netty是最近项目要用到的nio框架，找了各种资料，发现称赞它的有点多，所以决定用它：其实也就二选一嘛，mina或netty或自己写。对于mina，也不熟，不过看各种介绍，貌似netty干活还是很不错的，尤其是最新的4.x和5.x重构后，且使用结构清晰就先了解了解了。
首先要把应用跑起来啦（官网的例子比较多），我这是一个关于mqtt的一个例子：
1 m_bossGroup = new NioEventLoopGroup(); 2 m_workerGroup = new NioEventLoopGroup(); 3 4 final NettyMQTTHandler handler = new NettyMQTTHandler(); 5 handler.setMessaging(messaging); 6 7 ServerBootstrap b = new ServerBootstrap(); 8 b.group(m_bossGroup, m_workerGroup) 9 .channel(NioServerSocketChannel.class) 10 .childHandler(new ChannelInitializer<SocketChannel>() { 11 @Override 12 public void initChannel(SocketChannel ch) throws Exception { 13 ChannelPipeline pipeline = ch.pipeline(); 14 //pipeline.addFirst("metrics", new BytesMetricsHandler(m_metricsCollector)); 15 pipeline.addFirst("idleStateHandler", new IdleStateHandler(0, 0, Constants.DEFAULT_CONNECT_TIMEOUT)); 16 pipeline.addAfter("idleStateHandler", "idleEventHandler", new MoquetteIdleTimoutHandler()); 17 //pipeline.addLast("logger", new LoggingHandler("Netty", LogLevel.ERROR)); 18 pipeline.addLast("decoder", new MQTTDecoder()); 19 pipeline.addLast("encoder", new MQTTEncoder()); 20 pipeline.addLast("metrics", new MessageMetricsHandler(m_metricsCollector)); 21 pipeline.addLast("handler", handler); 22 } 23 }) 24 .option(ChannelOption.SO_BACKLOG, 128) 25 .option(ChannelOption.SO_REUSEADDR, true) 26 .childOption(ChannelOption.SO_KEEPALIVE, true); 27 try { 28 // Bind and start to accept incoming connections. 29 ChannelFuture f = b.bind(Constants.PORT); 30 LOG.info("Server binded"); 31 f.sync(); 32 } catch (InterruptedException ex) { 33 LOG.error(null, ex); 34 }
再回想下，我们自己写serversocket的时候是怎么写的呢（这是一个笨拙的实例代码）：
ServerSocket socket; channel = ServerSocketChannel.open(); // 打开通道 socket = channel.socket(); //得到与通到相关的socket对象 socket.bind(new InetSocketAddress(port)); //将scoket榜定在制定的端口上 //配置通到使用非阻塞模式，在非阻塞模式下，可以编写多道程序同时避免使用复杂的多线程 channel.configureBlocking(false); channel.register(selector, SelectionKey.OP_ACCEPT); try { while (true) { this.selector.select(); Iterator<SelectionKey> iter = this.selector.selectedKeys().iterator(); while (iter.hasNext()) { SelectionKey key = iter.next(); iter.remove(); this.handleKey(key); } } } catch (IOException ex) { ex.printStackTrace(); }
原理还是那些，channel.open(),然后register key，然后遍历，再然后才进行handleKey()的干活。
那netty的写法为什么那么潇洒呢，怀着这个莫名的疑问，我先不管它的结构什么的，直接进行search，发现了这么个东东：
1 NioEventLoop(NioEventLoopGroup parent, ThreadFactory threadFactory, SelectorProvider selectorProvider) { 2 super(parent, threadFactory, false); 3 if (selectorProvider == null) { 4 throw new NullPointerException("selectorProvider"); 5 } 6 provider = selectorProvider; 7 selector = openSelector(); 8 }
其中第8行从名称上来看，有点点意思了，往下看：
1 private Selector openSelector() { 2 final Selector selector; 3 try { 4 selector = provider.openSelector();
其中的provider就是我们熟悉的:java.nio.channels.spi.SelectorProvider类。
所以这个就是做了selector.open的工作。
接下来能看到NioEventLoop：
1 protected void run() { 2 for (;;) { 3 oldWakenUp = wakenUp.getAndSet(false); 4 try { 5 if (hasTasks()) { 6 selectNow(); 7 } else { 8 select();
再继续看，该类中处理的selectedKey：
1 final NioUnsafe unsafe = ch.unsafe(); 2 if (!k.isValid()) { 3 // close the channel if the key is not valid anymore 4 unsafe.close(unsafe.voidPromise()); 5 return; 6 } 7 8 try { 9 int readyOps = k.readyOps(); 10 if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) { 11 unsafe.read(); 12 if (!ch.isOpen()) { 13 // Connection already closed - no need to handle write. 14 return; 15 } 16 } 17 if ((readyOps & SelectionKey.OP_WRITE) != 0) { 18 // Call forceFlush which will also take care of clear the OP_WRITE once there is nothing left to write 19 ch.unsafe().forceFlush(); 20 } 21 if ((readyOps & SelectionKey.OP_CONNECT) != 0) { 22 // remove OP_CONNECT as otherwise Selector.select(..) will always return without blocking 23 // See https://github.com/netty/netty/issues/924 24 int ops = k.interestOps(); 25 ops &= ~SelectionKey.OP_CONNECT; 26 k.interestOps(ops); 27 28 unsafe.finishConnect(); 29 } 30 } catch (CancelledKeyException e) { 31 unsafe.close(unsafe.voidPromise()); 32 } 33
现在明白了吧，其实netty也是走这么一套逻辑。
然后再网上看，逻辑是这样：
NioEventLoopGroup extends MultithreadEventExecutorGroup，其初始化了n个单线程的线程池(children = new SingleThreadEventExecutor[nThreads];)
每个单线程的对象child[i]=NioEventLoop对象，每个NioEventLoop有一个Selector字段。
其run方法是该group都需要干活的具体业务逻辑代码。
后续再加上别的类说明。