java - 为什么我不能访问我的静态列表中的数据？

Question

我正在尝试访问我的列表中的数据，但它总是以 null 或其中没有任何内容结束。我运行我的 Class Server那有 public static ArrayList<String> clientUsernameList = new ArrayList<>();在代码中。如果我的服务器接受套接字，则应该出现以下代码。

package multi.threaded_server_application;

import java.io.DataInputStream;
import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;
import java.time.Clock;
import java.util.ArrayList;
import java.util.logging.Level;
import java.util.logging.Logger;
import jdk.nashorn.internal.codegen.CompilerConstants;

/**
*
* @author Seeya
*/
public class Server {

public static ArrayList<String> clientUsernameList = new ArrayList<>();

/**
 * @param args the command line arguments
 * @throws java.io.IOException
 */
public static void main(String[] args) throws IOException {

    int port = 8900;    //Port number the ServerSocket is going to use
    ServerSocket myServerSocket = null;  //Serversocket for sockets to connect
    Socket clientSocket = null;    //Listerner for accepted clients
    ClientThread[] clientsConnected = new ClientThread[20]; //Max clients in this server
    DataInputStream IN = null;

    try {
        myServerSocket = new ServerSocket(port);
        System.out.println("Server waiting for clients on port:" + port);
    } catch (IOException ex) {
        Logger.getLogger(Server.class.getName()).log(Level.SEVERE, null, ex);
    }

    while (true) {            
        try {   //Freezes while-loop untill a socket have been accepted or if some failure occur
            clientSocket = myServerSocket.accept();
            IN = new DataInputStream(clientSocket.getInputStream());
            String userName = IN.readUTF();
            Server.clientUsernameList.add(userName);

            System.out.println("Client have connected from:" + clientSocket.getLocalAddress().getHostName());
            System.out.println("Username:" + userName);
            System.out.println(Server.clientUsernameList);

        } catch (Exception e) {
            //Print out exception
            System.out.println(e);
        }
        //For-loop that counts every element in Array-clientsConnected
        for (int i = 0; i < clientsConnected.length; i++) {
            //If-statement checks if current element is null 
            if(clientsConnected[i] == null){
                //If current element in the Array is null then create a new object from ClientThread class
                //With a socket and the object of itself as parameter.
                (clientsConnected[i] = new ClientThread(clientSocket, clientsConnected)).start();

                //Must have a break otherwise it will create 20 objects (Exit for-loop)
                break;
            }   //Exit if-statement
        }   //Exit for-loop
    }   //Exit while-loop

}

} 我从我的 Class Client 收到用户名当它运行时。我把它打印出来，看看我是否在列表中添加了用户名。现在，当我尝试从我的 Class Client 访问数据时我什么也没收到/null。

package Server_application;

import java.io.DataInputStream;
import java.io.DataOutput;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.Socket;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.GregorianCalendar;
import java.util.Scanner;
import java.util.TimeZone;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
*
* @author Seeya
*/
public class Client extends Thread {

ClientGUI GUI = null;
DataInputStream IN = null;
DataOutputStream OUT = null;
Socket SOCKET = null;
String Message = null; 
String userName = null;
String ipAdress = "localhost";
int port = 8900;

public Client(ClientGUI gui,String username) {
    this.userName = username;
    this.GUI = gui;
}



@Override
public void run() {
    try {
            //Handle Input/Output from client
            SOCKET = new Socket(ipAdress, port);
            IN = new DataInputStream(SOCKET.getInputStream());
            OUT = new DataOutputStream(SOCKET.getOutputStream());

            //Sends username to server!
            OUT.writeUTF(userName);                

            System.out.println(Server.clientUsernameList);
            GUI.addToList(Server.clientUsernameList.toString());
            while (true) {
                CheckForMessages(IN);
            }

    } catch (IOException ex) {
        Logger.getLogger(Client.class.getName()).log(Level.SEVERE, null, ex);
    }

 }
//-----------------------------------------------------------------
//Method for sending Messages!
public void write(String s) throws IOException{
    //Writes message to server so it can send it to other clients.
    //Also sends to clients self GUI
    String A = s;
    OUT.writeUTF(A);
    GUI.writeGUI("You said: " + A);
}
//------------------------------------------------------------------
//Method for listening on messages
private void CheckForMessages(DataInputStream in) throws IOException {
    try {
        String IncomingMessage = in.readUTF();        
        System.out.println(IncomingMessage);
        GUI.writeGUI(IncomingMessage);
    } catch (Exception e) {
        System.err.println(e);
    }
}
}

打印出来的都是[]来 self 的 Class Client .我尝试访问静态变量的方式有问题吗？

编辑!这是我用来向服务器中的所有客户端发送/接收消息的代码。

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package multi.threaded_server_application;

import com.sun.security.ntlm.Client;
import java.awt.List;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.Socket;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.time.Clock;
import java.util.ArrayList;
import java.util.Date;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.swing.DefaultListModel;

