服务器：

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.Arrays;
import java.util.Iterator;
public class TrainAgain {

private int port;
private Selector selector;

public TrainAgain(int port) {
    this.port = port;
}

public void startServer() {
    try {
        //open serversocketchannel, bind to port,config non-blocking and register on selector .
        selector = Selector.open();
        ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
        serverSocketChannel.socket().bind(new InetSocketAddress("localhost",port));
        serverSocketChannel.configureBlocking(false);
        serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
        System.out.println("listen on port " + port);

        while (true) {
            //blocking here listening to channel that connected
            int r = selector.select();
            if (r == 0) {
                continue;
            }
            //after connected, use iterator to scan them all
            Iterator<SelectionKey> itr = selector.selectedKeys().iterator();
            while (itr.hasNext()) {
                SelectionKey key = itr.next();
                itr.remove();
                if (key.isAcceptable()) {
                    //channel is ready to accept a new socket channel
                    ServerSocketChannel ssc = (ServerSocketChannel) key.channel();
                    ssc.configureBlocking(false);
                    //get the socket channel and registed it to selector;
                    SocketChannel sc = ssc.accept();
                    sc.configureBlocking(false);
                    if (sc == null) {
                        continue;
                    }
                    sc.register(selector, SelectionKey.OP_READ);
                    System.out.println("connected to a new socket channel :" + sc);
                }else if(key.isConnectable()){
                    ((SocketChannel)key.channel()).finishConnect();
                } else if (key.isReadable()) {
                    //channel is ready for reading
                    SocketChannel sc = (SocketChannel) key.channel();
                    sc.configureBlocking(false);
                    ByteBuffer buffer = ByteBuffer.allocate(48);
                    while (true) {
                        buffer.clear();
                        int n = sc.read(buffer);
                        if (n <= 0) {
                            break;
                        }
                        System.out.println("receive data :" + Arrays.toString(buffer.array())+" from "+sc);
                    }
                    buffer = ByteBuffer.wrap("welcome!".getBytes());
                    buffer.flip();
                    //I write buffer here
                    sc.write(buffer);
                    System.out.println("write data :" + Arrays.toString(buffer.array())+" to "+sc);
                }

            }

        }

    } catch (IOException e) {
        e.printStackTrace();
    }

}

public static void main(String[] args) {
    TrainAgain again = new TrainAgain(8000);
    again.startServer();
}

}

客户：

import java.io.IOException;
import java.io.RandomAccessFile;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.SocketChannel;
import java.util.Arrays;

public class Client {

private void startClient(int port) {
    try {
        //get socketchannel and connect to server
        SocketChannel sc = SocketChannel.open();
        sc.connect(new InetSocketAddress("localhost",port));
        System.out.println("connect to server from "+sc);
        sc.configureBlocking(false);
        RandomAccessFile randomAccessFile = new RandomAccessFile("D:\\nio-data.txt","rw");
        FileChannel fc = randomAccessFile.getChannel();
        //transfer data from filechannel to socketchannel
        fc.transferTo(0, randomAccessFile.length(), sc);

        ByteBuffer buffer = ByteBuffer.allocate(48);
        if(sc.isConnectionPending()){
            sc.finishConnect();
        }
        System.out.println("connect to server from "+sc);
        while(true){
            buffer.clear();
            System.out.println("read data from "+sc);
            //blocking here, why can't I read buffer
            int r = sc.read(buffer);
            if(r <= 0){
                break;
            }
            System.out.println(Arrays.toString(buffer.array()));
        }

    } catch (IOException e) {
        e.printStackTrace();
    }

}

public static void main(String[] args) {
    Client client = new Client();
    client.startClient(8000);
}
}

我是 java 大学的新生，谢谢你的帮助。我已经调试过了，下面是输出：
从服务器：
侦听连接到新套接字通道的端口8000:java.nio.channels.socketchannel[connected local=/127.0.0.1:8000 remote=/127.0.1:62694]接收数据：[82、101、116、117、116、104、101、110、32、105、110、118、111、107、105、110、103、32、116、104、105、115、32、109、101、116、104、111、100、32、104、97、115、32、110、111、32、101、102，101，99，116，46，0，0，0]从java.nio.channels.socketchannel[connected local=/127.0.0.1:8000 remote=/127.0.0.1:62694]将数据：[119，101，108，99，111，109，101，33]写入java.nio.channels.socketchannel[connected local=/127.0.0.1:8000 remote=/127.0.0.1:62694]
来自客户：
从java.nio.channels.socketchannel[connected local=/127.0.0.1:62694 remote=localhost/127.0.0.1:8000]连接到服务器从java.nio.channels.socketchannel[connected local=/127.0.0.1:62694 remote=localhost/127.0.1:8000]从java.nio.channels.socketchannel[connected local=/127.0.1:62694]读取数据远程=本地主机/127.0.0.1:8000]
但最终，客户端阻塞了read（buffer）语句。