CODEWITHSUNDEEP
javasocketstcpconnection
Chain Cross
Chain Cross

Reputation: 351

Server application hangs Java socket programming

My application keeps loading (circle cursor). Here is my code

public class ControllerW {

@FXML
private Label statusLbl, timeLbl, textLbl;
@FXML
private Button exitBtn, linkBtn;

private ServerSocket serverSckt;
private Socket clientSckt;
DataInputStream dis;
String text;

@FXML
private void exit() {
    System.exit(0);
}

@FXML
private void linkAndroid() {
    try {
        serverSckt = new ServerSocket(5678);
        statusLbl.setText("Server started");
        clientSckt = serverSckt.accept();
        statusLbl.setText("client connected successfully");
        dis = new DataInputStream(clientSckt.getInputStream());
        text = dis.toString();
        textLbl.setText(text);
    } catch (Exception e) {
        statusLbl.setText("No connection on port 5678");
        e.printStackTrace();
    }
}

}

I am new to socket programming, could someone please help me if there is anything wrong with my code

Upvotes: 0

Views: 315

Answers (2)

user9875877
user9875877

Reputation:

First you run the socket with the GUI/JFX thread. so accept() will block it (UI won't be responsible, dat circle you know).

You need to run the IO/socket with another thread. but beware, you need to change the UI stuffs with UI thread. (for example textLbl.setText(text); may fail if is called from another thread directly)

Also this line text = dis.toString(); is nonsense, you supposed to get a string sent from client? So use(recommended) readLine() method from BufferedReader instead of DataInputStream. (readUTF may work from DataInputStream too).

Upvotes: 0

L.Spillner
L.Spillner

Reputation: 1772

Your application is stopping to respond because it's not threaded. Without multithreading your application will 'stop' the moment you click your "link" button because the call to ServerSocket#accept.

The reason for that is the blocking nature of the accept method. Until another socket connects the ServerSocket is waiting and keeps his current thread 'suspended'. Think of it as an inner while loop similiar to this:

while(!isConnected())
{
    // check if there is a new request
}

Imagine no client would ever connect. How would this loop ever stop? Never.

A possible workaround is to either thread the entire UI part or 'data model' part.

My prefered way is to create a custom class which only handles the ServerSocket#accept method and delegates incoming connections to another thread which takes care of the rest. I do that to keep my server accessible at any time which might not be the best approach for your problem.

This is how my helper class commonly looks like:

ClientAccepter.java:

public class ClientAccepter
    implements Runnable
{
  // the ServerSocket
  private final ServerSocket server;
  // ExecutorServices ease the pain of threading your application quite a lot
  private final ExecutorService es = Executors.newCachedThreadPool();
  
  private boolean isAlive = true;

  public ClientAccepter( ServerSocket server )
  {
    this.server = server;
  }

  @Override
  public void run()
  {
    //This is where you specify your desired behaviour.
    //Example:
    while ( isAlive )
    {
      try
      {
        es.execute( new ClientHandler( server.accept() ) );
      }
      catch ( IOException e )
      {
        e.printStackTrace();
      }
    }
  }
}

The call to such a class could look like this:

Server.java:

public class Server
{
  //ExecutorServices are the best and easy to use.
  private static ExecutorService serverThreader = Executors.newSingleThreadExecutor();
  
  public static void main( String[] args )
  {
    try
    {
      serverThreader.execute( new ClientAccepter( new ServerSocket( 8080 ) ) );
    }
    catch ( IOException e )
    {
      e.printStackTrace();
     //further exception handling.
    }
  }
}

To be fair this might not be the best approach but it's my preferred way of handling blocking operations. You should consider working through the orcale concurrency tutorial and definitly take a look at ExecutorServices because they are an incredible framework to handle threadpools with ease.

Also specificly for JavaFX there is a concurrency tutorial.

Upvotes: 2

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