TCP Java server, Python client String not equal

Question

I have a TCP connection between a Java server and Python Client. when 'q' is sent from any to the other connection must be closed. It works when I send 'q' from python to java. However, when I sent 'q' from java to python it does not work. I tried typecasting but no use.

Java Server:

public class Sender {

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

    ServerSocket Server = new ServerSocket(25000);

    System.out.println("TCPServer Waiting for client on port 25000");

    while (true) {
        Socket connected = Server.accept();
        System.out.println(
                " THE CLIENT" + " " + connected.getInetAddress() + ":" + connected.getPort() + " IS CONNECTED ");

        PrintWriter out = new PrintWriter(connected.getOutputStream(), true);
        Scanner sc = new Scanner(System.in);


        while (true) {
            String input = sc.nextLine();
            out.println(input);
        }
    }

}

}

Python Client:

client_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
client_socket.connect(("localhost", 25000))
while True:
    r, _, _ = select.select([client_socket], [], [])
    if r:
        data = client_socket.recv(4096)
        string_data = data.decode('utf-8')
        print str(string_data) == 'q'
        if str(string_data) == 'q' or string_data == 'Q':
            print ('Connection closed')
            client_socket.close()
            break
        else:
            print (string_data)

The statement print str(string_data) == 'q' always returns false

SubOptimal · Accepted Answer · 2016-03-29 10:27:21Z

2

You need to change your Java server code from out.println(input) to out.print(input). Add also an out.flush() to force that the data are send immediately not only after the buffer is filled.

println - would also send the linebreak to the client

With print it works as expected.

edit Some additonal. In such a case it helps on the Python client to print out the bytes in data.

Amend the Python client code as

data = client_socket.recv(4096)
for i in bytearray(data):
    sys.stdout.write(hex(i) + " ")
print

Assumed input on Java is q+ENTER. See below the output on the Pathon client side.

using on Java server side out.println

0x71 0xd 0xa
False
q

using on Java server side out.print

0x71
True
Connection closed

That way it's easy to spot the additional bytes (0xd 0xa) which were sent to the client.

edited Mar 29, 2016 at 10:27

answered Mar 29, 2016 at 10:00

SubOptimal

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Comments

Shiva · Accepted Answer · 2016-03-29 09:36:11Z

-1

I think the encoding is the problem. Just comment out the encoding part and the code should work.

answered Mar 29, 2016 at 9:36

Shiva

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4 Comments

Omar Amr Over a year ago

Still the same problem

Shiva Over a year ago

Findout the encoding of the java side using the Charset.defaultCharset() or system property or InputStreamReader.getEncoding and use the same on python side

Omar Amr Over a year ago

the encoding is UTF-8

Shiva Over a year ago

Then check the standard encoding alias please.

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