serverSocketChannel.accept（） 返回 null

问：

我正在尝试用Java NIO编写一个简单的客户端服务器程序。

代码如下：

package org.lb;

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.Set;

public class Test {
  public static void main(String[] args) throws IOException {
    ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
    serverSocketChannel.bind(new InetSocketAddress(8080));
    serverSocketChannel.configureBlocking(false);

    Selector selector = Selector.open();
    serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);

    while (true) {
      selector.select();
      Set<SelectionKey> selectionKeySet = selector.selectedKeys();
      for (SelectionKey selectionKey : selectionKeySet) {
        if (selectionKey.isAcceptable()) {
          SocketChannel socketChannel = serverSocketChannel.accept();
          if (socketChannel == null) {
            System.out.println("No connection...");
          } else {
            socketChannel.configureBlocking(false);
            socketChannel.register(selector, SelectionKey.OP_READ);
            System.out.println("Connection done!");
          }
        }

        if (selectionKey.isReadable()) {
          System.out.println("isReadable");
          SocketChannel socketChannel = (SocketChannel) selectionKey.channel();
          socketChannel.close();
        }
      }
    }
  }
}

您可以看到，我只在返回 true 时调用。因此，我认为它应该始终返回一个有效的实例，但有时它也会返回 null。serverSocketChannel.accept()selectionKey.isAcceptable()serverSocketChannel.accept()SocketChannel

我正在尝试使用 Postman 发送请求，但浏览器请求也是如此。

使用 Postman 单个请求输出

Connection done!
No connection...
isReadable
Connection done!
No connection...
isReadable

我以为它打印了一次，但它打印了两次。另外，为什么要打印？Connection done!No connection...

使用 curl single 请求输出

Connection done!
isReadable
No connection...