Best How To :

Here is the modified code using a new thread (though not tried):

 public FrmServer() throws HeadlessException {
    setTitle("Server");
    setLayout(new BorderLayout());

    addContent();
    addSend();

    setSize(400, 300);
    setVisible(true);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    chat();
}
private ServerSocket serverInfo;
private Socket clientInfo;
private ObjectInputStream ois;
private ObjectOutputStream oos;

private void chat() {
    Runnable runnable = new Runnable() {
        public void run() {
            try {
                serverInfo = new ServerSocket(12345);

                clientInfo = serverInfo.accept();
                ois = new ObjectInputStream(clientInfo.getInputStream());

                oos = new ObjectOutputStream(clientInfo.getOutputStream());

                while (true) {
                    String data = ois.readObject().toString();
                    SwingUtilities.invokeLater(new Runnable() {
                        public void run(){
                            txtContent.append("Client :" + data + "\n");
                        }
                    });
                }
            } catch (Exception ex) {
            }

        }
    };
    //You can use a ThreadPool too.
    new Thread(runnable).start();
}

With Java 8 you can improve the syntax.

The important thing is you should not do anything time consuming on the Swing/AWT Event Dispatch Thread. Connecting to a server, waiting for reply certainly time consuming. Though the catch is, you cannot update the UI from other threads. So you need the SwingUtilities class to execute code on EDT.

Concurrency in Swing tutorial might help understanding the details.