NIO's Selector execution process

一、Seletor是什么？

selector 单从字面意思不好理解,Seletor是一个监听器,它可以监听Channel中发生的事件.Channelcan be registered atSeletor中,when these are registeredChannel在事件发生时,Seletor的select 方法就会返回这些事件交给 thread 来处理.

二、Seletordifference from multithreading

It is not easy to understand just from the above explanation,需要结合服务器的设计演化来理解它的用途

多线程版设计

to clients in the networksocket请求,If each request opens a thread to handle,Then the system memory usage is high,Thread context switching costs are also high,The stress on the system can be very high.

线程池版设计

使用线程池,Optimized thread creation,But in blocking mode,线程仅能处理一个 socket 连接, 仅适合短连接场景

selector版本

selector 的作用就是配合一个线程来管理多个 channel,获取这些 channel 上发生的事件,这些 channel 工作在非阻塞模式下,不会让线程吊死在一个 channel 上.适合连接数特别多,但流量低的场景（low traffic）

三、selector单线程处理多socket案例

服务端代码：

@Slf4j
public class ChannelDemo6 {
    
    public static void main(String[] args) {
    
        try (ServerSocketChannel channel = ServerSocketChannel.open()) {
    
            channel.bind(new InetSocketAddress(8080));
            System.out.println(channel);
            Selector selector = Selector.open();
            channel.configureBlocking(false);
            channel.register(selector, SelectionKey.OP_ACCEPT);

            while (true) {
    
                int count = selector.select();
                 log.debug("select count: {}", count);


                // 获取所有事件
                Set<SelectionKey> keys = selector.selectedKeys();

                // 遍历所有事件,逐一处理
                Iterator<SelectionKey> iter = keys.iterator();
                while (iter.hasNext()) {
    
                    SelectionKey key = iter.next();
                    // 判断事件类型
                    if (key.isAcceptable()) {
    
                        ServerSocketChannel c = (ServerSocketChannel) key.channel();
                        // 必须处理
                        SocketChannel sc = c.accept();
                        sc.configureBlocking(false);
                        sc.register(selector, SelectionKey.OP_READ);
                        log.debug("连接已建立: {}", sc);
                    }
                    else if (key.isReadable()) {
    
                        SocketChannel sc = (SocketChannel) key.channel();
                        ByteBuffer buffer = ByteBuffer.allocate(128);
                        int read = sc.read(buffer);
                        if(read == -1) {
    
                            key.cancel();
                            sc.close();
                        } else {
    
                            buffer.flip();
                            debugAll(buffer);
                        }
                    }
                    // 处理完毕,必须将事件移除
                    iter.remove();
                }
            }
        } catch (IOException e) {
    
            e.printStackTrace();
        }
    }
}

客户端代码:

public class Client {
    
    public static void main(String[] args) {
    
        try (Socket socket = new Socket("localhost", 8080)) {
    
            System.out.println(socket);
            socket.getOutputStream().write("world".getBytes());
            System.in.read();
        } catch (IOException e) {
    
            e.printStackTrace();
        }
    }
}

方法解释:

• ServerSocketChannelused to create the serverChannel,它有点像ServerSocket.
• channel.configureBlocking(false);设置channel为非阻塞,这样如果accpet的时候,如果没有连接,就会返回null.
• channel.register(selector, SelectionKey.OP_ACCEPT)将channel注册到selector中,监听accept连接事件
• selector.select(),会阻塞,等待客户端连接
• selector.selectedKeys(),event production,will add events to this collection

四、selector事件

• accept Fired when a connection is made
• connect Fired when a client establishes a connection
• read Raises a readable event when a client message is received
• 可写事件