請問一個java nio問題

在學習一本java網路程式設計的書的時候有一個程式看不懂，程式如下，
Example 12-2. A non-blocking chargen server
import java.nio.*;

import java.nio.channels.*;

import java.net.*;

import java.util.*;

import java.io.IOException;

public class ChargenServer {

public static int DEFAULT_PORT = 19;

public static void main(String[] args) {

int port;

try {

port = Integer.parseInt(args[0]);

}

catch (Exception ex) {

port = DEFAULT_PORT;

}

System.out.println("Listening for connections on port " + port);

byte[] rotation = new byte[95*2];

for (byte i = ' '; i <= '~'; i++) {

rotation[i-' '] = i;

rotation[i+95-' '] = i;

}

ServerSocketChannel serverChannel;

Selector selector;

try {

serverChannel = ServerSocketChannel.open( );

ServerSocket ss = serverChannel.socket( );

InetSocketAddress address = new InetSocketAddress(port);

ss.bind(address);

serverChannel.configureBlocking(false);

selector = Selector.open( );

serverChannel.register(selector, SelectionKey.OP_ACCEPT);

}

catch (IOException ex) {

ex.printStackTrace( );

return;

}

while (true) {

try {

selector.select( );

}

catch (IOException ex) {

ex.printStackTrace( );

break;

}

Set readyKeys = selector.selectedKeys( );

Iterator iterator = readyKeys.iterator( );

while (iterator.hasNext( )) {

SelectionKey key = (SelectionKey) iterator.next( );

iterator.remove( );

try {

if (key.isAcceptable( )) {

ServerSocketChannel server = (ServerSocketChannel) key.channel( );

SocketChannel client = server.accept( );

System.out.println("Accepted connection from " + client);

client.configureBlocking(false);

SelectionKey key2 = client.register(selector, SelectionKey.

OP_WRITE);

ByteBuffer buffer = ByteBuffer.allocate(74);

buffer.put(rotation, 0, 72);

buffer.put((byte) '\r');

buffer.put((byte) '\n');

buffer.flip( );

key2.attach(buffer);

}

else if (key.isWritable( )) {

SocketChannel client = (SocketChannel) key.channel( );

ByteBuffer buffer = (ByteBuffer) key.attachment( );

if (!buffer.hasRemaining( )) {

// Refill the buffer with the next line

buffer.rewind( );

// Get the old first character

int first = buffer.get( );

// Get ready to change the data in the buffer

buffer.rewind( );

// Find the new first characters position in rotation

int position = first - ' ' + 1;

// copy the data from rotation into the buffer

buffer.put(rotation, position, 72);

// Store a line break at the end of the buffer

buffer.put((byte) '\r');

buffer.put((byte) '\n');

// Prepare the buffer for writing

buffer.flip( );

}

client.write(buffer);

}

}

catch (IOException ex) {

key.cancel( );

try {

key.channel( ).close( );

}

catch (IOException cex) {}

}

}

}

}

}

請問
1是否可以多個channel在selector中註冊？
2這個程式那裡體現了使用java nio的好處，這個程式的目的是什麼？
謝謝

請問一個java nio問題

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