使用 Selector 等待事件（例如使用 OP CONNECT）

NIO 出现在 Java 1.4 中并引入了 Channels 的概念，它应该比常规 I / O 更快。在网络方面， SelectableChannel 是最有趣的，因为它允许监视频道的不同状态。它的工作方式与 C select() 系统调用类似：当发生某些类型的事件时，我们会被唤醒：

• 接收连接（OP_ACCEPT）
• 连接实现（OP_CONNECT）
• 读 FIFO 中可用的数据（OP_READ）
• 可以推送数据写入 FIFO（OP_WRITE）

它允许在检测插槽 I / O（可以读/写/ …）和执行 I / O（读/写/ …）之间分离。特别是，所有 I / O 检测都可以在一个线程中为多个套接字（客户端）完成，而执行 I / O 可以在线程池或其他任何地方进行处理。这允许应用程序容易地扩展到连接的客户端的数量。

以下示例显示了基础知识：

1. 创建一个 Selector
2. 创建一个 SocketChannel
3. 注册 SocketChannel 到 Selector
4. 使用 Selector 循环检测事件

Selector sel = Selector.open(); // Create the Selector
SocketChannel sc = SocketChannel.open(); // Create a SocketChannel
sc.configureBlocking(false); // ... non blocking
sc.setOption(StandardSocketOptions.SO_KEEPALIVE, true); // ... set some options

// Register the Channel to the Selector for wake-up on CONNECT event and use some description as an attachement
sc.register(sel, SelectionKey.OP_CONNECT, "Connection to google.com"); // Returns a SelectionKey: the association between the SocketChannel and the Selector
System.out.println("Initiating connection");
if (sc.connect(new InetSocketAddress("www.google.com", 80)))
    System.out.println("Connected"); // Connected right-away: nothing else to do
else {
    boolean exit = false;
    while (!exit) {
        if (sel.select(100) == 0) // Did something happen on some registered Channels during the last 100ms?
            continue; // No, wait some more
        
        // Something happened...
        Set<SelectionKey> keys = sel.selectedKeys(); // List of SelectionKeys on which some registered operation was triggered
        for (SelectionKey k : keys) {
            System.out.println("Checking "+k.attachment());
            if (k.isConnectable()) { // CONNECT event
                System.out.print("Connected through select() on "+k.channel()+" -> ");
                if (sc.finishConnect()) { // Finish connection process
                    System.out.println("done!");
                    k.interestOps(k.interestOps() & ~SelectionKey.OP_CONNECT); // We are already connected: remove interest in CONNECT event
                    exit = true;
                } else
                    System.out.println("unfinished...");
            }
            // TODO: else if (k.isReadable()) { ...
        }
        keys.clear(); // Have to clear the selected keys set once processed!
    }
}
System.out.print("Disconnecting ... ");
sc.shutdownOutput(); // Initiate graceful disconnection
// TODO: emtpy receive buffer
sc.close();
System.out.println("done");

会给出以下输出：

Initiating connection
Checking Connection to google.com
Connected through 'select()' on java.nio.channels.SocketChannel[connection-pending remote=www.google.com/216.58.208.228:80] -> done!
Disconnecting ... done