java网络通信：异步非阻塞I/O (NIO)
首先是channel，是一个双向的全双工的通道，可同时读写，而输入输出流都是单工的，要么读要么写。Channel分为两大类，分别是用于网络数据的SelectableChannel和用于文件操作的FileChannel。
注意：在java NIO库中，所有的数据都是用缓冲区处理，常用的是ByteBuffer。
多路复用器Selector：
Selector会不断轮询注册在其上的Channel，如果某个Channel上又新的连接接入、读和写事件，这个Channel就处于就绪状态，通过SelectorKey可以获取就绪Channel的集合。底层使用了epoll()实现，没有最大连接句柄的限制。
服务端代码：
public class TimeServer { public static void main(String[] args) throws IOException { int port = 8080; MultiplexerTimeServer timeServer = new MultiplexerTimeServer(port); new Thread(timeServer, "NIO-MultiplexerTimeServer-001").start(); } } 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);//设置非阻塞模式 servChannel.socket().bind(new InetSocketAddress(port), 1024); servChannel.register(selector, SelectionKey.OP_ACCEPT);//将Channel注册到selector，监听accept事件 System.out.println("The time server is start in 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);//每隔一秒轮询一次 Set<SelectionKey> selectedKeys = selector.selectedKeys(); Iterator<SelectionKey> it = selectedKeys.iterator(); SelectionKey key = null; while (it.hasNext()) {//如果有新的客户端接入 key = it.next(); it.remove(); try { handleInput(key);//处理请求 } catch (Exception e) { if (key != null) { key.cancel(); if (key.channel() != null) key.channel().close(); } } } } catch (Throwable t) { t.printStackTrace(); } } // 多路复用器关闭后，所有注册在上面的Channel和Pipe等资源都会被自动去注册并关闭，所以不需要重复释放资源 if (selector != null) try { selector.close(); } catch (IOException e) { e.printStackTrace(); } } private void handleInput(SelectionKey key) throws IOException { if (key.isValid()) { // 处理新接入的请求消息 if (key.isAcceptable()) { // 获取客户端的连接通道 ServerSocketChannel ssc = (ServerSocketChannel) key.channel(); SocketChannel sc = ssc.accept(); sc.configureBlocking(false); //将新连接注册到selector，并监听读事件 sc.register(selector, SelectionKey.OP_READ); } if (key.isReadable()) { //读取通道数据写入字节缓存 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);//字节缓存写入字节数组 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);//向socketchannel写入数据 } else if (readBytes < 0) { // 对端链路关闭 key.cancel(); sc.close(); } } } } 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); } } }
客户端：
public class TimeClient { public static void main(String[] args) { int port = 8080; new Thread(new TimeClientHandle("127.0.0.1", port), "TimeClient-001").start(); } } public class TimeClientHandle implements Runnable { private String host; private int port; private Selector selector; private SocketChannel socketChannel; private volatile boolean stop; public TimeClientHandle(String host, int port) { this.host = host == null ? "127.0.0.1" : host; this.port = port; try { selector = Selector.open(); socketChannel = SocketChannel.open(); socketChannel.configureBlocking(false); } catch (IOException e) { e.printStackTrace(); System.exit(1); } } @Override public void run() { try { doConnect(); } catch (IOException e) { e.printStackTrace(); System.exit(1); } while (!stop) { try { selector.select(1000); Set<SelectionKey> selectedKeys = selector.selectedKeys(); Iterator<SelectionKey> it = selectedKeys.iterator(); SelectionKey key = null; while (it.hasNext()) { key = it.next(); it.remove(); try { handleInput(key); } catch (Exception e) { if (key != null) { key.cancel(); if (key.channel() != null) key.channel().close(); } } } } catch (Exception e) { e.printStackTrace(); System.exit(1); } } // 多路复用器关闭后，所有注册在上面的Channel和Pipe等资源都会被自动去注册并关闭，所以不需要重复释放资源 if (selector != null) try { selector.close(); } catch (IOException e) { e.printStackTrace(); } } private void doConnect() throws IOException { // 如果直接连接成功，则注册到多路复用器上，发送请求消息，读应答 if (!socketChannel.connect(new InetSocketAddress(host, port))) { socketChannel.register(selector, SelectionKey.OP_READ); doWrite(socketChannel); } else//没有直接连接成功，不代表失败，而是说明服务器还没有返回TCP握手的应答消息，所以注册OP_CONNECT事件，监听消息。 socketChannel.register(selector, SelectionKey.OP_CONNECT); } private void handleInput(SelectionKey key) throws IOException { if (key.isValid()) { // 判断是否连接成功 SocketChannel sc = (SocketChannel) key.channel(); if (key.isConnectable()) {//判断是否有连接事件，是的话，说明未连接 if (sc.finishConnect()) {//再连接 sc.register(selector, SelectionKey.OP_READ); doWrite(sc); } else System.exit(1);// 连接失败，进程退出 } if (key.isReadable()) { 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("Now is : " + body); this.stop = true; } else if (readBytes < 0) { // 对端链路关闭 key.cancel(); sc.close(); } } } } private void doWrite(SocketChannel sc) throws IOException { byte[] req = "QUERY TIME ORDER".getBytes(); ByteBuffer writeBuffer = ByteBuffer.allocate(req.length); writeBuffer.put(req); writeBuffer.flip(); sc.write(writeBuffer); if (!writeBuffer.hasRemaining()) System.out.println("Send order 2 server succeed."); } }
