Question

I have a java application that people can log into (and do various things with a database with, but that’s not important). I can use the .getInetAddress() to get the IP addresses of the people logged into the server, however I am not sure how exactly to communicate between the threads. I am trying to add a chat service to this pre-existing program.

My program uses a standard multi-threaded server, and the clinet is a single threaded AWT Action Listener program. What is the best way to do this? My code includes a class containing main that creates a new “Client Handler” class and creates a new thread to handle it. Currently there is no inter-thread communication, the thread spawns on it’s own socket port and runs independently on the server. My previous thoughts include:

Put the client into a blocking state with ObjectInputStream and wait for the client to receive a message (instead of busy waiting for a button to be pressed), with the condition that when the client goes to perform an action (edit a field), it would release the blocking I/O and perform the method, then go back to the blocked I/O “Waiting” stage of the program.

Things I’m not sure about this:

1. If i invoke the blocking I/O, will the AWT ActionListener unblock the blocking I/O and jump to the event handler?

2. If the client is in code (not on the blocking InputStream) and a message is sent, will the server know to wait until the program is back on the inputStream before it sends the message, or will it send the message and create a queue, or worse cashe, problem.

3. The server is multithreaded, If John is at 192.168.1.100 and Larry is at 192.168.1.152 and John wants to send a message to Larry, how do I get the message from thread handling *.100, to thread handling *.152 so it can be outputted to the right client on the right socket.

My other thought (and maybe the easier one) would be to make the client multi-threaded, connect to the server on a different port, and use a whole different set of socket connections to handle the events. In this case i can just use a blocking I/O wait, when i get the message, output it, and then go back to the blocking I/O. There wouldn’t be any issues in miss communication between the production code and the chat code, and from that stance it is better, however this means I would need to have 2 open connections on different ports for every client that connects. The program will eventually have thousands of users connected to it at one time, and I do not want to hog all the server ports with one application.

Are their other ways of doing this without flooding the ports or risking communication errors?

Answer 1

Your client should be using 3 threads. All gui interactions should be done from the EDT. Then, you should have a thread for the socket InputStream and a thread for the socket OutputStream (both of which block can potentially block). when you want to send a message, your gui thread should use some sort of thread-safe queue to pass the message to the OutputStream thread. any messages received by the InputStream thread should use something like SwingUtilities.invokeLater to push the message to the gui.

i’m not exactly sure what you are saying about the sockets and ports. every client which is connected to the server will require a separate socket connection (which will involve a separate port)