Thread: simple soclet communication between 2 PCs

Hi

I have the following code which runs successfully as a stand alone example

when I try to incorporate it into my java program I get an error message

Error message (on Server): Cannot make a static reference to the non-static field server

Please advise what I need to change in the code to get it to work ?

Bob M
New Zealand

SocketServer console:
Waiting for the client request
Message Received: GBPAUDtest-data
Waiting for the client request
 
SocketClient console:
Sending request to Socket Server
Message: GBPAUDtest-data
 
SocketServer:
 
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.ServerSocket;
import java.net.Socket;
 
/**
 * This class implements java Socket server
 *
 */
public class SocketServer {
    // static ServerSocket variable
    private static ServerSocket server;
    // socket server port on which it will listen
    private static int port = 9876;
 
    public static void main(String args[]) throws IOException,
            ClassNotFoundException {
        // create the socket server object
        server = new ServerSocket(port);
        // keep listens indefinitely until receives 'exit' call or program
        // terminates
        while (true) {
            System.out.println("Waiting for the client request");
            // creating socket and waiting for client connection
            Socket socket = server.accept();
            // read from socket to ObjectInputStream object
            ObjectInputStream ois = new ObjectInputStream(
                    socket.getInputStream());
            // convert ObjectInputStream object to String
            String message = (String) ois.readObject();
            System.out.println("Message Received: " + message);
            // create ObjectOutputStream object
            ObjectOutputStream oos = new ObjectOutputStream(
                    socket.getOutputStream());
            // write object to Socket
            oos.writeObject(message);
            // close resources
            ois.close();
            oos.close();
            socket.close();
            // terminate the server if client sends exit request
            if (message.equalsIgnoreCase("exit"))
                break;
        }
        System.out.println("Shutting down Socket server!!");
        // close the ServerSocket object
        server.close();
    }
} // end of SocketServer
 
SocketClient
 
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.InetAddress;
import java.net.Socket;
import java.net.UnknownHostException;
 
/**
 * This class implements java socket client
 *
 */
public class SocketClient {
    public static void main(String[] args) throws UnknownHostException, IOException,
            ClassNotFoundException, InterruptedException {
 
        String Trading_Decision_2 = "test-data";
        // get Computer 1 (server) IP address
        InetAddress host = (InetAddress.getByName("127.0.0.2"));
        Socket socket = null;
        ObjectOutputStream oos = null;
        ObjectInputStream ois = null;
        // establish socket connection to server
        socket = new Socket(host.getHostName(), 9876);
        // write to socket using ObjectOutputStream
        oos = new ObjectOutputStream(socket.getOutputStream());
        // Socket
        System.out.println("Sending request to Socket Server");
        oos.writeObject("GBPAUD" + Trading_Decision_2);
        // read the server response message
        ois = new ObjectInputStream(socket.getInputStream());
        String message = (String) ois.readObject();
        System.out.println("Message: " + message);
        // close resources
        ois.close();
        oos.close();
        Thread.sleep(100);
    }
} // end of SocketClient

yes - compilation errors

3 problems (3 errors)
2019-06-06 15:26:00 ----------
2019-06-06 15:26:00 3. ERROR in C:\Users\...............java (at line 1683)
2019-06-06 15:26:00 public static void main(String args[]) throws IOException, ClassNotFoundException {
2019-06-06 15:26:00 ___________________^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2019-06-06 15:26:00 The method main cannot be declared static; static methods can only be declared in a static or top level type
2019-06-06 15:26:00 ----------
2019-06-06 15:26:00 2. ERROR in C:\Users\.............java (at line 1678)
2019-06-06 15:26:00 public class SocketServer () {
2019-06-06 15:26:00 _____________________________^
2019-06-06 15:26:00 Syntax error, insert "}" to complete Block
2019-06-06 15:26:00 ----------
2019-06-06 15:26:00 1. ERROR in C:\Users\.............java (at line 1678)
2019-06-06 15:26:00 public class SocketServer () {
2019-06-06 15:26:00 _______^^^^^
2019-06-06 15:26:00 Syntax error on token "class", @ expected
2019-06-06 15:25:59 Compiling xxxx.java

Line 1678 = public class SocketServer {
Line 1683 = public static void main(......etc.

Hi

have done that and now on compilation I get 2 problems on amended line

public static class SocketServer() {

Error 1] Syntax error on token "class", @ expected
Error 2] Syntax error, insert } to complete block

I don't understand the second error
when I highlight the { at the end of the offending line
I see the matching } on the second line after server.close(); where it should be (as per listing)

will do.....

Bob M

questions:-......................

Is that main in a nested class (you asked)
Does it appear to be in the code I posted ?

Make the class static (you advised)
I added the word 'static' - is that all I needed to do ?

