Java socket - Read & Write

Question

The problem: Client doesn't receive any message.

Here is the full code for both client and server:

CLIENT

public class Client extends Socket{


public Client(String hostName, int port) throws UnknownHostException, IOException {
    super(hostName,port);

    BufferedReader in = new BufferedReader(new InputStreamReader(getInputStream()));

    while(true) {
        String line = in.readLine();
        System.out.println("Text received: " + line);
    }

}

SERVER

public final class Server extends ServerSocket{

public Server(int port) throws IOException {
    super(port);

    System.out.println("Server waiting for client 1");
    Socket client1 = accept();
    PrintWriter writer = new PrintWriter(client1.getOutputStream(), true);
    writer.write("Hello user 1");

    System.out.println("Server waiting for client 2");
    Socket client2 = accept();
    PrintWriter writer2 = new PrintWriter(client2.getOutputStream(), true);
    writer2.write("Hello user 2");

    System.out.println("Clients connected");

}

I start the server to listen to port 4444
I start the clients with hostname of "localhost" and port 4444

Edited my answer. Is the connection going through? Try adding a println() statement after Socket client1 = accept(); and see if it prints. — Alexander Guyer
– Alexander Guyer, Commented Jul 29, 2016 at 23:17
You're reading lines but you aren't writing lines. Add a line terminator, or use println(). NB Your read loop is not adequate. It doesn't test for end of stream. It should be while ((line = in.readLine()) != null) {...}. — user207421
– user207421, Commented Jul 30, 2016 at 1:26

Haifeng Zhang · Accepted Answer · 2016-08-01 16:59:45Z

15

You have to include a newline character at the end of the message, and also flush the PrintWriter if the connection is not immediately altered, forcing an automatic flush:

writer.write("Hello user 1\n");
writer.flush();

EDIT:

It is possible to enable automatic flushing on a PrintWriter using the constructor new PrintWriter(someOutputStream, true)

However, as explained in the documentation:

if automatic flushing is enabled it will be done only when one of the println, printf, or format methods is invoked, rather than whenever a newline character happens to be output

This means that even with automatic flushing, the PrintWriter would still have to be manually flushed after write is called, and a newline character (\n) would still have to be included at the end of the string.

EDIT 2:

Here is a small example of a fully functional client/server application:

Client:

public static void main(String[] args){
    try{
        Socket socket = new Socket(HOST_ADDRESS, PORT);
        BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        System.out.println(bufferedReader.readLine());
        bufferedReader.close();
        socket.close();
    }catch(IOException e){}
}

Server:

public static void main(String[] args){
    try{
        ServerSocket serverSocket = new ServerSocket(PORT);
        Socket socket = serverSocket.accept();
        PrintWriter printWriter = new PrintWriter(socket.getOutputStream());
        printWriter.write("Hello user!\n");
        printWriter.flush();
        printWriter.close();
        socket.close();
        serverSocket.close();
    }catch(IOException e){}
}

edited Aug 1, 2016 at 16:59

Haifeng Zhang

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answered Jul 29, 2016 at 22:17

Alexander Guyer

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2 Comments

Alexander Guyer Over a year ago

Edited to include a fully functional example

ShlomiRex Over a year ago

Thank you. You'r edit number 2 is working. It like magic. I have no idea why its working. The same functionality, same code...

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