Writing Java Server to handle multiple client concurrently (simultaneously)

Question

I am working on a program where I have a Server and Client class, but at the moment it only handles only one client at a time. I need the server to be able to handle multiple clients concurrently (simultaneously), using multithreading.

Here is my Server code; how can I change it to handle multiple clients concurrently?

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

       ServerSocket socket = new ServerSocket(8945);
       Server serverInstance = new Server();

       System.out.println("Server is running. Waiting for client.");

       while(true) {
           server.socket = s.accept();
           System.out.println("Client connected");
           serverInstance.run();
           System.out.println("Client disconnected. Waiting for new client.");
       }
   }

   public void run() {
       try {
           try {
               in = new Scanner(socket.getInputStream());
               out = new PrintWriter(socket.getOutputStream());
               RequestHandlingMethod();  

           } finally {
               socket.close();
           }

       } catch (IOException e) {
           System.err.println(e);
       }

}

jbx · Accepted Answer · 2019-02-26 12:35:39Z

Create a separate class that handles the client. Make it implement Runnable so that you can just start a separate Thread with it.

public class ClientHandler implements Runnable {

   private final Socket socket;

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

    public void run() {
       try (Socket socket = this.socket;
            Scanner in = new Scanner(socket.getInputStream();
            PrintWriter out = new PrintWriter(socket.getOutputStream()) {

               //todo: do whatever you need to do


       } catch (IOException ex) {
           ex.printStackTrace();
       }

       System.out.println("Client disconnected.");
    }  
}

Then in your server you do:

       System.out.println("Waiting for new client connection");
       Socket clientSocket = s.accept();
       System.out.println("Client connected");
       new Thread(new ClientHandler(clientSocket)).start();

If you don't want to create a lot of disposable Threads, you might want to consider using an ExecutorService with a cached thread pool (or another thread pool of your choice if you prefer).

You would just create a new ExecutorService with ExecutorService executor = ExecutorService.newCachedThreadPool() and then inside your loop you do:

       System.out.println("Waiting for new client connection");
       Socket clientSocket = s.accept();
       System.out.println("Client connected");
       executor.submit(new ClientHandler(clientSocket));

If you think you are going to have a lot of concurrent clients, you might want to look at using a non-blocking server with NIO instead. It will have 1 single event loop thread instead (doesn't block on the accept) and handles all I/O events in there, and you can have a pool of worker threads that do the client handling logic.

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