Why the key should be removed in `selector.selectedKeys().iterator()` in java nio?

Question

I found some sample code of java nio:

 ServerSocketChannel server = ServerSocketChannel.open();  
 Selector selector = Selector.open();  
 server.socket().bi         


        

Answer 1

Because until you do it you can't detect new event repetitions.

Answer 2

if you dont invoke remove method you will find it also work fine。

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.InetSocketAddress;
import java.net.ServerSocket;
import java.net.Socket;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
@SuppressWarnings({ "unchecked" })
public class NioServer {
    ServerSocketChannel serverChannel;
    ServerSocket serverSocket;
    public final int port;
    private Selector selector;
    ByteBuffer buffer = ByteBuffer.allocate(1024);
    NioServer(final int port) {
        this.port = port;
    }
    void init() throws Exception {
        // 创建 ServerSocketChannel、ServerSocket
        serverChannel = ServerSocketChannel.open();
        serverSocket = serverChannel.socket();
        serverSocket.bind(new InetSocketAddress(port));
        // 设置通道为非阻塞模式
        serverChannel.configureBlocking(false);
        // 开启通道选择器，并注册 ServerSocketChannel
        selector = Selector.open();
        serverChannel.register(selector, SelectionKey.OP_ACCEPT);
    }

    void go() throws Exception {
        while (true) {
            int num = selector.select();
            if (num <= 0)
                continue;
            Iterator<SelectionKey> keyIter = selector.selectedKeys().iterator();
            System.out.println(selector.selectedKeys().size()); //等于线程数量
            while (keyIter.hasNext()) {
                final SelectionKey key = keyIter.next();
                System.out.println("所有的key"+key);
                // 接收一个Socket连接
                // key.isAcceptable（）如果为true，说明channnel支持accept（）,也就是说明是一个ServerSocketChannel
                if (key.isAcceptable()) {
                    System.out.println("可以连接的key:"+key);
                    SocketChannel clientChannel = serverChannel.accept();
                    if (clientChannel != null) {
                        clientChannel.configureBlocking(false);
                        clientChannel.register(selector, SelectionKey.OP_READ | SelectionKey.OP_WRITE);
                    }
                }
                // 如果isReadable()为true,说明是一个SocketChannel
                if (key.isReadable()) {
                    String requestContent = read(key);
                    // 业务处理
                    // responseContent=doSomthing(requestContent);
                    write(key, "ok" /* responseContent */);
                }
//          keyIter.remove();
            }
        }
    }

    // 从通道读取数据
    String read(SelectionKey key) throws Exception {
        SocketChannel socketChannel = (SocketChannel) key.channel();
        buffer.clear();// 这一步必须有
        int len = 0;
        StringBuffer str = new StringBuffer();
        while ((len = socketChannel.read(buffer)) > 0) {
            byte[] bs = buffer.array();
            String block = new String(bs, 0, len);
            System.out.println("Server read: " + block);
            str.append(block);

        }
        buffer.clear();
        return str.toString();
    }

    // 写数据到通道

    void write(SelectionKey key, String str) throws Exception {
        SocketChannel socketChannel = (SocketChannel) key.channel();
        buffer.clear();
        buffer.put(str.getBytes());
        buffer.flip();// 这一步必须有
        socketChannel.write(buffer);

    }

    public static void main(String[] args) throws Exception {
        final int port = 10000;
        NioServer server = new NioServer(port);
        server.init();
        /// ========================================================
        // 接下来模拟3个Client并发访问服务器
        int poolsize = 1;
        ExecutorService pool = Executors.newFixedThreadPool(poolsize);
        Collection<Callable> tasks = new ArrayList<Callable>(10);
        final String clientname = "clientThread";
        for (int i = 0; i < poolsize; i++) {
            final int n = i;
            // 若每一个Client都保持使用BIO方式发送数据到Server，并读取数据。
            tasks.add(new Callable() {
                @Override
                public Object call() throws Exception {
                    Socket socket = new Socket("127.0.0.1", port);
                    final InputStream input = socket.getInputStream();
                    final OutputStream out = socket.getOutputStream();
                    final String clientname_n = clientname + "_" + n;
                    // BIO读取数据线程
                    new Thread(clientname_n + "_read") {
                        @Override
                        public void run() {
                            byte[] bs = new byte[1024];
                            while (true) {
                                try {

                                    Thread.sleep(1000);

                                } catch (InterruptedException e) {
                                    e.printStackTrace();
                                }
                                int len = 0;
                                try {
                                    while ((len = input.read(bs)) != -1) {

                                        System.out.println("Clinet thread " + Thread.currentThread().getName()
                                                + " read: " + new String(bs, 0, len));
                                    }

                                } catch (IOException e) {

                                    e.printStackTrace();
                                }
                            }
                        }

                    }.start();
                    // BIO写数据线程
                    new Thread(clientname_n + "_write") {
                        @Override
                        public void run() {
                            int a = 0;
                            while (true) {
                                try {
                                    Thread.sleep(100);

                                } catch (InterruptedException e) {
                                    e.printStackTrace();
                                }

                                String str = Thread.currentThread().getName() + " hello, " + a;
                                try {
                                    out.write(str.getBytes());
                                    a++;

                                } catch (IOException e) {
                                    e.printStackTrace();
                                }
                            }
                        }

                    }.start();

                    return null;

                }

            });

        }
        pool.invokeAll((Collection<? extends Callable<Object>>) tasks);
        server.go();

    }

}

just print it and test it 。you will find even you not remove，use key.isAcceptable() can filter corrent result

Answer 3

Because the Selector never does that, it only adds to the set, so if you don't do it you will reprocess the event yourself next time the Selector returns.

Answer 4

avoid publicSelectedKeys private final Set<SelectionKey> publicSelectedKeys;

Answer 5

Because removing key from selected set, as it's been Handled, it will wait for next selection event.

selected set contains keys of the ready channels.

 selector.select(); //This may block for a long time. Upon returning, the elected set contains keys of the ready channels.

Get an iterator over the set of selected keys and Perform the Business Information

Iterator it = selector.selectedKeys().iterator(  );

Finally Remove key from selected set; it's been handled

 it.remove(  );

Keys can be directly removed from this set, but not added. Attempting to add to the selected key set throws java.lang.UnsupportedOperationException.