使用java中的套接字创建简单的客户端/代理/服务器程序

Question

I'm working on an assignment where I have to make 3 programs, a client, a proxy, and a server. The client is supposed to send messages to the server by sending them to the proxy, which sends them to the server, and vice versa. The server waits for the proxy to connect to it, and the proxy waits for the client to connect to it, then the client sends a message to the proxy which gets sent to the server. When I compile them though, while the proxy gets the message fine, the serve gets an error.

Exception in thread "main" java.net.SocketException: Connection reset
at java.net.SocketInputStream.read(SocketInputStream.java:196)
at java.net.SocketInputStream.read(SocketInputStream.java:122)
at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:283)
at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:325)
at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:177)
at java.io.InputStreamReader.read(InputStreamReader.java:184)
at java.io.BufferedReader.fill(BufferedReader.java:154)
at java.io.BufferedReader.readLine(BufferedReader.java:317)
at java.io.BufferedReader.readLine(BufferedReader.java:382)
at Server.run(Server.java:18)
at Server.main(Server.java:8)

Here is the client code

import java.net.*;
import java.io.*;


public class Client {

public static void main(String[] args) throws Exception{
    Client serv = new Client();
    serv.run();
    }

    public void run() throws Exception{
    Socket sock = new Socket("localhost", 7777);
    PrintStream ps = new PrintStream(sock.getOutputStream());
    ps.println("Hello Server");
    sock.close();
    }

}

Here is the proxy,

import java.net.*;
import java.io.*;

public class Proxy {

public static void main (String[] args) throws Exception{
    Proxy serv = new Proxy();
    serv.run();
}

    public void run() throws Exception{
    ServerSocket ss = new ServerSocket(7777);
    Socket sock = ss.accept();

    InputStreamReader ir = new InputStreamReader(sock.getInputStream());
    BufferedReader br = new BufferedReader(ir);

    String message = "";
    message = br.readLine();
    System.out.println(message);

    Socket sck = new Socket("localhost", 8888);
    PrintStream ps = new PrintStream(sock.getOutputStream());

    boolean waiting = true;
    while(waiting){
        if(message != null){
        ps.println("Hey Server, Client says: " + message);
        waiting = false;
        }
    }
    sock.close();
    }

}

and here is the server

import java.net.*;
import java.io.*;

public class Server {

    public static void main (String[] args) throws Exception{
    Server serv = new Server();
    serv.run();
    }

    public void run() throws Exception{
    ServerSocket ss = new ServerSocket(8888);
    Socket sock = ss.accept();

    InputStreamReader ir = new InputStreamReader(sock.getInputStream());
    BufferedReader br = new BufferedReader(ir);

    System.out.println(br.readLine());
    sock.close();
    ss.close();
    }

}

Does anybody know what I'm doing wrong?

Answer 1

Look at your proxy. First the sock Socket represents the connection with the client, then the sck Socket represents the connection with the server. However, when you create your PrintStream ps to connect with the server, you are using sock.getOutputStream as opposed to sck.getOutputStream . You are telling it to send information back to the client, which does not have an open BufferedReader to interpret, nor an open Socket at that point. Attempting to write to a closed connection results in a connection reset exception.

Secondly (though this is not causing the error), your proxy while loop is all jacked up. If you examine it, it will obviously cycle through at least once because waiting is defaulted to true. Then, you are checking to see if message != null . In this case, message will never be null because the client is set to always send the same message, and only one message. Most importantly, however, if for whatever reason message was null, there is nothing in the while loop to change that -- it does not attempt to read the next value from br or anything, so you will simply be stuck in an infinite while loop.

I believe what you were looking for was this:

String message;
while((message = br.readLine()) != null){
    ps.println("Hey server, Client says: " + message);
}

But as I said, a while loop is not necessary in this case because the client is set to send only one message anyways, so you can simply do this:

ps.println("Hey server, Client says: " + message);

... without any sort of looping.