选择器（Selector） 是 SelectableChannle 对象的多路复用器，Selector 可以同时监控多个 SelectableChannel 的 IO 状况，也就是说，利用 Selector可使一个单独的线程管理多个 Channel，selector 是非阻塞 IO 的核心。

选择器（Selector）的应用：

当通道使用register（Selector sel, int ops）方法将通道注册选择器时，选择器对通道事件进行监听，通过第二个参数指定监听的事件类型。

其中可监听的事件类型包括以下：

读 : SelectionKey.OP_READ （1）写 : SelectionKey.OP_WRITE （4）连接 : SelectionKey.OP_CONNECT （8）接收 : SelectionKey.OP_ACCEPT （16）

服务端：

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.Iterator;import java.util.Set;public class WebServer { public static void main(String[] args) { try { ServerSocketChannel ssc = ServerSocketChannel.open(); ssc.socket().bind(new InetSocketAddress("127.0.0.1", 8000)); ssc.configureBlocking(false); Selector selector = Selector.open(); // 注册 channel，并且指定感兴趣的事件是 Accept ssc.register(selector, SelectionKey.OP_ACCEPT); ByteBuffer readBuff = ByteBuffer.allocate(1024); ByteBuffer writeBuff = ByteBuffer.allocate(128); writeBuff.put("received".getBytes()); writeBuff.flip(); while (true) { int nReady = selector.select(); Set<SelectionKey> keys = selector.selectedKeys(); Iterator<SelectionKey> it = keys.iterator(); while (it.hasNext()) { SelectionKey key = it.next(); it.remove(); if (key.isAcceptable()) { // 创建新的连接，并且把连接注册到selector上，而且， // 声明这个channel只对读操作感兴趣。 SocketChannel socketChannel = ssc.accept(); socketChannel.configureBlocking(false); socketChannel.register(selector, SelectionKey.OP_READ); } else if (key.isReadable()) { SocketChannel socketChannel = (SocketChannel) key.channel(); readBuff.clear(); socketChannel.read(readBuff); readBuff.flip(); System.out.println("received : " + new String(readBuff.array())); key.interestOps(SelectionKey.OP_WRITE); } else if (key.isWritable()) { writeBuff.rewind(); SocketChannel socketChannel = (SocketChannel) key.channel(); socketChannel.write(writeBuff); key.interestOps(SelectionKey.OP_READ); } } } } catch (IOException e) { e.printStackTrace(); } }}

客户端:

package selector;import java.io.IOException;import java.net.InetSocketAddress;import java.nio.ByteBuffer;import java.nio.channels.SocketChannel;public class WebClient { public static void main(String[] args) throws IOException { try { SocketChannel socketChannel = SocketChannel.open(); socketChannel.connect(new InetSocketAddress("127.0.0.1", 8000)); ByteBuffer writeBuffer = ByteBuffer.allocate(32); ByteBuffer readBuffer = ByteBuffer.allocate(32); writeBuffer.put("hello".getBytes()); writeBuffer.flip(); while (true) { writeBuffer.rewind(); socketChannel.write(writeBuffer); readBuffer.clear(); socketChannel.read(readBuffer); } } catch (IOException e) { } }}