Java（网络）：在客户端和服务器之间传递增量值

Question

I am supposed to have multiple clients connected to localhost and after the client is being connected, he can press a add button to increment a value by 1. Let take for example there are 2 clients (A and B). A clicks on Add button, The value(ctr) is incremented by 1(ctr=1) and if B clicks on Add Button, the value(ctr) is incremented by 1(ctr) again(ctr =2)

But I am struggling to get the correnct results for just one client so I will just stick to solving getting the correct results for one client first before moving on to getting correct results from multiple clients. But I do wish that anyone could provide me help on the multiple clients part as well.

I am not very good in networking and tried to implement this and here my logic for just ONE client

-In my button actionListener, Each time the Add button is pressed, I will send the message "add" from the client to the server.

-The server receives the message "add" and will increment the "ctr" variable by 1;

-The server will pass the incremented ctr to the client.

But the output I got from my codes is rather wierd and unstable after I clicked 3 times on the Add Button.

My Output

Starting SomeProcess  
1500476704 <--Incremented value passed back to client from the server, it should return 1
SomeProcess took 4 ms
Starting SomeProcess
1751217765 <--Incremented value passed back to client from the server, it should return 2
SomeProcess took 0 ms
Starting SomeProcess
543387502 <--Incremented value passed back to client from the server, it should return 3
SomeProcess took 0 ms

My codes

Client.java

import java.awt.EventQueue;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.BufferedReader;
import java.io.DataInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

@SuppressWarnings("serial")
public class Client extends JPanel {
    private JButton addBtn = new JButton("Add");
    private long start = 0;
    private PrintWriter output = null;
    private BufferedReader input = null;
    private Socket socket;

    public Client() {
        setLayout(new FlowLayout());
        add(addBtn);

        try {
            socket = new Socket("localhost",4444);
            output = new PrintWriter(socket.getOutputStream(), true);
            input = new BufferedReader(new InputStreamReader(socket.getInputStream()));

        }catch (IOException exception) {
            System.out.println("Error: " + exception);
        }

        addBtn.addActionListener(new AddBtnListener());
    }

    public class AddBtnListener implements ActionListener {

        @Override
        public void actionPerformed(ActionEvent e) {
            Thread t = new Thread(new Runnable() {
                public void run() {

                    System.out.println("Starting SomeProcess");
                    start = System.currentTimeMillis();
                    output.println("add");

                    try {
                        DataInputStream inputInt = new DataInputStream((socket.getInputStream()));    
                        System.out.println(inputInt.readInt());
                    } catch (IOException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                    long duration = System.currentTimeMillis() - start;
                    System.out.printf("SomeProcess took %,d ms%n", duration );
                }
            });
            t.start();
        }
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                JFrame frame = new JFrame();
                frame.add(new Client());
                frame.pack();
                frame.getPreferredSize();
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setVisible(true);
            }
        });
    }
}

Server.java

import java.awt.EventQueue;
import java.io.BufferedReader;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.Date;
import javax.swing.JFrame;
import javax.swing.JPanel; 
import javax.swing.JTextArea;

@SuppressWarnings("serial")
public class Server extends JPanel {

    private Socket threadSocket;
    private JTextArea textArea = new JTextArea(30,30);

    public Server() {
        add(textArea);
        textArea.setEditable(false);
        Thread t = new Thread(new acceptClient());
        t.start();
    }

    class acceptClient implements Runnable {

        @Override   
        public void run() {
            try {
                ServerSocket sSocket = new ServerSocket(4444);
                textArea.append("Server started at: " + new Date());
                while(true) {
                    Socket socket = sSocket.accept();
                    ClientThread cT = new ClientThread(socket);
                    new Thread(cT).start();

                } 
            } catch(IOException exception) {
                System.out.println("Error: " + exception);
            }
        }
    }

    class ClientThread extends Thread {
        String temp = " ";
        DataOutputStream outputInt = null;
        int ctr = 0;
        PrintWriter output;

        public ClientThread(Socket socket) {
            threadSocket = socket;
        }

        @Override  
        public void run() { 
            while(true) {
                try {
                    output = new PrintWriter(threadSocket.getOutputStream(), true);
                    BufferedReader input = new BufferedReader(new InputStreamReader(threadSocket.getInputStream()));
                    output.println("You have connected at: " + new Date());
                    textArea.append("\nClient connected\n");
                    temp = input.readLine();

                    if(temp.equals("add")) {
                        synchronized(this) {
                            ctr++;
                            textArea.append(Integer.toString(ctr));
                            outputInt.write(ctr);
                        }
                    }

                } catch(IOException exception) {
                    System.out.println("Error: " + exception);
                }
            }
        }
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                JFrame frame = new JFrame();
                frame.add(new Server());
                frame.setLocationRelativeTo(null);
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.pack();
                frame.setVisible(true);
                frame.setResizable(false);
            }
        });
    }
}

Answer 1

Take a look at you write/read process...

First, you server does...

output = new PrintWriter(threadSocket.getOutputStream(), true);
output.println("You have connected at: " + new Date());
//...
ctr++;
outputInt.write(ctr);

But your client only does...

DataInputStream inputInt = new DataInputStream((socket.getInputStream()));
System.out.println(inputInt.readInt());

...This means what you have actually read is the "You have connected at: " + new Date() response from the server.

You need to make sure you are reading the content in the same order your are writing it...

What I might consider doing is simplifying the process...Instead of introducing the DataOutputStream , I would simply continue to use the PrintWriter you've already set up and simple do something like...

ctr++;
textArea.append(Integer.toString(ctr));
output.println(ctr);

within the server instead...

On the client, you could then do

System.out.println(input.readLine());
System.out.println(input.readLine());

Which will read the "connect" message and the counter...

Side notes

There are a few things about your server ClientThread which concern me...

1. There is no need to re-create the PrintWriter and BufferedReader within the while-loop , in fact, output doesn't really need to be class instance variable...
2. If, for some reason, an Exception is raised, the run method will never exit, this means you could be caught in a never ending loop
3. I', not sure you really need the synchronized block as ctr is an instance field within the context of this thread, so unless something is going to try and alter it externally, this is just additional overhead...but I'll leave it alone for the moment...

Instead, I might be tempted to do something like...

try (PrintWriter output = new PrintWriter(threadSocket.getOutputStream(), true)) {
    try (BufferedReader input = new BufferedReader(new InputStreamReader(threadSocket.getInputStream()))) {

        while (true) {
            output.println("You have connected at: " + new Date());
            textArea.append("\nClient connected\n");
            temp = input.readLine();

            if (temp != null) {

                if (temp.equals("add")) {
                    synchronized (this) {
                        ctr++;
                        textArea.append(Integer.toString(ctr));
                        output.println(ctr);
                    }
                }

            }

        }

    }
} catch (IOException exp) {
    exp.printStackTrace();
}

instead.

While I know you can use a single try-with-resource statement, I wanted to separate the two stream creations for clarity...