How to Handle Multiple Clients Connecting to a Single Server Using Socket Programming

Question

How to handle multiple clients connecting to a single server in socket programming?

import javax.net.ssl.*;
import javax.net.*;
import java.io.*;
import java.net.*;

public class LogServer {
  private static final int PORT_NUM = 5000;
  public static void main(String args[]) {
    ServerSocketFactory serverSocketFactory =
      ServerSocketFactory.getDefault();
    ServerSocket serverSocket = null;
    try {
      serverSocket =
        serverSocketFactory.createServerSocket(PORT_NUM);
    } catch (IOException ignored) {
      System.err.println("Unable to create server");
      System.exit(-1);
    }
    System.out.printf("LogServer running on port: %s%n", PORT_NUM);
    while (true) {
      Socket socket = null;
      try {
        socket = serverSocket.accept();
        InputStream is = socket.getInputStream();
        BufferedReader br = new BufferedReader(
          new InputStreamReader(is, "US-ASCII"));
        String line = null;
        while ((line = br.readLine()) != null) {
          System.out.println(line);
        }
      } catch (IOException exception) {
        // Just handle next request.
      } finally {
        if (socket != null) {
          try {
            socket.close();
          } catch (IOException ignored) {
          }
        }
      }
    }
  }
}

Answer

To enable multiple clients to connect to one server in socket programming, you need to handle each client's connection in a separate thread. The given LogServer.java implementation currently processes one client at a time; it requires modification to accommodate concurrent client connections.

import javax.net.ssl.*;
import javax.net.*;
import java.io.*;
import java.net.*;

public class LogServer {
  private static final int PORT_NUM = 5000;
  public static void main(String args[]) {
    ServerSocketFactory serverSocketFactory =
      ServerSocketFactory.getDefault();
    ServerSocket serverSocket = null;
    try {
      serverSocket =
        serverSocketFactory.createServerSocket(PORT_NUM);
    } catch (IOException ignored) {
      System.err.println("Unable to create server");
      System.exit(-1);
    }
    System.out.printf("LogServer running on port: %s%n", PORT_NUM);
    while (true) {
      try {
        Socket socket = serverSocket.accept();
        // Handle each client in a new thread
        new ClientHandler(socket).start();
      } catch (IOException e) {
        e.printStackTrace();
      }
    }
  }
}

class ClientHandler extends Thread {
  private Socket socket;

  public ClientHandler(Socket socket) {
    this.socket = socket;
  }

  public void run() {
    try (InputStream is = socket.getInputStream();
         BufferedReader br = new BufferedReader(new InputStreamReader(is, "US-ASCII"))) {
      String line;
      while ((line = br.readLine()) != null) {
        System.out.println(line);
      }
    } catch (IOException e) {
      e.printStackTrace();
    } finally {
      try {
        socket.close();
      } catch (IOException ignored) {}
    }
  }
}

Causes

The server is only processing one connection in a blocking manner.
When a client connects, the server waits for that connection to close before accepting new ones.

Solutions

Use multithreading to handle each client connection separately.
For each accepted socket, create a new thread that processes the communication with that specific client.

Common Mistakes

Mistake: Not using multithreading, causing the server to handle a single connection at a time.

Solution: Implement a separate thread for each client connection using the thread class or implement runnable.

Mistake: Immediate socket closure without handling exceptions properly.

Solution: Ensure graceful handling and logging of exceptions during socket operations.

Helpers

socket programming
Java server multiple clients
multithreaded server example
handle client connections
Java socket example