Java TCP Socket While循环和计数器接收数据X次

Question

I'm a amateur programmer and writing Java TCP socket client program that receives Integer and String type messages from the server(plc). Currently it works fine and returns correct values if using basic try catch without any loops to keep the program running.

But... I've been trying to add while loop with counter of 5 receive times that ends the program after that but it returns on ONE read:

Int: 705 //correct value that it should receive every time
String: testi3 //correct value that it should receive every time



Int: 0 // prints also this duplicate value
String:  // prints also this duplicate value

How I should use the while loop in this case so it returns the correct values 5 times and then ends the program correctly?

My program looks like this:

import java.io.*;
import java.net.Socket;
import java.util.Arrays;

public class GetData2 {

    private static int counter =0;
    private Socket clientSocket;
    //private PrintWriter out;
    private String host = "192.168.0.1";
    private int port2 = 2000;
    private DataInputStream inFromServer;




    public void start() throws IOException{
        System.out.println("Client started");
        clientSocket = new Socket(host, port2);
    }




    public void run() throws IOException {

        while (counter <= 5) {

            inFromServer = new DataInputStream(clientSocket.getInputStream());
            //BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));

            int length = 100; // read length of incoming message
            byte[] messageFromServer = new byte[length];


                for (int i = 0; i < messageFromServer.length; i++) {
                    messageFromServer[i] = (byte) inFromServer.read();             //read message from the server
                }
                System.out.println("\n");

                //System.out.println(Arrays.toString(messageFromServer));
                byte[] copyOfData = Arrays.copyOf(messageFromServer, messageFromServer.length); //copies byte array from messageFromServer[] to copyOfData[]



                System.out.println("\n");
                //Integer
                short value = 0;
                                                                                            // reads bytes 0-1
                value = (short) (copyOfData[0] * 256 + (short) copyOfData[1]);              // moves received bytes
                                                                                          // shows the order of received byes

                System.out.println("Int: " + value);                                     // return bytes as an Integer

                //String
                System.out.print("String: ");
                for (int i = 4; i < copyOfData.length && i < 10; i++) {            // reads bytes 4-10 from array

                    System.out.printf("%c", copyOfData[i]);                   // returns String testi2 from pythondemo 2 plc application
                }
                counter++;
            System.out.println(counter);



        }


    }
    public void stop() throws IOException{

        //out.close();

        System.out.println("Application stopped");
        clientSocket.close();
        inFromServer.close();

    }




    public static void main(String[] args) throws IOException {
        GetData2 getData2 =new GetData2();
        getData2.start();
        getData2.run();


        if(counter == 5){
            getData2.stop();
        }


    }

}

EDIT:

Seems like if i change the received byte array length from [100] to [10], the program executes and prints this;

Client started




Int: 705
String: testi3
counter: 1




Int: 0
String:       
counter: 2




Int: 0
String:       
counter: 3




Int: 0
String:       
counter: 4




Int: 0
String:       
counter: 5
Application stopped

EDIT #2

So, I made server program that runs in localhost and sends byte[100] and it works correctly. Seems like problem is most likely in my PLC program, not in Java.

Server Class:

import java.io.*;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.Scanner;

public class GetData2_Server {
    public static void main(String[] args) throws IOException {
        System.out.println("server started");
        Scanner scan = new Scanner(System.in);
        ServerSocket server = new ServerSocket(8080);
        Socket socket = server.accept();
        System.out.println("server accepted");
        System.out.println("1 == out n flush, 0 == close program");
        int value = scan.nextInt();
        byte[] bytesOut = new byte[100];
        bytesOut[0]=0;
        bytesOut[1]=3;

        DataOutputStream out = new DataOutputStream(socket.getOutputStream());

        while (value == 1)
            {
                out.write(bytesOut);
                out.flush();
                value = scan.nextInt();
                if(value == 0){
                    out.close();
                    socket.close();
                }
            }
    }
}

Answer 1

in your while loop, you loop from 0 to 5, which means 6 times. you may want to change this:

while (counter <= 5)

to this, so that it loops only 5 times:

while (counter < 5)

Answer 2

You are not taking into account that the device sending the data (I'll refer to it as "server" for short) may close the stream on its end. When this happens, inFromServer.read() will return -1. You're not checking for this value, so you will end up filling the messageFromServer array with -1 values.

If you know that the messages are supposed to be 100 bytes, you can read a message with a single DataInputStream method call, without a for loop. Replace your for (int i = 0; i < messageFromServer.length; i++) with:

byte[] messageFromServer = new byte[length];
inFromServer.readFully(messageFromServer);

If the server stream is closed before the full message is read, this method will throw an exception (EOFException).