NIO核心三：Selector

一、基本概念

选择器提供一种选择执行已经就绪的任务的能力。selector选择器可以让单线程处理多个通道。如果程序打开了多个连接通道，每个连接的流量都比较低，可以使用Selector对通道进行管理。

二、如何创建选择器

1.创建Selector

Selector selector = Selector.open();

2.必须将通道设置为非阻塞模式才能注册到选择器上

Channel.configureBlocking(false);

3.把通道注册到选择器上，会返回一个选择键

SelectionKey selectionKey = socketChannel.register(selector, SelectionKey.OP_READ);

SelectionKey的操作有:

• SelectionKey.OP_CONNECT,指某个通道连接到服务器
• SelectionKey.OP_ACCEPT,只有ServerSocketChannel有这个事件，查看是否有新的连接
• SelectionKey.OP_READ，是否有可读的通道就绪
• SelectionKey.OP_WRITE，写数据的通道是否就绪

注册完成后，可以调用select()方法轮询是否有就绪的通道

int count = selector.select();

select()方法，返回就绪的通道数量

三、服务器端模板

//服务器端模板代码public static void Server_Standard_Code_template() {try {ServerSocketChannel ssc=ServerSocketChannel.open();ssc.socket().bind(new InetSocketAddress("localhost",80));//只有设置为非阻塞才能注册到选择器中ssc.configureBlocking(false);//创建一个选择器Selector selector = Selector.open();//通道注册进选择器中---监听客户端连接事件ssc.register(selector, SelectionKey.OP_ACCEPT);while(true){//获取以及就绪的通道数量int select = selector.select();//没有通道就绪if(select==0){continue;}//获取已经就绪的Set<SelectionKey> selectionKeys = selector.selectedKeys();Iterator<SelectionKey> iterator = selectionKeys.iterator();while (iterator.hasNext()){SelectionKey selectionKey = iterator.next();//客户端连接请求事件if(selectionKey.isAcceptable()){//接收连接}else if(selectionKey.isReadable()){//读取数据}else if(selectionKey.isWritable()){//写数据}//移除iterator.remove();}}} catch (IOException e) {e.printStackTrace();}}

四、NIO通讯实例

服务器端

public class NIOServer {//通道管理器private Selector selector;/** * 获取一个ServerSocket通道，并对该通道做一些初始化工作 * @param port 端口号 * @throws IOException */public void initServer(int port) throws IOException {//获取一个ServerSocket通道ServerSocketChannel socketChannel = ServerSocketChannel.open();//设置通道为非阻塞socketChannel.configureBlocking(false);//将通道对应的ServerSocket绑定到port端口socketChannel.socket().bind(new InetSocketAddress(port));//获取一个通道管理器this.selector = Selector.open();/** * 将通道管理器和该通道绑定，并为该通道注册SelectionKey.OP_ACCEPT事件 * 注册该事件后，当该事件到达时，selector.select()会返回 * 如果该事件没有到达，selector.select()会一直阻塞 */socketChannel.register(selector, SelectionKey.OP_ACCEPT);}public void listen() throws IOException {while (true){//当注册的事件到达时，方法返回，否则该方法一直阻塞selector.select();//获取selector中选项的迭代器Iterator<SelectionKey> iterator = this.selector.selectedKeys().iterator();while (iterator.hasNext()){SelectionKey key = iterator.next();//删除已经选择的key,防止重复处理iterator.remove();//客户端连接请求事件if(key.isAcceptable()){//接收连接ServerSocketChannel serverSocketChannel = (ServerSocketChannel) key.channel();//获取客户端连接的通道SocketChannel channel = serverSocketChannel.accept();//设置为非阻塞channel.configureBlocking(false);//向客户端发送数据源ByteBuffer buf = ByteBuffer.allocate(1024);String message = "你好我是服务器端，我接收到了你的消息";buf.put(message.getBytes(StandardCharsets.UTF_8));//把缓冲区切换成读取模式buf.flip();//将buffer写入channelwhile (buf.hasRemaining()){channel.write(buf);}//和客户端连接成功后，为了接收到客户端的信息，需要给通道设置读取权限channel.register(this.selector,SelectionKey.OP_READ);}else if(key.isReadable()){//读取数据read(key);}}}}public void read(SelectionKey key) throws IOException {//得到事件发生的socket通道SocketChannel channel = (SocketChannel) key.channel();//创建读取的缓冲区ByteBuffer buffer = ByteBuffer.allocate(1024);//将数据读取到缓冲区channel.read(buffer);// 4、把缓冲区切换成写出模式buffer.flip();String rs = new String(buffer.array(),0,buffer.remaining());System.out.println(rs);}public static void main(String[] args) throws IOException {NIOServer server = new NIOServer();server.initServer(8100);server.listen();}}

客户端

public class NIOClient {//通道管理器private Selector selector;public static void main(String[] args) throws IOException {NIOClient client = new NIOClient();client.initClick("127.0.0.1",8100);client.listen();}public void initClick(String ip,int port) throws IOException {//获取一个socketSocketChannel channel = SocketChannel.open();//设置通道为非阻塞channel.configureBlocking(false);//获取一个通道管理器this.selector = Selector.open();channel.connect(new InetSocketAddress(ip,port));channel.register(this.selector, SelectionKey.OP_CONNECT);}public void listen() throws IOException {while (true){selector.select();Iterator<SelectionKey> iterator = this.selector.selectedKeys().iterator();while (iterator.hasNext()){SelectionKey key = iterator.next();iterator.remove();if(key.isConnectable()){SocketChannel channel = (SocketChannel) key.channel();if(channel.isConnectionPending()){channel.finishConnect();}//设置为阻塞channel.configureBlocking(false);//向客户端发送数据源ByteBuffer buffer = ByteBuffer.allocate(1024);Stringmessage = "服务器端你好，我是客户端";buffer.put(message.getBytes(StandardCharsets.UTF_8));//把缓冲区切换成读取模式buffer.flip();//将buffer写入channelwhile (buffer.hasRemaining()){channel.write(buffer);}channel.register(this.selector,SelectionKey.OP_READ);}else if(key.isReadable()){read(key);}}}}public void read(SelectionKey key) throws IOException {SocketChannel channel = (SocketChannel) key.channel();ByteBuffer buffer = ByteBuffer.allocate(1024);channel.read(buffer);byte[] data = buffer.array();// 4、把缓冲区切换成写出模式buffer.flip();String rs = new String(data,0,buffer.remaining());System.out.println(rs);}}