性能优化的巧妙处理，当工作需要时，能够灵活运用

二、NioEventLoopGroup 对象的创建过程

2.1 创建流程图

可以看到，其实我们传的线程数量实际控制的是NioEventLoop对象创建的数量，而每个 NioEventLoop 其实是一个Executor执行器，那么至此，我们只是相当于创建了两个 NioEventLoopGroup 对象，他们分别有自己的children执行器 NioEventLoop 数组，同一个数组内的 NioEventLoop 共享一个ThreadPerTaskExecutor执行器，但是现在这个执行器后续如何处理事件和如何调度还不知道，后续会讲到，本文先看看创建NioEventLoopGroup对象都做了什么