/**
 *
 * @author Seeya
 */

//THIS CLASS TAKES CARE OF HANDLING MESSAGES TO OTHER CLIENTS!
//-------------------------------------------------------
public class ClientThread extends Thread {

private ClientThread[] clientsConnected;
private Socket SOCKET = null;
private DataInputStream IN = null;
private DataOutputStream OUT = null;
private String userName = null;
//-------------------------------------------------------    
//Constructor
public ClientThread(Socket socket, ClientThread[] clientThread) {
    this.SOCKET = socket;
    this.clientsConnected = clientThread;
}


//This starts as soon as the constructor is done/been created.
@Override
public void run() {
    try {
        //Handleing the messages
        IN = new DataInputStream(SOCKET.getInputStream());
        OUT = new DataOutputStream(SOCKET.getOutputStream());          
        String message = null;

        while (true) {             
            //This will be read second from the socket DataOutPutStream
            //We will use this string to send a message to every client that is online
            message = IN.readUTF();
            //For-loop to check how many people are actually online
            //for (ClientThread k : clientsConnected) = For each client in clientsConnected ends up in variable k
            for (int i = 0; i < clientsConnected.length; i++) {
                    //don't send message to yourself
                if (clientsConnected[i]!= null && clientsConnected[i].userName != this.userName){ 
                    // loops through all the list and calls the objects sendMessage method.
                    clientsConnected[i].sendMessage(message, userName); 
                }
            }
        }

    } catch (IOException ex) {
        Logger.getLogger(ClientThread.class.getName()).log(Level.SEVERE, null, ex);
    }


}

private void sendMessage(String message, String userName) {

    try {            
        Date date = new Date();
        DateFormat format = new SimpleDateFormat("HH:mm:ss");
        OUT.writeUTF(format.format(date) + " " + userName + " says: " + message);
        OUT.flush();

    } catch (IOException ex) {
        Logger.getLogger(ClientThread.class.getName()).log(Level.SEVERE, null, ex);
        System.out.println("Something failed to send message");
    }
}   
}

Answer 1

如果您希望从服务器字符串中获取客户端用户名 static关闭它，您将需要通过使用循环创建的套接字向客户端发送用户名，客户端将需要代码来读取用户名列表。

---

我不知道这是否会神奇地让您的程序执行您想要的操作，但这些代码足以让您理解我认为的概念。

在您的服务器类中

while (true) {            
    try {   //Freezes while-loop until a socket have been accepted or if some failure occur
        clientSocket = myServerSocket.accept();
        IN = new DataInputStream(clientSocket.getInputStream());
        String userName = IN.readUTF();
        OnConnect(userName); // Where to call the below method.

        System.out.println("Client have connected from:" + clientSocket.getLocalAddress().getHostName());
        System.out.println("Username:" + userName);
        System.out.println(Server.clientUsernameList);

    } catch (Exception e) {
        //Print out exception
        System.out.println(e);
    }


//.... More of your code.
String userNamesCSVs = ""

private void OnConnect(String theNewConnectionName){
    //For-Each loop says for every ClientThread object in clientsConnected do this block of code
    for(ClientThread client : clientsConnected){
        //Construct a long string with comma's in it.
        userNamesCSVs = userNamesCSVs + client.userName + ",";
    }
    //Don't forget to count the new name! This was added so you didn't have to do 
    //tricky stuff in the above for loop with the trailing comma. Also the server
    //doesn't have your user name yet.
    userNamesCSVs = userNamesCSVs + theNewConnectionName;
    //New for loop to start sending a mass message. Same as before.
    for(ClientThread client : clientsConnected){
        client.sendMessage(userNamesCSVs, userName);
    }
    //Remember take that static off clientUsernameList. Look up static it's not what you want here.

clientUsernameList.add(userName);
}

---

在您从服务器接收消息的客户端类中。

String[] userNameArray = new String[20]; //Max users and what not.
//Same method you are using named check for messages. It's needed to get the data from the server.
private void getUserNames(DataInputStream in) throws IOException{
    try {
        String userNames = in.readUTF();        
        System.out.println(IncomingMessage);
        userNameArray = userNames.split(",")
    } catch (Exception e) {
        System.err.println(e);
    }
    for(String user : userNames){
        GUI.writeGuid("User connected: " + user);
    }
}

---

旁注:

要查找的另一件事是 'serializable'和字节流和其他类似的很酷的东西。一旦您掌握了通过客户端和服务器来回发送字符串，这就很容易做到。基本上您可以发送整个对象，而不仅仅是字符串，只要它是可序列化的即可。