Netty入门--NIO
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JavaNIO库是在JDK1.4中引入的。NIO弥补了原来同步阻塞I/O的不足。
下面是NIO的三个主要概念
1.缓冲区Buffer
Buffer是一个对象，它包含一些要写入或者要读取的数据
在NIO库中，所有数据都是用缓冲区处理的。在读取数据时，他是直接读到缓冲区中；在写入数据时，写入到缓冲区中。任何时候访问NIO中的数据，都是通过缓冲区进行操作。
缓冲区实质上是一个数组。通常是一个是字节数组ByteBuffer
2.通道Channel
Channel是一个通道，网络数据通过Channel读取和写入。通道和流的不同之处在于通道是双向的，流只是在一个方向上移动（InputStream或OutputStream），而通道可以用于读、写或者二者同时进行。
3.多路复用器Selector
Selector会不断轮询注册在其上的Channel，如果某个Channel上面发生读或写时间，这个Channel就处于就绪状态，会被Selector出来，然后通过SelectionKey可以获取就绪Channel的集合，进行后续的I/O操作。
一个多路复用器Selector可以同时轮询多个Channel，由于JDK使用了epoll()代替了传统的select实现，所以它并没有最大连接句柄1024/2048的限制。
只需要一个线程负责Selector的轮询，就可以接入成千上万的客户端。
下面用一个例子来演示NIO
package com.hueason.netty.n1.nio;/** * Created by Administrator on 2017/4/19. */public class TimeServer { public static void main(String[] args) { int port = 8080; if(args != null && args.length >0){ port = Integer.valueOf(args[0]); } MultiplexerTimeServer timeServer = new MultiplexerTimeServer(port); new Thread(timeServer,"NIO-MultiplexerTimeServer-001").start(); }}package com.hueason.netty.n1.nio;import java.io.IOException;import java.net.InetSocketAddress;import java.nio.ByteBuffer;import java.nio.channels.SelectionKey;import java.nio.channels.Selector;import java.nio.channels.ServerSocketChannel;import java.nio.channels.SocketChannel;import java.util.Date;import java.util.Iterator;import java.util.Set;/** * Created by Administrator on 2017/4/19. */public class MultiplexerTimeServer implements Runnable{ private Selector selector; private ServerSocketChannel servChannel; private volatile boolean stop; public MultiplexerTimeServer(int port) { try { selector = Selector.open(); servChannel = ServerSocketChannel.open(); servChannel.configureBlocking(false); servChannel.socket().bind(new InetSocketAddress(port),1024); servChannel.register(selector, SelectionKey.OP_ACCEPT); System.out.println("The time server is start in port :"+port); } catch (IOException e) { e.printStackTrace(); System.exit(1); } } public void stop(){ this.stop = true; } @Override public void run() { while(!stop){ try { selector.select(1000); Set<SelectionKey> selectionKeys = selector.selectedKeys(); Iterator<SelectionKey> it = selectionKeys.iterator(); SelectionKey key = null; while (it.hasNext()){ key = it.next(); it.remove(); try { handleInput(key); } catch (Exception e) { if(key != null){ key.cancel(); if(key.channel() != null){ key.channel().close(); } } } } } catch (IOException e) { e.printStackTrace(); } } //多路复用器关闭后，所有注册在上面的Channel和Pipe等资源都会被自动去注册并关闭，所以不需要重复释放资源 if(selector != null){ try { selector.close(); } catch (IOException e) { e.printStackTrace(); } } } private void handleInput(SelectionKey key) throws IOException { if(key.isValid()){ //处理新接入的请求信息 if(key.isAcceptable()){ //Accept the new connection ServerSocketChannel ssc = (ServerSocketChannel) key.channel(); SocketChannel sc = ssc.accept(); sc.configureBlocking(false); //Add the new connection to the selector sc.register(selector,SelectionKey.OP_READ); } if(key.isReadable()){ //Read the data SocketChannel sc = (SocketChannel) key.channel(); ByteBuffer readBuffer = ByteBuffer.allocate(1024); int readBytes = sc.read(readBuffer); if(readBytes >0){ readBuffer.flip(); byte[] bytes = new byte[readBuffer.remaining()]; readBuffer.get(bytes); String body = new String(bytes,"UTF-8"); System.out.println("The time server receive order "+body); String currentTime = "QUERY TIME ORDER".equalsIgnoreCase(body)?new Date(System.currentTimeMillis()).toString():"BAD ORDER"; doWrite(sc,currentTime); }else if(readBytes<0){ //对端链路关闭 key.cancel(); sc.close(); }else{ //读到0字节，忽略 } } } } private void doWrite(SocketChannel channel, String response) throws IOException { if(response != null && response.trim().length()>0){ byte[] bytes = response.getBytes(); ByteBuffer writeBuffer = ByteBuffer.allocate(bytes.length); writeBuffer.put(bytes); writeBuffer.flip(); channel.write(writeBuffer); } }}客户端
