Question

我目前正在开发Java局域网游戏。我希望能够在按下按钮后启动服务器。当我按下按钮时，变量startServer和listenToClients变为true。但是，当我开始监听客户端时，我得到一个空指针异常。我该如何解决这个问题？

这是主类的代码：

public static void main(String args[]) throws IOException {

    ServerSocket ss = null;
    Socket cs = null;
    String address=Inet4Address.getLocalHost().getHostAddress();
    InetAddress addr = InetAddress.getByName(address);




    java.awt.EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
                new TablaPrinc().setVisible(true);
            } catch (IOException ex) {
                Logger.getLogger(Game.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
    });

    while (true) {

        if (startServer == true) {

            ss = new ServerSocket(5555,50,addr);
            System.out.println("Server started on "+address);
            startServer=false;  

        }

        try {
            Thread.sleep(10);
        } catch (InterruptedException ex) {
            Logger.getLogger(TablaPrinc.class.getName()).log(Level.SEVERE, null, ex);
        }

        if (listenToClients==true){
            cs = ss.accept();         
            System.out.println("\n we have a new client. "); 
            Connection con=new Connection(cs, ++i);

        }

    }


}

这是连接类：

static class Connection extends Thread {

int identity;
Socket cs = null;
DataInputStream is = null;
DataOutputStream os = null;


    public Connection(Socket client, int i) throws IOException {
    cs = client;
    identity = i;
    is = new DataInputStream(cs.getInputStream());
    os = new DataOutputStream(cs.getOutputStream());

    start();
    }
    public void run(){

       while(true){

      /* Interaction between the server and the clients */
       }


    }
}

Answer 1

首先确保首先将startServer变量设置为true，然后等待它再次变为false，这意味着ServerSocket已初始化，最后将listenToClients设置为true。否则，当ServerSocket为null时，您可能遇到listenToClients为true的情况，得到您正在讨论的NullPointerException。

你能为我们提供完整的堆栈跟踪打印异常吗？

Answer 2

好的，我修改了代码，摆脱了异常，这就是我现在所拥有的：

服务器：

package game;

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.Inet4Address;
import java.net.InetAddress;
import java.net.ServerSocket;
import java.net.Socket;
import java.net.UnknownHostException;
import java.util.ArrayList;


public class GServer {
 static int i=0;
 static int numberOfPlayers=0;
 static ArrayList <String> players=new ArrayList <String> ();



 public GServer(int nrj) throws UnknownHostException, IOException{
    this.numberOfPlayers=nrj;
    ServerSocket ss = null;
    Socket cs = null;
    String address=Inet4Address.getLocalHost().getHostAddress();
    InetAddress addr = InetAddress.getByName(address);
    ss = new ServerSocket(5555,50,addr);
    MainFrame.jLabel18.setText("Server started on "+address);




    for (int j=0;i<nrj;j++) {
        cs = ss.accept();     
        System.out.println("\n New Client"); 
        Connection con=new Connection(cs, ++i);
        players.add(Connection.is.readUTF());
    }

   for (int i=0;i<players.size();i++){
       System.out.println("\n Player:"+players.get(i));
   }


 }

}


class Connection extends Thread {

int identity;
Socket cs = null;
static DataInputStream is = null;
static DataOutputStream os = null;


    public Connection(Socket client, int i) throws IOException {
    cs = client;
    identity = i;
    is = new DataInputStream(cs.getInputStream());
    os = new DataOutputStream(cs.getOutputStream());

    start();
    }
    public void run(){

       while(true){

       }


    }
}

这是客户：

package game;

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.Socket;


public class GClient {       
public GClient() throws IOException{

    String address=MainFrame.jTextField18.getText();
    Socket cs = new Socket(address, 5555);
    MainFrame.jLabel20.setText("Connected to "+address);
    final DataOutputStream os = new DataOutputStream(cs.getOutputStream());
    DataInputStream is = new DataInputStream(cs.getInputStream());
    os.writeUTF(TablaPrinc.jTextField19.getText());
 }
}

这是主框架中的主要类：

public static void main(String args[]) throws IOException {

   try {

    java.awt.EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
                new TablaPrinc().setVisible(true);
             } catch (IOException ex) {
            Logger .getLogger(TablaPrinc.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
    });


    while(true){


        if (serverStart==true){
            int numberOfPlayers=Integer.parseInt(jTextField19.getText());
            GServer gs=new GServer(numberOfPlayers);
            serverStart=false;

     jButton2.setEnabled(false);
        }

        try {
            Thread.sleep(15);
        } catch (InterruptedException ex) {
            Logger.getLogger(TablaPrinc.class.getName()).log(Level.SEVERE, null, ex);
        }

    }




}

我想要做的是在客户端侦听结束后在所有帧上禁用jButton2。使用此代码，这仅发生在第一帧

按下此按钮后客户端连接到服务器：

private void jButton11ActionPerformed(java.awt.event.ActionEvent evt) {                                          

    jPanel3.setVisible(false);
    jLabel16.setVisible(false);
    jLabel17.setVisible(false);
    jLabel18.setVisible(false);
    jTextField18.setVisible(false);
    jTextField19.setVisible(false);
    jTextField20.setVisible(false);
    jButton5.setVisible(false);
    jButton6.setVisible(false);
    jButton7.setVisible(true);
    jLabel20.setVisible(true);
    jLabel7.setVisible(true);
    jTextField3.setVisible(true);
       try {
           GClient gc=new GClient();
       } catch (IOException ex) {
           jLabel20.setText("Connection unsuccesfull");
           Logger.getLogger(TablaPrinc.class.getName()).log(Level.SEVERE, null, ex);
       }



}

按下按钮然后等待客户端时启动Java套接字服务器

2 个答案: