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Send message from Java client to Python server using sockets and vice versa

I am trying to send receive data using a Python server and a Java client. First, Java sends a JSON in string to Python Server. After the string received, Python server will send a JSON back to the client. After the client receives the JSON from the server, it again sends a JSON in string to server. (Client sends the same message all the time) This is a recursive process.
ISSUE: After when I execute both Python server and Java, Python server receives the message sent by the Java Client and it sent back the JSON. But in the client side, the message sent by the server didnt receive.
Server.py
import socket
import threading
import json
import numpy
HEADER_INITIAL = 25
PORT = 1234
SERVER = socket.gethostbyname(socket.gethostname())
ADDR = (SERVER, PORT)
FORMAT = 'utf-8'
def handle_client(self, conn, addr):
print(f"[NEW CONNECTION] {addr} connected.")
connected = True
while connected:
msg = conn.recv(HEADER_INITIAL).decode(FORMAT)
if msg:
print("[DATA] RECEIVED"+ str(msg))
x = {
"Sentence": "This is a value"
}
y = json.dumps(x)
conn.send(y.encode(FORMAT))
conn.send("\n".encode(FORMAT));
conn.close()
Client.java
try (Socket socket = new Socket(Address, Port)) {
InputStream input = socket.getInputStream();
InputStreamReader reader = new InputStreamReader(input);
OutputStream output = socket.getOutputStream();
PrintWriter writer = new PrintWriter(output, true);
int character;
StringBuilder data = new StringBuilder();
while(true){
Thread.sleep(4000);
String strJson = "{'message':'Hello World'}";
JSONObject jsonObj = new JSONObject(strJson);
writer.println(jsonObj.toString());
while((character = reader.read()) != -1) {
data.append((char) character);
}
System.out.println(data);
}
} catch (UnknownHostException ex) {
System.out.println("Server not found: " + ex.getMessage());
} catch (IOException ex) {
System.out.println("I/O error: " + ex.getMessage());
}
UPDATE
Here is the debug output.
I first started the server and then started client. Initially server receives the {'message':'Hello World'} value which is sent by the client and the server sends back the value of the x variable to the client. Then the server receives nothing from the client, but the client prints the value of x continuously.( System.out.println(data);) I tried to send dynamic values from the server to client, but the client prints only the value which is sent by the server in the first time.

You don't provide any debugging output so it's difficult to be 100% sure this is the entire cause. However, it seems pretty evident that this section of your client code isn't correct:
while((character = reader.read()) != -1) {
data.append((char) character);
}
System.out.println(data);
The server is holding the connection open forever (nothing ever sets connected to false). And so in the loop above, the character returned by reader.read will never be -1 because -1 is only returned at "end of stream". End of stream will only occur when the server closes its socket -- or is otherwise disconnected.
You should add a check for the newline to break out of the read loop:
if (character == '\n')
break;
or you could add it to the while condition:
while ((character = reader.read()) != -1 && character != '\n') {
...
Your code overall lacks appropriate handling of possible exceptional conditions. For example, if the client disconnects, your server will never exit its loop. It will call recv, get back an empty string (signifying "end of file" on the connection), and so will correctly bypass sending a response, but it will then simply go back and execute recv again, get an empty string again, and so forth forever.
Also, your python code makes the implicit assumption that the recv returns exactly the single string that was sent by the client, which is not guaranteed. If the client sends a 20 character string for example, it's possible that the first server recv call returns the first 10 characters, and the next call returns the rest.
(In practice, given the sleep in the client side code, that's unlikely to be a problem in this snippet of code, but one should program defensively because in a real production program, there will inevitably be a race or edge case that will do exactly this and it will cause the client and server to get out of sync and be difficult to debug.)

Java communication with TCP socket and PIC stuck in read()

I try to communicate with a java application to a µController in wifi (Flyport).
I have a problem with the java application :
It first create a socket to communicate with the Flyport server, then send a message and receive the Flyport answer.
Everything work fine until the read part. I'm polling the read() function of the BufferedReader until it return -1, but it doesn't. The first read works fine, all the answer are red, but the application stay stuck when it tries to read again.
My code is very simple :
Java application :
try (
Socket clientSocket = new Socket(hostName, portNumber);
PrintWriter out =
new PrintWriter(clientSocket.getOutputStream(), true);
BufferedReader in =
new BufferedReader(
new InputStreamReader(clientSocket.getInputStream()));
)
{
...//Connection and sending message works fine
}
char[] buffer = new char[500];
while ((in.read(buffer)) != -1) { // first read() works fine, second read() stay stuck...
System.out.println(buffer); // display all answer sent by flyport
}
The code in the flyport :
while(isClientConnected){
//check if client is still connected
...
//read client message
while((RxLen=TCPRxLen(sock))>0)
{
TCPRead(sock,bff,RxLen);
strcat(msg,bff);
}
//write back to the client that the order is received
TCPWrite(sock, msg, strlen(msg));
//process the client order
...
//Write to the client that the process is done
TCPWrite(sock, msg2, strlen(msg2));
}
The java application read msg and msg2 with the first read(). msg and msg2 have "\r\n" at the end.
Doesn't somebody can tell me where I am wrong ?
Is there a function from BufferedReading that tells how much data there is left to read ?
Thanks and regards.
NB : I try with a small buffer in the java application, the problem is the same, read() is stuck when there is nothing left to read...

You're reading from the socket until end of stream, and you're never causing end of stream, as you are never closing the socket at the sender. Either close the socket or don't read until end of stream.