package com.hueason.netty.n1.nio;/** * Created by Administrator on 2017/4/19. */public class TimeClient { public static void main(String[] args) { int port = 8080; if(args != null && args.length>0){ port = Integer.valueOf(args[0]); } new Thread(new TimeClientHandler("127.0.0.1",port),"TimeClient-001").start(); }}package com.hueason.netty.n1.nio;import java.io.IOException;import java.net.InetSocketAddress;import java.nio.ByteBuffer;import java.nio.channels.SelectionKey;import java.nio.channels.Selector;import java.nio.channels.SocketChannel;import java.util.Iterator;import java.util.Set;/** * Created by Administrator on 2017/4/19. */public class TimeClientHandler implements Runnable { private String host; private int port; private Selector selector; private SocketChannel socketChannel; private volatile boolean stop; public TimeClientHandler(String host, int port) { this.host = host == null?"127.0.0.1":host; this.port = port; try { selector = Selector.open(); socketChannel = SocketChannel.open(); socketChannel.configureBlocking(false); } catch (IOException e) { e.printStackTrace(); System.exit(1); } } @Override public void run() { try { doConnect(); } catch (Exception e) { e.printStackTrace(); System.exit(1); } while(!stop){ try { selector.select(1000); Set<SelectionKey> selectedKeys = selector.selectedKeys(); Iterator<SelectionKey> it = selectedKeys.iterator(); SelectionKey key = null; while(it.hasNext()){ key = it.next(); it.remove(); try { handleInput(key); } catch (Exception e) { if(key != null){ key.cancel(); if(key.channel() != null){ key.channel().close(); } } e.printStackTrace(); } } } catch (IOException e) { e.printStackTrace(); System.exit(1); } } //多路复用器关闭后，所有注册在上面的Channel和Pipe等资源都会被自动去注册并关闭，所以不需要重复释放资源 if(selector != null){ try { selector.close(); } catch (IOException e) { e.printStackTrace(); } } } private void handleInput(SelectionKey key) throws IOException { if(key.isValid()){ //判断是否连接成功 SocketChannel sc = (SocketChannel) key.channel(); if(key.isConnectable()){ if(sc.finishConnect()){ sc.register(selector,SelectionKey.OP_READ); doWrite(sc); }else{ System.exit(1);//连接失败，进程退出 } } if(key.isReadable()){ ByteBuffer readBuffer = ByteBuffer.allocate(1024); int readBytes = sc.read(readBuffer); if(readBytes>0){ readBuffer.flip(); byte[] bytes = new byte[readBuffer.remaining()]; readBuffer.get(bytes); String body = new String(bytes,"UTF-8"); System.out.println("Now is :"+body); this.stop = true; }else if(readBytes<0){ key.cancel(); sc.close(); }else{ //读到0字节，忽略 } } } } private void doWrite(SocketChannel sc) throws IOException { byte[] req = "QUERY TIME ORDER".getBytes(); ByteBuffer writeBuffer = ByteBuffer.allocate(req.length); writeBuffer.put(req); writeBuffer.flip(); sc.write(writeBuffer); if(!writeBuffer.hasRemaining()){ System.out.println("Send order 2 server succeed."); } } private void doConnect() throws IOException { //如果直接连接成功，则注册到多路复用器上，发送请求消息，读应答 if(socketChannel.connect(new InetSocketAddress(host,port))){ socketChannel.register(selector,SelectionKey.OP_READ); doWrite(socketChannel); }else { socketChannel.register(selector,SelectionKey.OP_CONNECT); } }}从代码可以看出来，NIO编程的难度比同步阻塞BIO大很多，而且这个例子没有考虑半包情况。所以它的劣势就是代码复杂，开发难度大
NIO的优点
1）客户端发起的连接操作是异步的，可以通过在多路复用器注册OP_CONNECT等待后续结果，不需要像BIO客户端那样被同步阻塞。
2）SocketChannel的读写操作都是异步的，如果没有可读写的数据它不会同步等待，直接返回，这样I/O通信线程就可以处理其他的链路，不需要同步等待这个链路可用
3）线程模型的优化：由于JDK的Selector在Linux上通过epoll实现，没有连接句柄数的限制（只受限于操作系统的最大句柄数或者对单个进程的句柄限制），这意味着一个Selector可以同属处理成千上万个客户端连接，而且性能不会随着客户端的增加而线性下降。非常适合做高性能、高负载的网络服务器。
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