Java NIO之Selector

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前面的篇文章介绍了NIO中的Buffer和Channel,有了之前的基础,这篇文章来介绍一下另一个比较重要的概念----Selector。我们知道系统线程的切换是消耗系统资源的,如果我们每一个连接都用一个线程来管理,资源的开销会非常大,这个时候就可以用Selector。通过Selector可以实现一个线程管理多个Channel,如下图:

Selector

Selector使用

打开

使用之前获得一个Selector对象

Selector selector = Selector.open();

注册

要把Channel注册到Selector上,Channel必需是非阻塞的。因此FileChannel是无法注册到Selector的。如果注册的时候不调用configureBlocking方法就会抛出IllegalBlockingModeException异常。

SelectionKey

SelectionKey共有四种

ServerSocketChannel注册

serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);

ServerSocketChannel的Operation Set只能是OP_ACCEPT,如果在注册的时候添加了OP_CONNECT、OP_WRITE或OP_READ会报异常。例如按照以下写法

serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT | SelectionKey.OP_CONNECT);

就会抛出下面的异常

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)

ServerSocketChannel的validOps可以看到只有OP_ACCEPT是合法的

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

SocketChannel注册

socketChannel.register(selector, SelectionKey.OP_CONNECT);

SocketChannel的Operation Set只能是OP_CONNECT、OP_WRITE和OP_READ,如果在注册的时候添加了OP_ACCEPT同样会报异常。

SocketChannel的validOps可以看到只有OP_READ、OP_WRITE、OP_CONNECT是合法的

public final int validOps() {
    return (SelectionKey.OP_READ
            | SelectionKey.OP_WRITE
            | SelectionKey.OP_CONNECT);
}

注册成功之后,我们通过一个demo实现,客户端和服务端交互:

服务端:

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 如果不设置非阻塞,register的时候会报异常
	// 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("接收来自客户端的数据:" + new String(buffer.array()));
					selectionKey.interestOps(SelectionKey.OP_WRITE);
				} else if (selectionKey.isWritable()) {
					System.err.println("Writable");
					SocketChannel channel = (SocketChannel) selectionKey.channel();
					String content = "向客户端发送数据 : " + System.currentTimeMillis();
					ByteBuffer buffer = ByteBuffer.wrap(content.getBytes());
					channel.write(buffer);
					selectionKey.interestOps(SelectionKey.OP_READ);
				}
			}
		}
	}
}

我们来看一下服务端的逻辑

1、服务端注册到selector,然后interest set(ops)设置为SelectionKey.OP_ACCEPT等待客户端连接。

2、客户端连接到达,调用到selectionKey.isAcceptable()方法,接收客户端连接,然后获得一个channel,并把

interest set设置为SelectionKey.OP_READ等待从通道中读数据。

3、当客户端发送的数据到达,selectionKey.isReadable() 被触发,接收客户端的数据并打印,然后把selectionKey.interestOps 设置为SelectionKey.OP_WRITE,向客户端发送数据。

4、当可写之后selectionKey.isWritable()被触发,向客户端发送数据,同时selectionKey.interestOps再次设置为

SelectionKey.OP_READ等待客户端数据到达。

客户端:

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("收到服务端数据" + 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("向服务端发送数据" + new String(buffer.array()));
				}

				iterator.remove();
			}
		}
	}
}

再来看一下服务端的逻辑

1、向服务端发起连接请求。

2、selectionKey.isConnectable()被触发,连接成功之后,selectionKey.interestOps设置为SelectionKey.OP_WRITE,准备向服务端发送数据。

3、channel可写之后selectionKey.isWritable()被触发,向服务端发送数据,之后selectionKey.interestOps设置为SelectionKey.OP_READ,等待服务端过来的数据。

4、服务端数据发过来之后,selectionKey.isReadable()被触发,读取服务端数据之后selectionKey.interestOps设置为SelectionKey.OP_WRITE向服务端写数据。

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