Java网络编程 - 非阻塞IO模型

1. Java NIO简介

NIO(Non-Block I/O)是在JDK1.4之后引入的。NIO弥补了同步阻塞I/O的缺点。提供了高速、面向块的I/O。在NIO库中，所有的数据都是用缓冲区来处理的，在读取数据的时候，他是直接读到缓冲区中的：在写数据时，写到缓冲区中。任何时候访问NIO中的数据都是通过缓冲区进行操作。Java NIO由三个核心部分组成，分别是Buffers, Channel和Selector。

1.1 Buffers

Java为每一种基础的数据类型都提供了一个对应的缓冲区（除了Boolean外），具体有：

• ByteBuffer 字节缓冲区
• CharBuffer 字符缓冲区
• ShortBuffer 短整型缓冲区
• IntBuffer 整型缓冲区
• LongBuffer 长整形缓冲区
• FloatBuffer 浮点型缓冲区
• DoubleBuffer 双精度浮点型缓冲区

1.2 Channels

JAVA NIO中包括以下几种Channel:

• FileChannel
• DatagramChannel
• SocketChannel
• ServerSocketChannel
  这些通道涵盖了UDP和TCP相关的网络IO，同时也包含有文件操作的IO。

1.3 Selector

Selector允许单线程处理多个Channel。如果你的应用打开了多个连接（通道），但每个连接的流量都很低，使用Selector就会很方便。Java NIO中Selector采用epoll的机制进行实现，因此不存在有最大文件句柄的限制，因此只需要一个监听的线程就可以完成对成千上万个连接进行处理。

2. Java NIO服务器时序

按照上述步骤，时间

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 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("Timer Server Start! 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){
                        key.cancel();
                        if(key.channel() != null){
                            key.channel().close();
                        }
                    }
                }
            }
            catch(Throwable t){
                t.printStackTrace();
            }
        }

        if(selector != null){
            try{
                selector.close();
            }catch(Exception e){
                e.printStackTrace();
            }
        }
    }


    private void handleInput(SelectionKey key) throws IOException{
        if(key.isValid()){
            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 java.util.Date(
                        System.currentTimeMillis()).toString()
                        : "BAD ORDER";
                    doWrite(sc, currentTime);
                } else if (readBytes < 0) {
                    key.cancel(); // 对端链路关闭
                    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);
        }
    }

}