I'm trying to create a socket server in Java that can communicate with a python client. I ran into the problem that everything executes fine if I start the server, send a message through python, and then immediately exit, but if the client sends a message and then listens to the socket again the server won't respond.
Here's the code, mostly taken from this example from oracle
public class PrimeServer {
public static void main(String[] args) throws IOException {
int portNumber = 4444;
try (
ServerSocket serverSocket = new ServerSocket(portNumber);
Socket clientSocket = serverSocket.accept();
PrintWriter out =
new PrintWriter(clientSocket.getOutputStream(), true);
BufferedReader in = new BufferedReader(
new InputStreamReader(clientSocket.getInputStream()))
) {
String inputLine;
String outputLine;
String rawMessage;
// Initiate conversation with client
PrimeProtocol ppc = new PrimeProtocol();
outputLine = String.format("%1022s", "Ready").replace(' ', '0');
out.println(outputLine);
System.out.println("Over while");
while ((inputLine = in.readLine()) != null) {
System.out.println("In while");
if (ppc.processInput(Integer.parseInt(inputLine))){
rawMessage = "True";
} else {
rawMessage = "False";
}
outputLine = String.format("%1024s", rawMessage).replace(' ', '0');
System.out.println("Outputting " + rawMessage);
out.println(outputLine);
}
System.out.println("Passed while.");
} catch (IOException e) {
System.out.println("Exception caught when trying to listen on port "
+ portNumber + " or listening for a connection");
System.out.println(e.getMessage());
}
}
}
And the python code for good measure:
msg_len = 1024
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect(("127.0.0.1", 4444))
init_message = sock.recv(msg_len).decode("UTF-8")
init_message = init_message.replace('0', '')
print(init_message)
print(len(init_message))
sock.send(b'3')
With this code the output from the java server is:
Over while
In while
Outputting True
Passed while.
And the client output is:
1024
Ready
After which the connection breaks.
But if I add this line to the client: new_message = sock.recv(msg_len).decode("UTF-8")
Making the new python:
msg_len = 1024
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect(("127.0.0.1", 4444))
init_message = sock.recv(msg_len).decode("UTF-8")
print(len(init_message))
init_message = init_message.replace('0', '')
print(init_message)
sock.send(b'3')
new_message = sock.recv(msg_len).decode("UTF-8")
The server jams and the output is only:
Over while
While the client outputs:
1024
Ready
While also jamming.
Any ideas what is going on here?
Turns out it was the python socket that needed to flush. New python code:
msg_len = 1024
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect(("127.0.0.1", 4444))
init_message = sock.recv(msg_len).decode("UTF-8")
print(len(init_message))
init_message = init_message.replace('0', '')
print(init_message)
sock.send(b'3\n')
new_message = sock.recv(msg_len).decode("UTF-8")
print(new_message)
(added \\n) to the message going out.
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