问题

我正在尝试编写一个简单的客户端/服务器程序，其中客户端是一个安卓应用，服务器是一个树莓派 4。我只想实现的是允许客户端输入一条消息，然后让树莓派在终端上显示出这条消息。然而，我的应用程序在`out.writeObject(message_text)`这一行代码处抛出异常，位于`sendMessage()`方法中。

以下是服务器端的 Java 程序：

```java
import java.io.ObjectInputStream;
import java.net.ServerSocket;
import java.net.Socket;

public class RaspPiServer {
    
    private ServerSocket server;

    public RaspPiServer(){
    }

    public static void main(String[] args){
        RaspPiServer server = new RaspPiServer();
        server.runServer();
    }

    public void runServer(){
        try{
            server = new ServerSocket(12345,100);
            while (true){
                new Controller(server.accept()).start();
            }
        } catch(Exception e){
            e.printStackTrace();
        }
    }

    private class Controller extends Thread {

        private Socket socket;
        private ObjectInputStream input;
        private String in;

        public Controller(Socket socket){
            this.socket = socket;
            System.out.println("New client at " + socket.getRemoteSocketAddress());
        }

        @Override
        public void run(){
            try{
                input = new ObjectInputStream(socket.getInputStream());
                while (!(in = (String)input.readObject()).equals("close")){
                    System.out.println(in);
                }
            } catch(Exception e){
                e.printStackTrace();
            } finally {
                closeConnection();
                System.out.println("Connection with client @ " + socket.getRemoteSocketAddress() + " closed");
            }
        }

        private void closeConnection() {
            try {
                input.close();
                socket.close();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }

    }

}

在这里是否有一个简单的修复方法，还是我忽略了更大的问题？

<details>
<summary>英文:</summary>

I&#39;m trying to write a simple client/server program, where the client is an android app and the server is a Raspberry Pi 4. All I want to do is allow the client to type a message and have the Raspberry Pi display the message on the terminal. However, my app keeps throwing an exception at the `out.writeObject(message_text)` line in the `sendMessage()` method.

public class MainActivity extends AppCompatActivity {

private EditText message;
private Button send;
private ObjectOutputStream out;
private Socket socket;
private String raspi_ip = "enter ip here";
private int raspi_portnum = 12345;
Client client;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    message = findViewById(R.id.message_text);
    send = findViewById(R.id.send_button);

    try{
        client = new Client(raspi_ip,raspi_portnum);
        client.start();

    } catch (Exception e){
        AlertDialog.Builder dialog = new AlertDialog.Builder(MainActivity.this);
        dialog.setTitle("Error! ").setMessage("Couldn't connect to server.").setNeutralButton("OK", null).create().show();
    }

    send.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            client.sendMessage();
        }
    });
}

private void closeConnection(){
    try{
        out.close();
        socket.close();
    } catch (Exception e){
        e.printStackTrace();
    }
}

@Override
protected void onStop(){
    super.onStop();
    closeConnection();
}

private class Client extends Thread {
    private String ip_address;
    private int port_number;

    public Client(String ipaddress,int portnum){
        this.ip_address = ipaddress;
        this.port_number = portnum;
    }

    @Override
    public void run() {
        super.run();
        connectToServer(ip_address,port_number);
    }

    public void connectToServer(String ipaddress, int portnum){
        try{
            socket = new Socket(InetAddress.getByName(ipaddress),portnum);
            out = new ObjectOutputStream(socket.getOutputStream());
            out.flush();
        }catch (Exception e){
            AlertDialog.Builder dialog = new AlertDialog.Builder(MainActivity.this);
            dialog.setTitle("Error! ").setMessage("Couldn't connect to server.").setNeutralButton("OK", null).create().show();
        }
    }

    public void sendMessage(){
        String message_text = message.getText().toString();
        try{
            out.writeObject(message_text);
            out.flush();
        } catch (Exception e) {
            AlertDialog.Builder dialog = new AlertDialog.Builder(MainActivity.this);
            dialog.setTitle("Error! ").setMessage("IO Exception.").setNeutralButton("OK", null).create().show();
        }
    }
}

}

Here is the server side java program:

import java.io.ObjectInputStream;
import java.net.ServerSocket;
import java.net.Socket;

public class RaspPiServer {

private ServerSocket server;

public RaspPiServer(){
}

public static void main(String[] args){
    RaspPiServer server = new RaspPiServer();
    server.runServer();
}

public void runServer(){
    try{
        server = new ServerSocket(12345,100);
        while (true){
            new Controller(server.accept()).start();
        }
    } catch(Exception e){
        e.printStackTrace();
    }
}

private class Controller extends Thread {

    private Socket socket;
    private ObjectInputStream input;
    private String in;

    public Controller(Socket socket){
        this.socket = socket;
        System.out.println("New client at " + socket.getRemoteSocketAddress());
    }

    @Override
    public void run(){
        try{
            input = new ObjectInputStream(socket.getInputStream());
            while (!(in = (String)input.readObject()).equals("close")){
                System.out.println(in);
            }
        } catch(Exception e){
            e.printStackTrace();
        } finally {
            closeConnection();
            System.out.println("Connection with client @ " + socket.getRemoteSocketAddress() + " closed");
        }
    }

    private void closeConnection() {
        try {
            input.close();
            socket.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

}

}

Is there an easy fix here, or am I missing something bigger? 

</details>