Java TCP server - android client packet loss

I'm trying to create a basic multiplayer game for android, using a Java TCP server and Android client. The problem is slow speed when sending TCP packets. When I put Thread.sleep(100) then it works.
server side:
for(int i = 0; i<50; i++) {
client.send("test_" + i);
}
client just received (~3 packet)
test_0
test_1
server with sleep:
for(int i = 0; i<50; i++) {
client.send("test_" + i);
Thread.sleep(100);
}
client received ~45
EDIT: client side:
while (true) {
if (!running)
break;
inFromServer = new BufferedReader(new InputStreamReader(socket.getInputStream(), "UTF-8"), 2 * 1024);
String rawRecervied = inFromServer.readLine();
if (rawRecervied == null) {
close();
break;
}
final String recervied = rawRecervied.substring(2); // for skip utf bom chars
new Thread() {
public void run() {
listener.dataRecervied(recervied);
Log.e("TCP recervied", recervied); // debug
}
}.start();
}

Maybe the key is in the BufferedReader. You're in a loop, and constantly create a BufferedReader to check if something has been sent from the server. Once data is detected, you start processing it, but data keeps coming, and is buffered in the BufferedReader. After processing the initially detected data, you create again a BufferedReader but, what happens with all the data that was already buffered in the BufferedReader created before? Maybe it's lost.
Could you try creating the BufferedReader outside the loop?

If it is a one-way protocol where packet loss is acceptable then use UDP instead of TCP as it is cheaper in terms of network resources. I think this is not your case however. If TCP, then implement a basic flow control where the client acknowledges the received packet with echoing its ID back to the server.
You should also revise your client and server code because this behaviour might be in the way you implemented that client.sent(..). Do you always close and reopen the connection? Or what?

Sending packets over TCP socket

I'm writing this tiny utility method to test sending raw packets to a specific messaging network (planning on developing a client to connect to it).
The network is the Deviantart messaging network (chat.deviantart.com:3900; TCP).
My class:
protected void connect() throws IOException{
Socket dAmn = null;
//BufferedWriter out = null;
PrintWriter out = null;
BufferedReader in = null;
/*
* Create Socket Connection
*/
try{
dAmn =
new Socket("chat.deviantart.com", 3900);
/*out =
new BufferedWriter(new OutputStreamWriter(dAmn.getOutputStream()));*/
out =
new PrintWriter(dAmn.getOutputStream(), true);
in =
new BufferedReader(new InputStreamReader(dAmn.getInputStream()));
}
catch(SocketException e){
System.err.println("No host or port for given connection");
//handle
}
catch(IOException e){
System.err.println("I/O Error on host");
//handle
}
String userInput;
BufferedReader userIn =
new BufferedReader(new InputStreamReader(System.in));
/*
* dAmn communication
*/
while((userInput = userIn.readLine()) != null){
out.write(userInput);
System.out.println(in.readLine());
}
if(in!=null)
in.close();
if(out!=null)
out.close();
if(dAmn!=null)
dAmn.close();
}
The server requires a handshake to be sent before the login may proceed. A typical login packet looks like thus:
dAmnclient damnClient (currently 0.3)
agent= agent
Every packet must end with a newline and a null.
My handshake packet would look something like:
dAmnClient 0.3\nagent=SomeAgent\n\0
However the server simply replies with disconnect
I think something is incorrectly being parsed, any advice? Also, if you're super intersted in helping me out: here's some quick documentation on the client -> server dAmn protocol:
http://botdom.com/wiki/DAmn#dAmnClient_.28handshake.29

You should use Wireshark
With Wireshark you can sniff traffic from/to hosts. It makes it really easy to spot where your application does something else than the standard client.
BTW you have a \n in front of agent=, it might be the problem

The line read from the user will not contain the actual line termination, and it will not contain any null-termination either. Typing \n at the input will actually transmit "\n" rather than a new-line.
You could add a new-line by replacing write with println (careful, it may use \n, \r\n or just \r depending on platform):
out.println(userInput);
You could support packet termination e.g. by checking for a specific user input, like so:
if (userInput.equals(".")) {
out.write((char) 0);
out.flush();
} else {
out.println(userInput);
}
The user can now terminate packets by typing a dot.
(Actually the code could perform the handshake automatically without waiting for user input, but that's another story.)

TCP socket communication

I am creating a Chat in java.
I have a method (onMouseRelease) inside an object that creates a tcp server and waits for a socket like this:
ServerSocket server = new ServerSocket(port);
Socket channel = server.accept();
Now I want to make a thread that will loop and read data from the socket, so that once the user on the other side sends me a string, I will extract the data from the socket (or is it called packet? Sorry, I am new to this) and update a textbox to add the additional string from the socket (or packet?).
I have no idea how to READ (extract) the information from the socket(/packet) and then update it into a JTextArea which is called userOutput. And how to send a string to the other client, so that it will also could read the new data and update its JTextArea.
From what I know, for a 2 sided TCP communication you need one computer to host a server
and the other to connect (as a client) and once the connection is set the client can also receive new information from the socket. Is that true? and please tell me how.
Any help is appreciated! I know this is a bit long but I have searched a lot and didn't understand it (I saw something like PrintWriter but failed to understand).

You would have to do something like this;
InputStream in = new BufferedInputStream(channel.getInputStream());
You can then read characters from the socket using a loop;
char ch;
while (!finished) {
ch = in.read(); //read from socket
if(ch = -1) {
//nothing left to read
finished = true;
}
else {
//do something with ch
}
}
I can continue if you'd like?
Say we saved the incoming chars to a String called input, to update your text area you would call;
textArea.setText(input);
And to send text back to the client you would use a similar method to receiving, using an outputstream;
OutputStream out = new BufferedOutputStream(clientSock.getOutputStream());
out.write(output);