I write out a simple messaging client / server. I have already written a console application, which works fine but I wanted to write desktop application.
And there is a problem because when I create a connection to the program crashes(freeze) and I could only determine what portion of code that causes. I can say that the same code in the console is working properly.
Note that to "crash" means to exit abruptly, so the program's no longer running. To "freeze" means to stop working, so that the program won't respond to input anymore. Your program is freezing, not crashing!
Anyway: accept() waits for a new connection to come in before returning, right? Well, you're using invokeLater() to call accept() directly on the event-processing thread, which is also responsible for painting the screen and responding to mouse clicks. As long as accept() is waiting for a connection, your GUI will be frozen. Never call any method that will take a long time to run on the event thread, or it will freeze your GUI.
Even worse, you're running it on the event thread many, many, many times! invokeLater() returns immediately, so your while loop is adding calls to accept() onto the event thread at the rate of many thousands per second! Finally, note that you don't need to -- in fact, can't! -- create a new ServerSocket for each accept() call; you should create just one, and call accept on it in a loop, once for each client.
Anyway, to fix your problem: don't call accept() on the event loop! It looks like you already know how to define a Runnable and create a thread. Do that -- create a separate thread just for calling accept() and servicing clients. Create the ServerSocket outside of that loop.
I followed your advice and my application is not freezing. But now I do not get the message that the new client has connected.
1. I do not know how to find outside the thread that the client initiated the connection. Is there any way to find out.
2. Maybe I made sth wrong in the thread initialization and jTextArea1 doesn't get message.
You have all the right code here, but it's not necessarily in the right place. The line that updates the textarea should execute on the event thread, so you should use SwingUtilities.invokeLater() for that. On the other hand, the code where you've asked "//HOW IT CAN BE CHECKED", that should move onto the network thread. The best way to know about a client connecting is just to put the code right after accept() returns, right?
Now, actually, the way this is often done is you have one thread that calls accept() in a loop; whenever accept() returns, yet another thread is created to service that one client (passing the Socket as a constructor argument to the new thread.) That way, the accept() loop can call accept() again right away while that first client is being processed.