Java NIO入門小例（短連接：客戶端和服務器一問一答）

例子中有些寫法參考自Netty4源碼，建議在實際運用中采用Netty，而非原生的Java NIO（小心epoll空轉）。

1. 服務器端

public class NioServer {
    static SelectorProvider provider = SelectorProvider.provider();
    static Selector selector = null;
    static ServerSocketChannel server = null;
    
    private static void accept() throws IOException {
        SocketChannel channel = null;
        try {
            channel = server.accept(); // 接受連接
            channel.configureBlocking(false); // 非阻塞模式
            channel.register(selector, SelectionKey.OP_READ, null); // 監聽讀就緒
        } catch (IOException e) {
            if (channel != null)
                channel.close();
        }
    }
    
    private static int read(SocketChannel channel) throws IOException {
        try {
            ByteBuffer buffer = ByteBuffer.allocate(1024); // 分配HeapByteBuffer
            int len = channel.read(buffer); // 直到沒有數據 || buffer滿
            if (len > 0)
                System.out.println(new String(buffer.array(), 0, len, Charset.forName("UTF-8"))); // buffer.array()：取HeapByteBuffer中的原始byte[]
            return len;
        } catch (IOException e) {
            if (channel != null)
                channel.close();
            return -1;
        }
    }
    
    private static void write(SocketChannel channel, String msg) throws IOException {
        try {
            byte[] bytes = msg.getBytes(Charset.forName("UTF-8"));
            ByteBuffer buffer = ByteBuffer.allocate(bytes.length); // 分配HeapByteBuffer
            buffer.put(bytes);
            buffer.flip(); // 切換為讀模式
            channel.write(buffer);
        } catch (IOException e) {
            if (channel != null)
                channel.close();
        }
    }
    
    public static void main(String[] args) throws IOException {
        try {
            selector = provider.openSelector();
            server = provider.openServerSocketChannel();
            server.configureBlocking(false); // 非阻塞模式
            SelectionKey key = server.register(selector, 0, null); // 注冊
            if (server.bind(new InetSocketAddress(8888)).socket().isBound()) // 綁定成功
                key.interestOps(SelectionKey.OP_ACCEPT); // 監聽連接請求
            while (true) {
                selector.select(); // 監聽就緒事件
                Iterator<SelectionKey> it = selector.selectedKeys().iterator();
                while (it.hasNext()) {
                    key = it.next();
                    it.remove(); // 從已選擇鍵集中移除key
                    if (key.isAcceptable()) { // 連接請求到來
                        System.out.println("accept...");
                        accept();
                    } else {
                        SocketChannel channel = (SocketChannel) key.channel();
                        if (key.isWritable()) { // 寫就緒
                            System.out.println("write...");
                            write(channel, "Hello NioClient!");
                            key.interestOps(key.interestOps() & ~SelectionKey.OP_WRITE); // 取消寫就緒，否則會一直觸發寫就緒（寫就緒為代碼觸發）
                            key.channel().close(); // 關閉channel（key將失效）
                        }
                        if (key.isValid() && key.isReadable()) { // key有效（避免在寫就緒時關閉了channel或者取消了key） && 讀就緒
                            System.out.println("read...");
                            int len = read(channel);
                            if (len >= 0)
                                key.interestOps(key.interestOps() | SelectionKey.OP_WRITE); // 寫就緒，准備寫數據
                            else if (len < 0) // 客戶端已關閉socket
                                channel.close(); // 關閉channel（key將失效）
                        }
                    }
                }
            }
        } finally {
            if (server != null)
                server.close();
            if (selector != null)
                selector.close();
        }
    }
}

2. 客戶端

public class NioClient {
    static SelectorProvider provider = SelectorProvider.provider();
    static Selector selector = null;
    static SocketChannel client = null;
    static boolean close = false;
    
    private static void write(String msg) throws IOException {
        byte[] bytes = msg.getBytes(Charset.forName("UTF-8"));
        ByteBuffer buffer = ByteBuffer.allocate(bytes.length); // 建立HeapByteBuffer（DirectByteBuffer以后有機會再討論）
        buffer.put(bytes);
        buffer.flip(); // 切換為讀模式
        client.write(buffer);
    }
    
    private static int read() throws IOException {
        ByteBuffer buffer = ByteBuffer.allocate(1024); // 分配HeapByteBuffer
        int len = client.read(buffer); // 直到沒有數據 || buffer滿
        if (len > 0)
            System.out.println(new String(buffer.array(), 0, len, Charset.forName("UTF-8"))); // buffer.array()：取HeapByteBuffer中的原始byte[]
        return len;
    }
    
    public static void main(String[] args) throws IOException {
        try {
            selector = provider.openSelector();
            client = provider.openSocketChannel();
            client.configureBlocking(false); // 非阻塞模式
            SelectionKey key = client.register(selector, 0, null); // 注冊
            if (client.connect(new InetSocketAddress("127.0.0.1", 8888))) { // 連接成功（很難）
                System.out.println("connected...");
                key.interestOps(SelectionKey.OP_READ | SelectionKey.OP_WRITE); // 監聽讀就緒和寫就緒（准備寫數據）
            } else // 連接失敗（正常情況下）
                key.interestOps(SelectionKey.OP_CONNECT); // 監聽連接就緒
            while (!close) {
                selector.select(); // 監聽就緒事件
                Iterator<SelectionKey> it = selector.selectedKeys().iterator();
                while (it.hasNext()) {
                    key = it.next();
                    it.remove(); // 從已選擇鍵集移除key
                    if (key.isConnectable()) { // 連接就緒
                        client.finishConnect(); // 完成連接
                        System.out.println("connected...");
                        key.interestOps(key.interestOps() & ~SelectionKey.OP_CONNECT); // 取消監聽連接就緒（否則selector會不斷提醒連接就緒）
                        key.interestOps(key.interestOps() | SelectionKey.OP_READ | SelectionKey.OP_WRITE); // 監聽讀就緒和寫就緒
                    } else {
                        if (key.isWritable()) { // 寫就緒
                            System.out.println("write...");
                            write("Hello NioServer!");
                            key.interestOps(key.interestOps() & ~SelectionKey.OP_WRITE); // 取消寫就緒，否則會一直觸發寫就緒（寫就緒為代碼觸發）
                        }
                        if (key.isValid() && key.isReadable()) { // key有效（避免在寫就緒時關閉了channel或者取消了key） && 讀就緒
                            System.out.println("read...");
                            if (read() < 0) // 服務器已關閉socket
                                close = true; // 退出循環
                        }
                    }
                }
            }
        } finally {
            if (client != null)
                client.close();
            if (selector != null)
                selector.close();
        }
    }
}