Exception in thread "Thread-1" java.lang.IllegalStateException: Scanner closed at java.base/java.util.Scanner.ensureOpen(Scanner.java:1150) at java.base/java.util.Scanner.next(Scanner.java:1573) at java.base/java.util.Scanner.nextInt(Scanner.java:2258) at java.base/java.util.Scanner.nextInt(Scanner.java:2212) at my_project/myproject.Client.lambda$1(Client.java:52) at java.base/java.lang.Thread.run(Thread.java:833) java.net.SocketException: Socket closed at java.base/sun.nio.ch.NioSocketImpl.endRead(NioSocketImpl.java:248) at java.base/sun.nio.ch.NioSocketImpl.implRead(NioSocketImpl.java:327) at java.base/sun.nio.ch.NioSocketImpl.read(NioSocketImpl.java:350) at java.base/sun.nio.ch.NioSocketImpl$1.read(NioSocketImpl.java:803) at java.base/java.net.Socket$SocketInputStream.read(Socket.java:966) at java.base/java.io.InputStream.read(InputStream.java:218) at my_project/myproject.Client.lambda$0(Client.java:33) at java.base/java.lang.Thread.run(Thread.java:833)

这个错误是由于在客户端代码中的线程，Scanner对象被关闭导致的。为了解决这个问题，你可以将Scanner对象的创建和关闭操作移到主线程中。下面是修改后的代码：

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.Socket;
import java.util.Scanner;

public class Client {
    private static final String SERVER_IP = "localhost";
    private static final int SERVER_PORT = 8888;
    private String username;

    public void start() {
        try {
            Socket socket = new Socket(SERVER_IP, SERVER_PORT);
            System.out.println("成功连接服务器");

            InputStream inputStream = socket.getInputStream();
            OutputStream outputStream = socket.getOutputStream();

            // 发送消息给服务器
            new Thread(() -> {
                try {
                    Scanner scanner = new Scanner(System.in);

                    while (true) {
                        String message = scanner.nextLine();
                        outputStream.write(message.getBytes());
                        outputStream.flush();
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }).start();

            // 接收服务器发送的消息
            try {
                byte[] buffer = new byte[1024];
                int len;

                while ((len = inputStream.read(buffer)) > 0) {
                    String message = new String(buffer, 0, len);
                    System.out.println("收到服务器消息：" + message);
                }
            } catch (IOException e) {
                e.printStackTrace();
            } finally {
                socket.close();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        Client client = new Client();
        client.start();
    }
}

在修改后的代码中，我们将接收服务器发送消息的逻辑移到了主线程中，并在最后关闭了Socket连接。这样就避免了在子线程中关闭Scanner对象和Socket连接的问题。