3. NIO--echoserver

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 EchoServer {

    public static void main(String[] args) throws IOException {

        ByteBuffer bytes = ByteBuffer.allocate(1024);

        ServerSocketChannel servChannel = ServerSocketChannel.open();
        servChannel.socket().bind(new InetSocketAddress(8888));
        servChannel.configureBlocking(false);

        Selector selector = Selector.open();
        servChannel.register(selector, SelectionKey.OP_ACCEPT);
        while (true) {
            int rds = selector.select();
            if (rds == 0)
                continue;
            Set<SelectionKey> selectedKeys = selector.selectedKeys();
            Iterator<SelectionKey> keyIterator = selectedKeys.iterator();
            while (keyIterator.hasNext()) {
                SelectionKey key = keyIterator.next();
                if (key.isAcceptable()) {
                    // a connection was accepted by a ServerSocketChannel.
                    ServerSocketChannel server = (ServerSocketChannel )key.channel();
                    SocketChannel cli = server.accept();
                    cli.configureBlocking(false);
                    cli.register(selector, SelectionKey.OP_READ);
                } else if (key.isConnectable()) {
                    // a connection was established with a remote server.
                } else if (key.isReadable()) {
                    // a channel is ready for reading
                    SocketChannel cli = (SocketChannel)key.channel();
                    bytes.clear();
                    int cnt = cli.read(bytes);
                    if(cnt == -1){
                        System.out.println(cli.getRemoteAddress() + "连接断开!");
                        cli.close();
                    }else{
                        bytes.flip();
                        byte[] dst = new byte[bytes.limit()];
                        bytes.get(dst);

                        String s = new String(dst,"UTF-8");
                        System.out.println(cli.getRemoteAddress() + ":" + s);
                        cli.register(selector, SelectionKey.OP_WRITE, s);
                    }
                } else if (key.isWritable()) {
                    // a channel is ready for writing
                    SocketChannel cli = (SocketChannel)key.channel();
                    String s = (String)key.attachment();
                    byte[] bs = s.getBytes("UTF-8");
                    bytes.clear();
                    bytes.put(bs);
                    bytes.flip(); //转读
                    cli.write(bytes);
                    cli.register(selector, SelectionKey.OP_READ);
                }
                keyIterator.remove();
            }
        }
    }

}