从套接字读取时获取空字符串 [英] Getting Null String when reading from a socket

问题描述

这里是一个使用Sockets在Java中编写客户端 - 服务器系统，但是我不能读取从服务器发送到客户端的数据。客户代码：

  public class ClientSocket 
 {
 Socket clientSocket = null; 
 PrintWriter out = null; 
 BufferedReader in = null; 
 
 
 //通过套接字4444建立到All Care服务器应用程序的连接（调整localhost以反映服务器
 //正在运行的IP地址）
 public ClientSocket（）
 {
 try 
 {
 clientSocket = new Socket（localhost，4445）; 
 
 out = new PrintWriter（clientSocket.getOutputStream（））; 
 in = new BufferedReader（new InputStreamReader（clientSocket.getInputStream（）））; 
} 
 catch（IOException e）
 {
 System.out.println（无法连接到所有Care Server应用程序）; 
} 
} 
 
 public void closeClientSocket（）
 {
 try 
 {
 clientSocket.close 
} 
 catch（IOException e）
 {
 System.out.println（无法关闭与所有Care Server应用程序的连接）; 
} 
} 
 
 public String getMessageFromServer（）
 {
 try 
 {
 String input = in.readLine ; 
 
返回输入; 
} 
 catch（IOException e）
 {
 System.out.println（无法从服务器读取消息）; 
} 
 
返回No Data; 
} 
 
 public void sendMessageToServer（String message）
 {
 out.write（message）; 
} 
} 
  

这里是服务器代码：

  public class ArFileServer {
 
 public static void main（String [] args）
 {
 ServerSocket serverSocket = null; 
 boolean listening = true; 
 
 try 
 {
 serverSocket = new ServerSocket（4445）; 
 
 //无限循环继续监听客户端的连接请求
 while（监听）
 {
 new ClientConnection（serverSocket.accept（））。start ）; 
 
 if（serverSocket！= null）
 {
 System.out.println（Connection to client established）; 
} 
} 
 
 serverSocket.close（）; 
} 
 catch（IOException e）
 {
 System.out.println（错误无法创建套接字连接到端口）; 
} 
} 
} 
 
 public class ClientConnection extends Thread 
 {
 private Socket socket = null; 
 
 public ClientConnection（Socket socket）
 {
 super（ClientConnection）; 
 this.socket = socket; 
} 
 
 //连接到服务器后运行的线程已被接受
 @Override 
 public void run（）
 {
 try 
 {
 PrintWriter out = new PrintWriter（socket.getOutputStream（））; 
 BufferedReader in = new BufferedReader（new InputStreamReader（socket.getInputStream（）））; 
 
 sendMessagetoClient（out，CONNECTION SUCCESS）; 
 
 //检查从客户端发送到服务器的登录凭据
 
 //如果有效，则返回其加密密码，否则输出登录错误消息
 
 //等待用户输入，然后根据他们的请求进行各种进程
 
 in.close（）; 
 out.close（）; 
 socket.close（）; 
} 
 catch（IOException e）
 {
 System.out.println（Client socket connection error）; 
} 
} 
 
 //向客户端发送消息
 void sendMessagetoClient（PrintWriter out，String message）
 {
 out。 write（message）; 
} 
 
 //监听来自客户端的消息
 String getMessageFromClient（BufferedReader in）
 {
 try 
 {
 String input = in.readLine（）; 
 return input; 
} 
 catch（IOException e）
 {
 System.out.println（无法从客户端读取消息）; 
} 
 
返回No Data; 
} 
  

这里是代码行im使用查看数据是否正在发送。

  System.out.println（clientSocket.getMessageFromServer（））;在您的 sendMessageToClient（）中，
  




解决方案

方法，您需要刷新：

  void sendMessagetoClient（PrintWriter out，String message）
 {
 out.write（message）; 
 out.flush（）; 
} 
  

---

PrintWriter ，使用 autoflush 的构造函数：

  PrintWriter out = new PrintWriter（socket.getOutputStream（），true）; 
  

当你写，而不是 out.write code>使用 printf（）或 println（）。

I am trying to write a client-server system using Sockets in java, however I cannot seem to read data sent from the server to the client.

Here is the code for the client:

public class ClientSocket 
{
    Socket clientSocket = null;
    PrintWriter out = null;
    BufferedReader in = null;


    // establish a connection to All Care's server application through socket 4444 (adjust localhost to reflect the IP address that the server
    // is being run from)
    public ClientSocket()
    {
        try
        {
            clientSocket = new Socket("localhost", 4445);

            out = new PrintWriter(clientSocket.getOutputStream());
            in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
        }
        catch (IOException e)
        {
            System.out.println("Could not connect to All Care Server Application");
        }
    }

    public void closeClientSocket()
    {   
        try
        {
            clientSocket.close();
        }
        catch (IOException e)
        {
            System.out.println("Could not close connection to All Care Server Application");
        }
    }

    public String getMessageFromServer()
    {
        try
        {
            String input = in.readLine();

            return input;
        }
        catch (IOException e)
        {
            System.out.println("Could not read message from server");
        }

        return "No Data";
    }

    public void sendMessageToServer(String message)
    {
        out.write(message);
    }
}

And here is the Server code:

public class ArFileServer {

    public static void main(String[] args) 
    {
        ServerSocket serverSocket = null;
        boolean listening = true;

        try
        {
            serverSocket = new ServerSocket(4445);

            // infinite loop to continually listen for connection requests made by clients
            while (listening)
            {
                new ClientConnection(serverSocket.accept()).start();

                if (serverSocket != null)
                {
                    System.out.println("Connection to client established");
                }
            }

            serverSocket.close();
        }
        catch (IOException e)
        {
            System.out.println("Error could not create socket connection to port");
        }
    }
}

public class ClientConnection extends Thread
{
    private Socket socket = null;

    public ClientConnection(Socket socket) 
    {
        super("ClientConnection");
        this.socket = socket; 
    }

    // the thread that runs after a connection to the server has been accepted
    @Override
    public void run()
    {
        try
        {
            PrintWriter out = new PrintWriter(socket.getOutputStream());
            BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));

            sendMessagetoClient(out, "CONNECTION SUCCESS");

            // check login credentials sent from client to the server

            // if valid send back their encrypted password, otherwise output a login error message

            // wait for user input and then do various processes based on their requests

            in.close();
            out.close();
            socket.close();
        }
        catch (IOException e)
        {
            System.out.println("Client socket connection error");
        }
    }

    // sends a message to the client
    void sendMessagetoClient(PrintWriter out, String message)
    {
        out.write(message);
    } 

    // listens for a message from the client
    String getMessageFromClient(BufferedReader in)
    {      
        try
        {
            String input = in.readLine();
            return input;
        }
        catch (IOException e)
        {
            System.out.println("Could not read message from client");
        }

        return "No Data";
    }

And here is the line of code im using to see if the data is being sent.

System.out.println(clientSocket.getMessageFromServer());

解决方案

In your sendMessageToClient() method, you need to flush:

void sendMessagetoClient(PrintWriter out, String message)
{
    out.write(message);
    out.flush();
} 

---

Or, when you create the PrintWriter, use the constructor with autoflush:

PrintWriter out = new PrintWriter(socket.getOutputStream(),true);

And when you write, instead of out.write(message) use printf() or println().

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