Error while reading data through socket communication

Question

Following scenario that explains my problem.
I've a PLC that acts as a server socket program. I've written a Client Java program to communicate through socket communication with the PLC.

Steps that take place in this process are:
1) For each second my Client program happen to communicate with the PLC, read the data in stream, store the data temporarily in a ByteArrayOutputStream and closing both input stream and socket. Following snippet gives the idea

    try {
        socket = new Socket(host, port);
        is = socket.getInputStream();
        outputBuffer = new ByteArrayOutputStream();

        byte[] buffer = new byte[1024];
        int read;
        if((read = is.read(buffer)) != -1) {
            outputBuffer.write(buffer, 0, read);
        }
    } catch (UnknownHostException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        try {
            System.out.println("Before closing the socket");
            try {
                is.close();
                socket.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
            System.out.println("After closing the socket");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

2) Processing stored data according to my requirement is what I'm trying to do. So for every 1 second, client program connects to Server, read the data(if data is present), store the data, close socket and process it. And it has to happen for a very long run, probably till the Server program is on. And that may happen till for every few weeks.

3) Problem what I'm facing is, I'm able to run the above show for 1-2 hours, but from then, Client Program unable to fetch the data from the Server Program(PLC in this case), though both are connected through socket. I.e 128 bytes of data present, but Client program isn't able to read that data. And this started happening after program run successfully for almost 2hours

4) Please find the brief code which may help for you to look into.

public class LoggingApplication {
    public static void main(String[] args) throws NumberFormatException {
        if (args.length > 0 && args.length == 2) {
            String ipAddress = mappingService.getIpAddress();
            int portNo = (int) mappingService.getPortNo();
            ScheduledExecutorService execService = Executors.newScheduledThreadPool(1);
            execService.schedule(new MyTask(execService, ipAddress, portNo, mappingService), 1000, TimeUnit.MILLISECONDS);
        } else {
            throw new IllegalArgumentException("Please pass IPAddress and port no as arguments");
        }
    }
}

Runnable Code:

public class MyTask implements Runnable {

    public ScheduledExecutorService execService;
    private String ipAddress;
    private int portNo;
    private ConfigurationMappingService mappingService;
    private MySocketSocketUtil mySocketSocketUtil;

    public MyTask(ScheduledExecutorService execService, String ipAddress, int portNo, ConfigurationMappingService mappingService) {
        this.execService = execService;
        this.ipAddress = ipAddress;
        this.portNo = portNo;
        this.mappingService = mappingService;
    }

    public void run() {
        MySocketSocketUtil mySocketSocketUtil = new MySocketSocketUtil(ipAddress, portNo);
        execService.schedule(new MyTask(execService, ipAddress, portNo, mappingService), 1000, TimeUnit.MILLISECONDS);

        mySocketSocketUtil.getData();  //It's able to fetch the data for almost 2 hours but from then, it's just getting empty data and it's keep on giving empty data from then. and so on.


        /*
         *
         *Some code
         */
    }
}

Here's where, I'm having the problem
mySocketSocketUtil.getData(); is able to fetch the data for almost 2 hours but from then, it's just getting empty data and it's keep on giving empty data from then. and so on. It's a big question I know, And I want to understand what might have gone wrong.

Edit: I'm ignoring the condition to check end of the stream and closing a socket based on it is because, I knew I'm going to read first 1024 bytes of data only always. And So, I'm closing the socket in finally block

Answer 1

I think there are couple of points you need to fix before getting to the solution to your problem. Please try to follow the following suggestions first:

1. As @EJP said this code block is not needed.

   if(socket != null && socket.isConnected()) {
   

2. also you are using a byte array of length 1024 and not using while or for loop to read the data stream. Are you expecting only a block of data which will never exceed 1024 bytes?

       byte[] buffer = new byte[1024];
       int read;
       if((read = is.read(buffer)) != -1) {
   

3. This is also not needed as it is unreachable.

   } else {
       System.err.println("Socket couldn't be connected");
   }
   

4. Can you explain the data stream behavior you are expecting?

5. Last but not the least is.read(buffer) is a blocking call so if there is no data to read yet, it will hold the thread execution at that point.

Please try to answer the questions I have asked.

@KishoreKumarKorada from your description in the comment section, it seems like you are monitoring the data change on server side. Socket stream works in a read-once fashion. So,

First thing is, you need to request from server every time and the server needs to RESEND the data on every request.

Second, the way you presented is more like you are operating on byte level, which is not very good way to do that unless you have any legitimate reason to do so. The good way is to wrap the data in JSON or XML format and send it over the stream. But to reduce bandwidth consumption, you may need to operate on byte stream sometimes. You need to decide on that.

Third, for monitoring the data change, the better way is to use some timestamp to compare when the data has changed on the server side and what is the timestamp stored on the client side, if they match, data has not changed. Otherwise fetch the data from the server side and update the client side.

Fourth, when there is data available that you are not able to read, can you debug the ins.read(...) statement to see if its getting executed and the execution goes inside the if block or if statement is evaluated to false? if true then examine the read value and let me know what you have found?

Thanks.

Answer 2

socket = new Socket(host, port);
if(socket != null && socket.isConnected())

It is impossible for socket to be null or socket.isConnected() to be false at this point. Don't write pointless code.

        if((read = is.read(buffer)) != -1) {
            outputBuffer.write(buffer, 0, read);
        };

Here you are ignoring a possible end of stream. If read() returns -1 you must close the socket. It will never not return -1 again. This completely explains your 'empty data':

from then, it's just getting empty data and it's keep on giving empty data from then, and so on

And you should not create a new Socket unless you have received -1 or an exception on the previous socket.

    } else {
        System.err.println("Socket couldn't be connected");
    }

Unreachable: see above. Don't write pointless code.

Answer 3

You should never disconnect from the established connection. Connect once in the LoggingApplication. Once the socket is connected keep it open. Reuse the socket on the next read.