Receiving python json through sockets with java

Question

I've been following multiple tutorials to connect a java code to python through sockets.

Sending from java to python works great, with json array. However, I can't seem to receive things with java. I don't really understand how the listening should be done. Now I just listen in a 15 second while loop (the python should send as soon as it receives an input), but I feel like I am doing something substantially wrong. Maybe somebody has an idea?

client.py:

import socket
import sys
import numpy as np
import json

# Create a TCP/IP socket
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

# Bind the socket to the port
server_address = ('localhost', 10004)
print >>sys.stderr, 'starting up on %s port %s' % server_address
sock.bind(server_address)

# Listen for incoming connections
sock.listen(1)

def mysum(x):
    return np.sum(x)

while True:
    # Wait for a connection
    print >>sys.stderr, 'waiting for a connection'
    connection, client_address = sock.accept()
    infile = sock.makefile();

    try:
        print >>sys.stderr, 'connection from', client_address

        # Receive the data in small chunks and retransmit it
        data = b''
        while True:
             new_data = connection.recv(16)
             if new_data:
                 # connection.sendall(data)
                 data += new_data
             else:
                 data += new_data[:]
                 print >>sys.stderr, 'no more data from', client_address
                 break
        data= data.strip('\n');
        print("data well received!: ")
        print(data,)
        print(np.array(json.loads(data)));

        #send a new array back

        sendArray =  np.array( [ (1.5,2,3), (4,5,6) ] );
        print("Preparing to send this:");
        print(sendArray);
        connection.send(json.dumps(sendArray.tolist()));

    except Exception as e:
        print(e)
        connection.close()
        print("closed");
    finally:
        # Clean up the connection
        connection.close()
        print("closed");

server.java:

import java.io.*;
import java.net.Socket;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;

import org.json.*;

import java.net.ServerSocket;

public class SocketTest {

    public static void main(String[] args) throws IOException {


        String hostName = "localhost";
        int portNumber = 10004;

        try (

                //open a socket
                Socket clientSocket = new Socket(hostName, portNumber);

                BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
                PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true);


        ) {

            System.out.println("Connected");
            Double[][] test2 = new Double[5][2];
            test2[1][1] = 0.1;
            test2[1][0] = 0.2;
            test2[2][1] = 0.2;
            test2[2][0] = 0.2;
            test2[3][1] = 0.1;
            test2[3][0] = 0.2;
            test2[4][1] = 0.2;
            test2[4][0] = 0.2;
            test2[0][1] = 0.2;
            test2[0][0] = 0.2;


            System.out.println("A");
            out.println(new JSONArray(test2).toString());
            System.out.println("B");


            long t = System.currentTimeMillis();
            long end = t + 15000;


            while (System.currentTimeMillis() < end) {
                String response;
                while ((response = in.readLine()) != null) {
                    System.out.println("receiving");
                    System.out.println( response );

                }


            }


            //listen for input continuously? 


            //clientSocket.close();
        } catch (JSONException e) {
            e.printStackTrace();
        }


    }


}

The output from python is:

data well received!: 
('[[0.2,0.2],[0.2,0.1],[0.2,0.2],[0.2,0.1],[0.2,0.2]]',)
[[ 0.2  0.2]
 [ 0.2  0.1]
 [ 0.2  0.2]
 [ 0.2  0.1]
 [ 0.2  0.2]]
Preparing to send this:
[[ 1.5  2.   3. ]
 [ 4.   5.   6. ]]
closed
waiting for a connection
connection from ('127.0.0.1', 40074)

Output from java:

A
B

The problem is that sendArray = np.array( [ (1.5,2,3), (4,5,6) ] ); is never received by java. I feel like I am missing something simple to make it listen... Thanks for any help.

Answer 1

This is happening because your java code is blocking. See how B is not printed to your logs? That is because it does't execute since this part is waiting for a flush command: out.println(new JSONArray(test2).toString()); . What you need to do is out.flush(); so it goes on.

Answer 2

You code has multiple issues.

Client.py is blocked at while True , keeps waiting for new data.
Server.java has sent the JSONArray(test2).toString() whose length is 51. Then keeps waiting at in.readLine().

See How to identify end of InputStream in java . The same idea works in python. It's better to know how many bytes you want to read.

Change: Because JSONArray(test2).toString() has length 51, so you replace

    while True:
         new_data = connection.recv(16)
         if new_data:
             # connection.sendall(data)
             data += new_data
         else:
             data += new_data[:]
             print >>sys.stderr, 'no more data from', client_address
             break

With

 data = connection.recv(51)

Change: send '\\n' so Server.java can successfully read a line.

    connection.send(json.dumps(sendArray.tolist()));
    connection.send("\n");

Changes: Make sure both sides don't send/receive data anymore, then close the socket together.

Code after changes of 1&2. (It requires more efforts to solve 3 and it's not the point of this question):

public class Server {

  public static void main(String[] args) throws IOException {


    String hostName = "localhost";
    int portNumber = 10004;

    try (

        //open a socket
        Socket clientSocket = new Socket(hostName, portNumber);

        BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
        PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true);


    ) {

      System.out.println("Connected");
      Double[][] test2 = new Double[5][2];
      test2[1][1] = 0.1;
      test2[1][0] = 0.2;
      test2[2][1] = 0.2;
      test2[2][0] = 0.2;
      test2[3][1] = 0.1;
      test2[3][0] = 0.2;
      test2[4][1] = 0.2;
      test2[4][0] = 0.2;
      test2[0][1] = 0.2;
      test2[0][0] = 0.2;


      System.out.println("A");
      out.println(new JSONArray(test2).toString());
      System.out.println("B");


      long t = System.currentTimeMillis();
      long end = t + 15000;


      while (System.currentTimeMillis() < end) {
        String response;
        while ((response = in.readLine()) != null) {
          System.out.println("receiving");
          System.out.println( response );

        }
      }
      //listen for input continuously?
      //clientSocket.close();
    } catch (JSONException e) {
      e.printStackTrace();
    }
  }
}

Client.py output:

starting up on localhost port 10004
waiting for a connection
connection from ('127.0.0.1', 53388)
data well received!: 
('[[0.2,0.2],[0.2,0.1],[0.2,0.2],[0.2,0.1],[0.2,0.2]]',)
[[ 0.2  0.2]
 [ 0.2  0.1]
 [ 0.2  0.2]
 [ 0.2  0.1]
 [ 0.2  0.2]]
Preparing to send this:
[[ 1.5  2.   3. ]
 [ 4.   5.   6. ]]
closed

Server.java output:

Connected
A
B
receiving
[[1.5, 2.0, 3.0], [4.0, 5.0, 6.0]]
Exception in thread "main" java.net.SocketException: Connection reset
    at java.net.SocketInputStream.read(SocketInputStream.java:209)
    at java.net.SocketInputStream.read(SocketInputStream.java:141)