使用 Selector 等待事件（例如使用 OP CONNECT）

NIO 出現在 Java 1.4 中並引入了 Channels 的概念，它應該比常規 I / O 更快。在網路方面， SelectableChannel 是最有趣的，因為它允許監視訊道的不同狀態。它的工作方式與 C select() 系統呼叫類似：當發生某些型別的事件時，我們會被喚醒：

• 接收連線（OP_ACCEPT）
• 連線實現（OP_CONNECT）
• 讀 FIFO 中可用的資料（OP_READ）
• 可以推送資料寫入 FIFO（OP_WRITE）

它允許在檢測插槽 I / O（可以讀/寫/ …）和執行 I / O（讀/寫/ …）之間分離。特別是，所有 I / O 檢測都可以在一個執行緒中為多個套接字（客戶端）完成，而執行 I / O 可以線上程池或其他任何地方進行處理。這允許應用程式容易地擴充套件到連線的客戶端的數量。

以下示例顯示了基礎知識：

1. 建立一個 Selector
2. 建立一個 SocketChannel
3. 註冊 SocketChannel 到 Selector
4. 使用 Selector 迴圈檢測事件

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");

會給出以下輸出：

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