不知歸期1，netty系列之:NIO和netty詳解

簡介

netty為什么快呢？這是因為netty底層使用了JAVA的NIO技術，並在其基礎上進行了性能的優化，雖然netty不是單純的JAVA nio，但是netty的底層還是基於的是nio技術。

nio是JDK1.4中引入的，用於區別於傳統的IO，所以nio也可以稱之為new io。

nio的三大核心是Selector,channel和Buffer，本文我們將會深入探究NIO和netty之間的關系。

NIO常用用法

在講解netty中的NIO實現之前，我們先來回顧一下JDK中NIO的selector，channel是怎么工作的。對於NIO來說selector主要用來接受客戶端的連接，所以一般用在server端。我們以一個NIO的服務器端和客戶端聊天室為例來講解NIO在JDK中是怎么使用的。

因為是一個簡單的聊天室，我們選擇Socket協議為基礎的ServerSocketChannel,首先就是open這個Server channel:

ServerSocketChannel serverSocketChannel = ServerSocketChannel.open(); serverSocketChannel.bind(new InetSocketAddress("localhost", 9527)); serverSocketChannel.configureBlocking(false); 

然后向server channel中注冊selector:

Selector selector = Selector.open(); serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT); 

雖然是NIO，但是對於Selector來說，它的select方法是阻塞方法，只有找到匹配的channel之后才會返回，為了多次進行select操作，我們需要在一個while循環里面進行selector的select操作：

while (true) { selector.select(); Set<SelectionKey> selectedKeys = selector.selectedKeys(); Iterator<SelectionKey> iter = selectedKeys.iterator(); while (iter.hasNext()) { SelectionKey selectionKey = iter.next(); if (selectionKey.isAcceptable()) { register(selector, serverSocketChannel); } if (selectionKey.isReadable()) { serverResponse(byteBuffer, selectionKey); } iter.remove(); } Thread.sleep(1000); } 

selector中會有一些SelectionKey,SelectionKey中有一些表示操作狀態的OP Status,根據這個OP Status的不同，selectionKey可以有四種狀態，分別是isReadable,isWritable,isConnectable和isAcceptable。

當SelectionKey處於isAcceptable狀態的時候，表示ServerSocketChannel可以接受連接了，我們需要調用register方法將serverSocketChannel accept生成的socketChannel注冊到selector中，以監聽它的OP READ狀態，后續可以從中讀取數據：

    private static void register(Selector selector, ServerSocketChannel serverSocketChannel) throws IOException { SocketChannel socketChannel = serverSocketChannel.accept(); socketChannel.configureBlocking(false); socketChannel.register(selector, SelectionKey.OP_READ); } 

當selectionKey處於isReadable狀態的時候，表示可以從socketChannel中讀取數據然后進行處理：

    private static void serverResponse(ByteBuffer byteBuffer, SelectionKey selectionKey) throws IOException { SocketChannel socketChannel = (SocketChannel) selectionKey.channel(); socketChannel.read(byteBuffer); byteBuffer.flip(); byte[] bytes= new byte[byteBuffer.limit()]; byteBuffer.get(bytes); log.info(new String(bytes).trim()); if(new String(bytes).trim().equals(BYE_BYE)){ log.info("說再見不如不見！"); socketChannel.write(ByteBuffer.wrap("再見".getBytes())); socketChannel.close(); }else { socketChannel.write(ByteBuffer.wrap("你是個好人".getBytes())); } byteBuffer.clear(); } 

上面的serverResponse方法中，從selectionKey中拿到對應的SocketChannel，然后調用SocketChannel的read方法，將channel中的數據讀取到byteBuffer中，要想回復消息到channel中，還是使用同一個socketChannel，然后調用write方法回寫消息給client端，到這里一個簡單的回寫客戶端消息的server端就完成了。

接下來就是對應的NIO客戶端，在NIO客戶端需要使用SocketChannel,首先建立和服務器的連接：

socketChannel = SocketChannel.open(new InetSocketAddress("localhost", 9527)); 

然后就可以使用這個channel來發送和接受消息了：

    public String sendMessage(String msg) throws IOException {
        byteBuffer = ByteBuffer.wrap(msg.getBytes()); String response = null; socketChannel.write(byteBuffer); byteBuffer.clear(); socketChannel.read(byteBuffer); byteBuffer.flip(); byte[] bytes= new byte[byteBuffer.limit()]; byteBuffer.get(bytes); response =new String(bytes).trim(); byteBuffer.clear(); return response; }

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