Bob M

the stand-alone example works

it is when I try to incorporate it into my large java program that i have problems

Incorporate = add server code (cut and paste) as a subroutine in my large java program and call it when required - on Computer 1

Add client code (cut and paste) as a subroutine in my large java program and call it when required on Computer 2

at this stage I experience compilation errors as posted

Bob M

I am logged on to Dukascopy's Trading platform where one runs java programs

I have removed () and get a clean compile

Sorry for my bad.........

thank you very much

Bob M
New Zealand

can you show me an example of what you are meaning please ?

hi again

I am now going with a non-static solution as follows.........

import java.io.*;
import java.net.*;
 
public class SocketClient {
 
    private int port;
    private String host;
 
    public static void main (String[] args) throws Exception {
        String tradingDecision = args.length == 0
                                ? "test"
                                : args[0];
 
        SocketClient client = new SocketClient("localhost", 9876);
        client.sendMessage(tradingDecision);
    }
 
    public SocketClient (String host, int port) {
        this.host = host;
        this.port = port;
    }
 
    protected void sendMessage (String msg) throws Exception {
        // establish socket connection to server
        Socket socket = new Socket(host, 9876);
        // write to socket using ObjectOutputStream
        ObjectOutputStream oos = new ObjectOutputStream(socket.getOutputStream());
        // Socket
        System.out.println("Sending request to Socket Server");
        oos.writeObject(msg);
        // read the server response message
        ObjectInputStream ois = new ObjectInputStream(socket.getInputStream());
        String message = (String) ois.readObject();
        System.out.println("Message: " + message);
        // close resources
        ois.close();
        oos.close();
    }
}

How do I call this subroutine from my main program in a new thread ?

Bob M

Hi

I have the following code (in my main java programs - which activate every hour on the hour)

Computer 2: (Client)
Calling code:-

// send a message to Computer 1 with your trading decision
 
        try {
            SocketClient client = new SocketClient("localhost", 9876);
            client.sendMessage(Trading_Decision_2);
            fw.writetoFile(("we get to here - zz: "), File_Name);
            }
        catch(Exception e) {
            myConsole.getOut().println("SocketClient exception occurred: " + e);
            e.printStackTrace();
        }

Socket Client code:-

public class SocketClient {
 
        private int port;
        private String host;
 
        public SocketClient (String host, int port) {
            this.host = host;
            this.port = port;
        }
 
        protected void sendMessage (String msg) throws Exception {
            // establish socket connection to server
            Socket socket = new Socket(host, 9876);
            // write to socket using ObjectOutputStream
            ObjectOutputStream oos = new ObjectOutputStream(socket.getOutputStream());
            // Socket
            System.out.println("Sending request to Socket Server");
            oos.writeObject(msg);
            // read the server response message
            ObjectInputStream ois = new ObjectInputStream(socket.getInputStream());
            String message = (String) ois.readObject();
            System.out.println("Message: " + message);
            // close resources
            ois.close();
            oos.close();
        }
    } // end of SocketClient

Computer 1: (Server)
Calling code:-

// initialise and start our thread
        NewThread t = new NewThread();
        t.start();
        fw.writetoFile(("we get to here  - zz"), File_Name);
 
        // setup socket connection on new background thread (to receive trading decision from Computer 2)
        try {
           SocketServer server = new SocketServer(9876);
           server.start();
           fw.writetoFile(("we get to here  - zzz"), File_Name);
           }
        catch(Exception e) {
           myConsole.getOut().println("SocketServer exception occured: " + e);
           e.printStackTrace();
        }

Socket Server code:-

public class SocketServer {
        private int port;
 
        public SocketServer (int port) {
            this.port = port;
        }
 
        protected void start() throws Exception {
            // create the socket server object
            ServerSocket server = new ServerSocket(port);
            // keep listens indefinitely until receives 'exit' call or program terminates
            while (true) {
                System.out.println("Waiting for the client request");
                // creating socket and waiting for client connection
                Socket socket = server.accept();
                // read from socket to ObjectInputStream object
                ObjectInputStream ois = new ObjectInputStream(socket.getInputStream());
                // convert ObjectInputStream object to String
                String message = (String) ois.readObject();
                System.out.println("Message Received: " + message);
                // create ObjectOutputStream object
                ObjectOutputStream oos = new ObjectOutputStream(socket.getOutputStream());
                // write object to Socket
                oos.writeObject(message);
                // close resources
                ois.close();
                oos.close();
                socket.close();
                // terminate the server if client sends exit request
                if (message.equalsIgnoreCase("exit"))
                    break;
            }
            System.out.println("Shutting down Socket server!!");
            // close the ServerSocket object
            server.close();
        }
    } // end of SocketServer

I am tangled up a bit here

Client error: (hourly)
SocketClient exception occurred: java.net.ConnectException: Connection refused: connect

Server error: (hourly)
SocketServer exception occured:java.net.BindException:Address already in use"JVM_Bind

This is my first attempt at explicitly using 'new' background thread !

Help

Bob M