Java TCP-Server socket connection keepalive

Question

I want to communicate as a TCP Server on Port 2000 and 2001 with my TCP Client (Machine which sends Bytestreams). Therefore I programmed a Spring Boot Application in Java.

This Question is only for Port 2001: I use Camunda as BPMN-Engine for executing and orchestrating. I start Threads like this:

package com.example.workflow;

import org.camunda.bpm.engine.delegate.DelegateExecution;
import org.camunda.bpm.engine.delegate.JavaDelegate;

public class StartTCPServersDelegate implements JavaDelegate {
    @Override
    public void execute(DelegateExecution delegateExecution) throws Exception {
        Runnable serverZyklisch = new ServerZyklisch();
        Runnable serverAzyklisch = new ServerAzyklisch((String) delegateExecution.getVariable("param"));
        Thread t1 = new Thread(serverZyklisch);
        t1.start();
        System.out.println("Thread Zyklisch gestartet");
        Thread t2 = new Thread(serverAzyklisch);
        t2.start();
        System.out.println("Thread Azyk. gestartet");
        String val1 = (String) delegateExecution.getVariable("param");
        int valueParam = Integer.parseInt(val1);
        System.out.println("Param ist: "+valueParam);

    }
}

This is my ServerAzyklisch Class:

public class ServerAzyklisch implements Runnable, JavaDelegate {
    private ServerSocket ssocket;
    String param;
    HexToByteConverter hexToByteConverter = new HexToByteConverter();
    public ServerAzyklisch(String Pparam) throws IOException {
        ssocket = new ServerSocket(2000);
        param = Pparam;
    }

    public void run() {
        System.out.println(param+"Paraaam");
        InputStream in;
        OutputStream out = null;
        Socket socket;
        while(true){
            try {
                socket = ssocket.accept();
                in = socket.getInputStream();
                out = socket.getOutputStream();
                byte []data = new byte[132];
                int numBytes = 0;
                byte[]durch = hexToByteConverter.hexStringToByteArray("333333330041006400040000000400000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000");
                byte[]durchlauf = hexToByteConverter.hexStringToByteArray("333333330041006400040000000400000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000");
                byte[]Pressen1hexdump111 = hexToByteConverter.hexStringToByteArray("33333333003d0064000600000004004001c9c78900010000006f00000000000000000000000000010000000000140000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000005");
                byte[]Pressen1hexdump110 = hexToByteConverter.hexStringToByteArray("33333333003d0064000600000004004001c9c78900010000006e0000000000000000000000000001000000000014000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"+param);
                byte[]Pressen2hexdump = hexToByteConverter.hexStringToByteArray("3333333300400065000a00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000");
                byte[]Pressen3hexdump = hexToByteConverter.hexStringToByteArray("3333333300400065001400000000003d01c9c7890001000000c9000000000000000000000000000100000000001e00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000");
                byte[]Pressen3hexdumpNextBohrer = hexToByteConverter.hexStringToByteArray("3333333300400065001400000000003f01c9c789000100000078000000000000000000000000000100000000001e00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000002");
                byte[]Pressen4hexdumpNextRSCIDBohrer = hexToByteConverter.hexStringToByteArray("33333333003f0065001400000000003d01c9c78900010000007a000000000000000000000000000100000000001e00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000");
                //gleichen Stream zurückschicken, der angekommen ist, für Durchlauf
                while((numBytes = in.read(data)) != -1){
                    System.out.println(Arrays.toString(data));
                    out.write(Pressen1hexdump110);
                    out.write(Pressen2hexdump);
                    out.write(Pressen3hexdumpNextBohrer);
                    //out.write(durchlauf);
                }
            } catch (SocketException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    @Override
    public void execute(DelegateExecution delegateExecution) throws IOException {
          }
}

I get everytime a different Result to my Client, so the behaviour is always another. But I want to send once all three bytearrays to my Client. I think something is wrong with my while loop. Do you have any idea ?

Why did you put "connection keepalive" in the title, there's no mention of it in the question? "I get everytime a different Result to my Client" - You're always sending the same thing. If the client prints out different things, that's on the client side. Can you show the code for the client? — Joni
– Joni, Commented Aug 10, 2020 at 14:07
the client is a machine, which sends a bytestream. I dont have code for that. I want to read it the Clients Message once and then send the client back three different messages (bytestreams) but I think the while loop is wrong or something, so the Client won't get my three messages one after the other ? Maybe its because I only have while loop while the client is sending != -1, isnt there something like while(connection is keepalive), Then -> Send the 3 messages -> then End — aw123
– aw123, Commented Aug 10, 2020 at 14:11
If the problem that the "messages" you send from the server get mixed together, or that you send more or fewer of them than the client expects? — Joni
– Joni, Commented Aug 10, 2020 at 14:21
I think that the client expects always one Message. Normally the communication is like a three way communication. It starts: Client sends message to Server -> Server answers to client with one message -> Client asks server -> Server answers -> Client send message -> Server answers -> "End of communication" — aw123
– aw123, Commented Aug 10, 2020 at 14:24

Joni · Accepted Answer · 2020-08-10 15:08:29Z

1

By the comments, the communication is based on request-response pairs. You need to read 3 messages from the client, and return a response for each message. To do this, replace the while loop with:

            readMessage(in, data);
            out.write(Pressen1hexdump110);

            readMessage(in, data);
            out.write(Pressen2hexdump);

            readMessage(in, data);
            out.write(Pressen3hexdumpNextBohrer);

where the readMessage method is a new method you must add, that reads a complete request from the client.

If the client requests are always 128 bytes, there is a convenient method in DataInputStream that you can use:

void readMessage(InputStream in, byte[] buffer) throws IOException {
    new DataInputStream(in).readFully(buffer, 0, 128);
}

In the general case the readMessage method would have to look something like this in pseudo-code:

void readMessage(InputStream in, byte[] buffer) {
    // Read a message
    while message is not complete:
        read from "in" into "buffer"
        if "in" was closed: throw an exception because the connection was closed mid-request
        else: incorporate newly read data from "buffer" in message
    done
}

edited Aug 10, 2020 at 15:08

answered Aug 10, 2020 at 14:36

Joni

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5 Comments

aw123 Over a year ago

The Message is a bytestream/bytearray which looks like: byte []bytesPressen = {51, 51, 51, 2, 0, 64, 0, 100, 0, 6, 0, 0, 0, 0, 0, 0, 0, -68, 97, 78, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}; its bytearray 128 length. can you show example java code, when message is completed if 128bytesend

aw123 Over a year ago

Now I get following Error: java.io.EOFException at java.base/java.io.DataInputStream.readFully(DataInputStream.java:202) at com.example.workflow.ServerAzyklisch.readMessage(ServerAzyklisch.java:90) at com.example.workflow.ServerAzyklisch.run(ServerAzyklisch.java:71) at java.base/java.lang.Thread.run(Thread.java:834) Do you have any idea maybe ? Im also not 100% sure if the clients sends always 128 Bytes.

Joni Over a year ago

That exception tells you that the client closed its end of the connection without sending 128 bytes.

aw123 Over a year ago

Do you have any idea ? Is there a way to get the length of the bytes, the client have sent ? Or how would be a way to solve this issue ?

Joni Over a year ago

TCP is a "stream" protocol - there is no built-in way to tell where one message ends and the next one begins. The messages don't have a built-in "length" field. This is a very common problem and also the reason why I asked if UDP or another protocol was a possibility. The documentation for the communication protocol your device is using should tell you how to read messages

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