Java NIO之Selector

前面兩篇文章介紹了NIO中的Buffer和Channel，有了以前的基礎，這篇文章來介紹一下另外一個比較重要的概念----Selector。咱們知道系統線程的切換是消耗系統資源的，若是咱們每個鏈接都用一個線程來管理，資源的開銷會很是大，這個時候就能夠用Selector。經過Selector能夠實現一個線程管理多個Channel，以下圖：

Selector使用

打開

使用以前得到一個Selector對象

Selector selector = Selector.open();
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註冊

要把Channel註冊到Selector上，Channel必需是非阻塞的。所以FileChannel是沒法註冊到Selector的。若是註冊的時候不調用configureBlocking方法就會拋出IllegalBlockingModeException異常。

SelectionKey

SelectionKey共有四種

• OP_ACCEPT
• OP_CONNECT
• OP_WRITE
• OP_READ

ServerSocketChannel註冊

serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
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ServerSocketChannel的Operation Set只能是OP_ACCEPT，若是在註冊的時候添加了OP_CONNECT、OP_WRITE或OP_READ會報異常。例如按照如下寫法

serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT | SelectionKey.OP_CONNECT);
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就會拋出下面的異常

Exception in thread "main" java.lang.IllegalArgumentException
	at java.nio.channels.spi.AbstractSelectableChannel.register(AbstractSelectableChannel.java:199)
	at java.nio.channels.SelectableChannel.register(SelectableChannel.java:280)
	at com.nio.sample.selector.SelectorServerSocketChannelSample.main(SelectorServerSocketChannelSample.java:27)
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ServerSocketChannel的validOps能夠看到只有OP_ACCEPT是合法的

public final int validOps() {
    return SelectionKey.OP_ACCEPT;
}
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SocketChannel註冊

socketChannel.register(selector, SelectionKey.OP_CONNECT);
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SocketChannel的Operation Set只能是OP_CONNECT、OP_WRITE和OP_READ，若是在註冊的時候添加了OP_ACCEPT一樣會報異常。

SocketChannel的validOps能夠看到只有OP_READ、OP_WRITE、OP_CONNECT是合法的

public final int validOps() {
    return (SelectionKey.OP_READ
            | SelectionKey.OP_WRITE
            | SelectionKey.OP_CONNECT);
}
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註冊成功以後，咱們經過一個demo實現，客戶端和服務端交互：

服務端：

public static void main(String[] args) throws Exception {

	ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();

	serverSocketChannel.socket().bind(new InetSocketAddress(9000));
	serverSocketChannel.configureBlocking(false);

	Selector selector = Selector.open();

	// configureBlocking 若是不設置非阻塞，register的時候會報異常
	// java.nio.channels.IllegalBlockingModeException
	serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);

	while (true) {

		int selected = selector.select();
		
		if (selected > 0) {

			Iterator<SelectionKey> iterator = selector.selectedKeys().iterator();
			while (iterator.hasNext()) {

				SelectionKey selectionKey = iterator.next();
				iterator.remove();

				if (selectionKey.isAcceptable()) {
					System.err.println("Acceptable");
					SocketChannel socketChannel = serverSocketChannel.accept();
					socketChannel.configureBlocking(false);
					socketChannel.register(selector, SelectionKey.OP_READ);
				} else if (selectionKey.isReadable()) {
					System.err.println("Readable");
					SocketChannel socketChannel = (SocketChannel) selectionKey.channel();
					ByteBuffer buffer = ByteBuffer.allocate(128);
					socketChannel.read(buffer);
					System.out.println("接收來自客戶端的數據：" + new String(buffer.array()));
					selectionKey.interestOps(SelectionKey.OP_WRITE);
				} else if (selectionKey.isWritable()) {
					System.err.println("Writable");
					SocketChannel channel = (SocketChannel) selectionKey.channel();
					String content = "向客戶端發送數據 : " + System.currentTimeMillis();
					ByteBuffer buffer = ByteBuffer.wrap(content.getBytes());
					channel.write(buffer);
					selectionKey.interestOps(SelectionKey.OP_READ);
				}
			}
		}
	}
}
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咱們來看一下服務端的邏輯

一、服務端註冊到selector，而後interest set（ops）設置爲SelectionKey.OP_ACCEPT等待客戶端鏈接。

二、客戶端鏈接到達，調用到selectionKey.isAcceptable()方法，接收客戶端鏈接，而後得到一個channel，並把

interest set設置爲SelectionKey.OP_READ等待從通道中讀數據。

三、當客戶端發送的數據到達，selectionKey.isReadable() 被觸發，接收客戶端的數據並打印，而後把selectionKey.interestOps 設置爲SelectionKey.OP_WRITE，向客戶端發送數據。

四、當可寫以後selectionKey.isWritable()被觸發，向客戶端發送數據，同時selectionKey.interestOps再次設置爲

SelectionKey.OP_READ等待客戶端數據到達。

客戶端:

public static void main(String[] args) throws IOException {

	SocketChannel socketChannel = SocketChannel.open();

	socketChannel.configureBlocking(false);
	Selector selector = Selector.open();
	socketChannel.register(selector, SelectionKey.OP_CONNECT);

	socketChannel.connect(new InetSocketAddress("127.0.0.1", 9000));

	while (true) {
		int select = selector.select();

		if (select > 0) {

			Iterator<SelectionKey> iterator = selector.selectedKeys().iterator();
			while (iterator.hasNext()) {

				SelectionKey selectionKey = iterator.next();

				if (selectionKey.isConnectable()) {
					System.err.println("Connectable");
					SocketChannel clientChannel = (SocketChannel) selectionKey.channel();
					clientChannel.finishConnect();
					selectionKey.interestOps(SelectionKey.OP_WRITE);

				} else if (selectionKey.isReadable()) {
					System.out.println("Readable");
					SocketChannel channel = (SocketChannel) selectionKey.channel();
					ByteBuffer buffer = ByteBuffer.allocate(128);
					channel.read(buffer);
					selectionKey.interestOps(SelectionKey.OP_WRITE);
					System.out.println("收到服務端數據" + new String(buffer.array()));

				} else if (selectionKey.isWritable()) {
					SocketChannel clientChannel = (SocketChannel) selectionKey.channel();
					String str = "qiwoo mobile";
					ByteBuffer buffer = ByteBuffer.wrap(str.getBytes());
					clientChannel.write(buffer);
					selectionKey.interestOps(SelectionKey.OP_READ);
					System.out.println("向服務端發送數據" + new String(buffer.array()));
				}

				iterator.remove();
			}
		}
	}
}
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再來看一下服務端的邏輯

一、向服務端發起鏈接請求。

二、selectionKey.isConnectable()被觸發，鏈接成功以後，selectionKey.interestOps設置爲SelectionKey.OP_WRITE，準備向服務端發送數據。

三、channel可寫以後selectionKey.isWritable()被觸發，向服務端發送數據，以後selectionKey.interestOps設置爲SelectionKey.OP_READ，等待服務端過來的數據。

四、服務端數據發過來以後，selectionKey.isReadable()被觸發，讀取服務端數據以後selectionKey.interestOps設置爲SelectionKey.OP_WRITE向服務端寫數據。

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