Develop Reference

How do I know If client wants to send the data or recieve the data from server

when I am trying to send the data from the server, my client side is not able to receive the data. The server is able to write the data to the JSON file. I think because I am executing the code for sending and receiving the data on the server inside the main method every time regardless of what request I am receiving from the client. Is there any built-in method in Java that helps us know what the client request is and execute the codes on the server depending on the request rather than executing the code for both read and write request. I hope You guys understand.
public static void main(String[] args) throws IOException {
ServerSocket serverSocket = new ServerSocket(666);
FileReader fileReader = new FileReader("MYJSON.json");
BufferedReader buff = new BufferedReader(fileReader);
while(true) {
Socket socket = serverSocket.accept();
InputStreamReader inputStreamReader = new
InputStreamReader(socket.getInputStream());
BufferedReader bufferedReader = new BufferedReader(inputStreamReader);
String str = bufferedReader.readLine();
writeJson(str);
String str;
OutputStreamWriter outputStreamWriter = new
OutputStreamWriter(socket.getOutputStream());
PrintWriter printWriter = new PrintWriter(outputStreamWriter);
str = buff.readLine();
printWriter.write(str);
printWriter.flush();
printWriter.close();
}
}

Basic Java Echo Socket Communication Error

I have been looking through the multitudes of explanation of basic Java Socket use, and have constructed the following basic code for my own Server/Client echo pair. However, there is some hangup in the client code that I cannot find for the life of me. Perhaps someone else can spot it?
// Server Code:
try (ServerSocket serverSocket = new ServerSocket(22222);
Socket cSocket = serverSocket.accept();
PrintWriter out = new PrintWriter(cSocket.getOutputStream());
BufferedReader in = new BufferedReader(
new InputStreamReader(cSocket.getInputStream()))) {
System.out.println("Client connected: " + cSocket.getInetAddress().getHostAddress());
// console DOES print ^this line and correct IP when client is run.
String inLine;
while (true) {
inLine = in.readLine();
out.println(inLine);
if (inLine.equals("exit")) break;
}
// client code
try (Socket socket = new Socket("localhost", 22222);
PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
BufferedReader consoleIn = new BufferedReader(new InputStreamReader(System.in));) {
String userIn;
while (true) {
System.out.print("Client> ");
userIn = consoleIn.readLine();
out.println(userIn); // code hangs here.
out.flush();
System.out.println("Server> " + in.readLine());
if (userIn.equals("exit")) break;
}

It isn't blocking there. It's blocking in the readLine() from the server. Try a flush() after the println() in the server.

data received on server from android app always null

I am trying to connect from a android emulator to a application on my desktop and send a line of text.
My app is able to connect to the server, but when ever i try to read data its always null
Server application running on my desktop:
ServerSocket ss = new ServerSocket(9001);
Socket cs = ss.accept();
if (cs.isConnected()) {
System.out.println("Client connected.");
}
BufferedReader reader = new BufferedReader(new InputStreamReader(cs.getInputStream()));
String str = reader.readLine();
System.out.println("Data:" + str);
Client application running in android app emulator:
InetAddress addr = InetAddress.getByName("10.0.2.2");
Socket socket = new Socket(addr, 9001);
if (socket.isConnected()) {
Log.d("APP", "socket connected");
}
PrintWriter pw = new PrintWriter(new OutputStreamWriter(socket.getOutputStream()));
String str = "this is a sample";
pw.write(str);
I can see that the isConnected function of the socket on both the client and server turns true.
But the Data printed on the server is always null.
Thanks

Try adding a flush call after the 'pw.write(str)' statement:
pw.flush();

Java socket does not receive data

I have to write server application that request questions from client and receives an answer. This is my client code:
clientSocket = new Socket("localhost", 1234);
System.err.println("Client started");
//get questions
ObjectInputStream in = new ObjectInputStream(clientSocket.getInputStream());
Question q = (Question)in.readObject();
//send answer
PrintWriter out = new PrintWriter(clientSocket.getOutputStream());
out.print("a1");
out.flush();
and server code:
//sending questions
ObjectOutputStream out = new ObjectOutputStream(client.getOutputStream());
List<Question> quest = Questions.getInstance().getQuestions();
out.writeObject(quest.get(0));
out.flush();
//get answer
BufferedReader in = new BufferedReader(new InputStreamReader(client.getInputStream()));
String temp = null;
while ((temp = in.readLine()) == null) {}
String answer = temp;
Questions successfully sent and later received by client, but server never get answer (infinite loop while reading temp variable). What is the problem?

Your calling out.print("a1"); on the client, but reading a line on the server using in.readLine(). Shouldn't you be writing out using println() on the client, else the server never gets to the end of the line? – CodeChimp Nov 21 at 21:07
Thanks for CodeChimp

Java Sockets - Server hangs after client sends its response

Just trying to get a handle on sockets. The server and client are running in two different programs.
They seem to be connecting fine to each other but the client will not properly send its output to the server. The server just hangs. Here's the code:
Server:
private ServerSocket serverSocket;
private Socket client;
public void run() throws Exception {
serverSocket = new ServerSocket(20005);
while(currentState == Game.State.NORMAL) {
client = serverSocket.accept();
PrintWriter out = new PrintWriter(client.getOutputStream(), true);
BufferedReader in = new BufferedReader(new InputStreamReader(client.getInputStream()));
String clientInput = in.readLine();
// Takes the client input string and does some simple game logic that returns a Gson object
Gson serverResponse = processInput(clientInput);
out.write(serverResponse.toString());
out.flush();
}
}
Client:
Socket clientSocket;
void run() throws Exception {
clientSocket = new Socket("192.168.0.24", 20005);
PrintWriter out;
BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
// Print the state of the game - returns false if state is win or lose.
while(printState()) {
out = new PrintWriter(clientSocket.getOutputStream(), true);
// This method just takes some input from the console
String clientInput = getInput();
out.write(clientInput);
out.flush();
String serverResponse = in.readLine();
updateState(serverResponse);
}
}
}
There is some underlying game logic that is happening but it's pretty minor and should be irrelevant. I imagine I am just misunderstanding something fundamental here.
Thanks all.

Make sure you send a newline character to match the in.readLine() statement in the Server.
out.write(clientInput + "\n");
The same applys when sending data from Server->Client.