NIO：为什么Selector的selectedKeys遍历处理事件后要移除？

package com.jielihaofeng.netty.c4;
​
import lombok.extern.slf4j.Slf4j;
​
import java.io.IOException;
import java.net.InetSocketAddress;
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.Iterator;
​
/**
 * @description Selector使用
 * @author Johnnie Wind
 * @date 2021/10/11 22:08
 */
@Slf4j
public class ServerSelector {
​
    public static void main(String[] args) throws IOException {
​
        // 1. 创建 selector，管理多个 channel
        Selector selector = Selector.open();
        ServerSocketChannel ssc = ServerSocketChannel.open();
        ssc.configureBlocking(false); // 一定要配置，否则报异常 java.nio.channels.IllegalBlockingModeException
​
        // 2. 建立 selector 和 channel 之间的联系
        // SelectionKey 就是将来事件发生后，通过它可以知道事件和哪个channel的事件
        SelectionKey sscKey = ssc.register(selector, 0, null);
        // 事件的四种类型：
        // accept - 会在有连接请求时触发
        // connect - 是客户端，连接建立后触发
        // read - 可读事件
        // write - 可写事件
        // key只关注 accept事件
        sscKey.interestOps(SelectionKey.OP_ACCEPT);
        log.debug("sscKey:{}",sscKey);
​
        ssc.bind(new InetSocketAddress(8080));
        while (true){
            // 3. select 方法，没有事件发生，线程阻塞，有事件，线程才会恢复运行
            // select 在事件未处理时，它不会阻塞，事件发生后要么处理要么取消，不能置之不理
            selector.select();
            // 4.处理事件，selectedKeys 内部包含了所有发生的事件
            Iterator<SelectionKey> iterator = selector.selectedKeys().iterator(); // accept,read
            while (iterator.hasNext()){
                SelectionKey key = iterator.next();
                log.debug("key:{}",key);
                // 5. 区分事件类型
                if (key.isAcceptable()){ // 如果是 accept
                    ServerSocketChannel channel = (ServerSocketChannel)key.channel();
                    SocketChannel sc = channel.accept();
                    sc.configureBlocking(false);
                    SelectionKey scKey = sc.register(selector, 0, null);
                    scKey.interestOps(SelectionKey.OP_READ);
                    log.debug("sc {}",sc);
                    log.debug("scKey:{}",scKey);
                }else if (key.isReadable()){ // 如果是 read
                    SocketChannel channel = (SocketChannel) key.channel(); // 拿到触发事件的channel
                    ByteBuffer buffer = ByteBuffer.allocate(16);
                    channel.read(buffer);
                    buffer.flip();
                    while(buffer.hasRemaining()){
                        System.out.println((char)buffer.get());
                    }
                    buffer.clear();
                }
            }
        }
    }
}

package com.jielihaofeng.netty.c4;
​
import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.channels.SocketChannel;
​
/**
 * @description 客户端
 * @author Johnnie Wind
 * @date 2021/10/11 22:17
 */
public class Client {
    public static void main(String[] args) throws IOException {
        SocketChannel sc = SocketChannel.open();
        sc.connect(new InetSocketAddress("localhost", 8080));
        System.out.println("waiting..."); // 注意，要在此处打断点进行调试启动
    }
}

21:11:44.400 [main] DEBUG com.jielihaofeng.netty.c4.ServerSelector - sscKey:sun.nio.ch.SelectionKeyImpl@c46bcd4
21:12:08.322 [main] DEBUG com.jielihaofeng.netty.c4.ServerSelector - key:sun.nio.ch.SelectionKeyImpl@c46bcd4
21:12:08.323 [main] DEBUG com.jielihaofeng.netty.c4.ServerSelector - sc java.nio.channels.SocketChannel[connected local=/127.0.0.1:8080 remote=/127.0.0.1:62001]
21:12:08.323 [main] DEBUG com.jielihaofeng.netty.c4.ServerSelector - scKey:sun.nio.ch.SelectionKeyImpl@4923ab24
21:23:57.723 [main] DEBUG com.jielihaofeng.netty.c4.ServerSelector - key:sun.nio.ch.SelectionKeyImpl@c46bcd4
Exception in thread "main" java.lang.NullPointerException
    at com.jielihaofeng.netty.c4.ServerSelector.main(ServerSelector.java:57)
Disconnected from the target VM, address: '127.0.0.1:64394', transport: 'socket'

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

if (key.isAcceptable()){ // 如果是 accept
// ...
}

SocketChannel sc = channel.accept(); // 此时的事件是sc的read，ssc获取sc为空！

sc.configureBlocking(false);

SelectionKey key = iterator.next();
// 处理完事件后一定要从 selectedKeys 集合中删除
iterator.remove();