Java静态变量值无法传输

Question

I have a homework, in which we should implement a server/client classes, so that we send 1400Bytes packets to the server and we get them back as reply. We must mess the throughput in kbit/s and it must be printed when there a timeout by the server! but the problem is that I'm calculating the throughput in the Client-Class, and trying to print it in the 'catch block' of the timeout-exception in the Server-Class, but this value is always transmitted/printed as 0.0, which is not the case when I print it in the Client-Class! I tried with simple static variable, with static get-method, but it doesnt work! Can anyone help me please? I must give it back today at 00:00! It Would be great! Thanks!

package blatt6;

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

public class UDPClient
{
static double startTime;
static double endTime;
static double dauer; 
static double paketGroesseKBit = (1400*8) / 1024;
private static double durchsatz;


public static void main(String args[])
{
    DatagramSocket sock = null;
    int port = 7777;
    String s = "";

    for (int i=0; i<1400; i++) {
        s = s + 'b';
    }

    //BufferedReader cin = new BufferedReader(new InputStreamReader(System.in));

    try
    {
        sock = new DatagramSocket();
        InetAddress host = InetAddress.getByName("localhost");

        int j=0;
        while(j<4)
        {
            //take input and send the packet
            echo("Enter message to send : ");
            //s = (String)cin.readLine();

            byte[] b = s.getBytes();    
            DatagramPacket  dp = new DatagramPacket(b , b.length , host , port);    
            byte[] buffer = new byte[1400];
            DatagramPacket reply = new DatagramPacket(buffer, buffer.length);

            sock.send(dp);
            startTime = System.nanoTime();      
            //buffer to receive incoming data
            sock.receive(reply);
            endTime = System.nanoTime();
            dauer = endTime - startTime;
            durchsatz = paketGroesseKBit / ((dauer/2) * Math.pow(10, -9));
            //System.out.println(dauer);

            j++;        
//              byte[] data = reply.getData();
//              s = new String(data, 0, reply.getLength());
//              
//              //echo the details of incoming data - client ip : client port - client message
//              echo(reply.getAddress().getHostAddress() + " : " + reply.getPort() + " - " + s);
        }
    }

    catch(IOException e)
    {
        System.err.println("IOException " + e);
    }
}

//simple function to echo data to terminal
public static void echo(String msg)
{
    System.out.println(msg);
}

public static double getDurchsatz() {
    return durchsatz;
}
}

Here Server:

package blatt6;

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

public class UDPServer
{

static double durchy = UDPClient.getDurchsatz();

public static void main(String args[])
{
    DatagramSocket sock = null;
    int timeout = 5000;

    try
    {
        //1. creating a server socket, parameter is local port number
        sock = new DatagramSocket(7777);
        sock.setSoTimeout(timeout);
        //buffer to receive incoming data
        byte[] buffer = new byte[1400];

        DatagramPacket incoming = new DatagramPacket(buffer, buffer.length);

        //2. Wait for an incoming data
        echo("Server socket created. Waiting for incoming data...");

        //communication loop
        while(true)
        {
            sock.receive(incoming);
            sock.send(incoming);                

//                byte[] data = incoming.getData();
//                String s = new String(data, 0, incoming.getLength());
//
//                //echo the details of incoming data - client ip : client port - client message
//                echo(incoming.getAddress().getHostAddress() + " : " + incoming.getPort() + " - " + s);       
//                s = "OK : " + s;
//
//                DatagramPacket dp = new DatagramPacket(s.getBytes() , s.getBytes().length , incoming.getAddress() , incoming.getPort());
//                sock.send(dp);
        }
    }

    catch(IOException e)
    {
        System.out.println(durchy);
        System.err.println("IOException " + e);
    }
}

//simple function to echo data to terminal
public static void echo(String msg)
{
    System.out.println(msg);
}
}

Answer 1

It doesn't matter both server and client are in the same project, since you are running two instances (using eclipse's play button twice) they not share anything.

They could share attributes if server and client where different threads of the same program, but it doesn't make sense for you homework (or any application I can think of).

As stated in my previous comment you should calculate throughput both in client and server.

You could make a third class, a throughput calculator , that keeps the common logic. Then both client and server can reuse the code. Keep in mind that at execution time they will be executing different copies of the same code, so they will not share values.

Answer 2

Well, one obvious error that I see is that you are using a static initializer to call UDPClient.getDurchsatz() from the server class. This works, because the two classes are in the same package, but it also does not work because that call is made before any non-static-initializer code in the server class is executed. Hence, you're reading the default value, which for a double is 0 . If you add a static initializer to the client class to set the value to something non-zero, you should get that value instead.

Before something like this has any hope of working, you'll need to remove the static initialization from the field and instead ask the client class for the value when you need it.

However, even without having looked at your code in detail, I think you'll also need some architectural changes. The normal way to calculate throughput is to time a (set of) send or receive call(s) where it is/they are made, and then calculate the throughput based on the amount of data transferred and elapsed time. That can be done either on the client or server side, and you don't need a separate communications channel between the two to do that.