connecting python socket and java socket

Question

I have been trying to send a simple string between a Java client socket and a Python server socket. This is the code for the server socket:

HOST=''
PORT=12000
s=socket.socket(socket.AF_INET,socket.SOCK_STREAM)
s.setsockopt(socket.SOL_SOCKET,socket.SO_REUSEADRR,1)
s.bind((HOST,PORT))
s.listen(5)
device=variador()
while True:

  conn,addr=s.accept()
  if data=="turn_on":
     respuesta=device.send_order(variador.start_order)
     conn.send(respuesta+'\n')
     conn.close()

and the client code is:

   try {

            Socket socket = new Socket("192.168.10.171", 12000);

            PrintWriter out = new PrintWriter(socket.getOutputStream(), true);

            BufferedReader in = new BufferedReader(new InputStreamReader(
                                       socket.getInputStream()));    

            BufferedReader stdIn = new BufferedReader(
                                          new InputStreamReader(System.in));

            out.print(command);

            out.close();
            in.close();
            socket.close();

        } catch (UnknownHostException e) {
            System.err.println("Unknown Host.");
           // System.exit(1);
        } catch (IOException e) {
            System.err.println("Couldn't get I/O for "
                               + "the connection.");
          //  System.exit(1);
        }

Everything works fine until I try to read the server's response, using this:

    String userInput;
  while ((userInput = stdIn.readLine()) != null) {
  out.println(userInput);
  System.out.println("echo: " + in.readLine());
  }

then the code hangs and the Python server does not receive any information, which I tested using print.

Is there a problem trying to send first and then wait for a response from the server in the Java client?

Any help will be much appreciated.

Answer 1

Well, I discovered that the Java client hangs because the messages sent by the python server were not explicitly finished with \r\n, so the Python code should have been something like this:

HOST = ''
PORT = 12000
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADRR, 1)
s.bind((HOST, PORT))
s.listen(5)
device = variador()
while True:

    conn, addr = s.accept()
    if data == "turn_on\r\n":
        respuesta = device.send_order(variador.start_order)
        conn.send(respuesta + '\r\n')
    conn.close()

I know it should have been quite obvious from the name of the methods in Java, readline() and println, both suggesting that java ends strings with the sequence \r\n

Answer 2

---

Connect Python And Java Sockets

---

Install the package jpysocket

pip install jpysocket

https://pypi.org/project/jpysocket

Import Library jpysocket

The Followings Are The Some Example

Python Server :

import jpysocket
import socket

host='localhost' #Host Name
port=12345    #Port Number
s=socket.socket() #Create Socket
s.bind((host,port)) #Bind Port And Host
s.listen(5) #Socket is Listening
print("Socket Is Listening....")
connection,address=s.accept() #Accept the Connection
print("Connected To ",address)
msgsend=jpysocket.jpyencode("Thank You For Connecting.") #Encript The Msg
connection.send(msgsend) #Send Msg
msgrecv=connection.recv(1024) #Recieve msg
msgrecv=jpysocket.jpydecode(msgrecv) #Decript msg
print("From Client: ",msgrecv)
s.close() #Close connection
print("Connection Closed.")

Java Client :

import java.io.*;  
  import java.net.*; 
  public class Client {
  public static void main(String[] args) {  
  try{      
  Socket soc=new Socket("localhost",12345);  
  DataOutputStream dout=new DataOutputStream(soc.getOutputStream());  
  DataInputStream in = new DataInputStream(soc.getInputStream());
  String msg=(String)in.readUTF();
  System.out.println("Server: "+msg);
  dout.writeUTF("Ok Boss");
  dout.flush();
  dout.close();
  soc.close();
  }
  catch(Exception e)
  {
      e.printStackTrace(); 
  }}}  

Python Client :

import jpysocket
import socket

host='localhost' #Host Name
port=12345    #Port Number
s=socket.socket() #Create Socket
s.connect((host,port)) #Connect to socket
print("Socket Is Connected....")
msgrecv=s.recv(1024) #Recieve msg
msgrecv=jpysocket.jpydecode(msgrecv) #Decript msg
print("From Server: ",msgrecv)
msgsend=jpysocket.jpyencode("Ok Boss.") #Encript The Msg
s.send(msgsend) #Send Msg
s.close() #Close connection
print("Connection Closed.")

Java Server :

import java.io.*;  
import java.net.*; 
public class Server {
  public static void main(String[] args) {
      try{ 
      ServerSocket serverSocket = new ServerSocket(12345);
      Socket soc = serverSocket.accept();
      System.out.println("Receive new connection: " + soc.getInetAddress());
      DataOutputStream dout=new DataOutputStream(soc.getOutputStream());  
      DataInputStream in = new DataInputStream(soc.getInputStream());
      dout.writeUTF("Thank You For Connecting.");
      String msg=(String)in.readUTF();
      System.out.println("Client: "+msg);
      dout.flush();
      dout.close();
      soc.close();
      }
      catch(Exception e)
      {
      e.printStackTrace(); 
  }
  }
  }