Question

I have two while conditions, one inside the other, with booleans to control them. Basically, one is for terminating a share and the other is to listen for a connection. A user can select to disable a share, and in this case, the server stops listening but it won’t terminate. If a user selects to terminate then both booleans are set to false and the cycle ends.

This is my code:

        public void run() {
            while (!terminate) {  
                while (listening) {
                    try {
                        // accept connection -> create a new thread for each client
                        ClientServerShareInstance clientServerShareInstance = new ClientServerShareInstance(serverSocket.accept(), ui);
                        Thread clientServerThread = new Thread(clientServerShareInstance);
                        clientSockets.add(clientServerShareInstance);
                        connectedClients++;
                        clientServerThread.start();
                    } catch (IOException ex) {
                    }
                }
            }
        }

        public void closeAllClientConnections() {
            for (Iterator it = clientSockets.iterator(); it.hasNext();) {
                ClientServerShareInstance clientServerShareInstance = (ClientServerShareInstance) it.next();
                clientServerShareInstance.closeAllConnections();
                it.remove();
            }
            try {
                this.serverSocket.close();
            } catch (IOException ex) {}
            this.setActive(false);
            this.connectedClients = 0;
        }


        public void openConnection() {
            try {
                serverSocket = new ServerSocket(portNumber, 0, Inet4Address.getLocalHost());
                setActive(true);
            } catch (IOException ex) {}
        }
    }

The closeAllClientConnections() method disables a share (does not terminate it), and the openConnection() reenables that share.

The problem is, if I disable a share, it should just loop the terminate while cicle indefinitely, testing the value of listening. When I set listening to true, it should re-enter that second while loop and start listening again, because I do open the server socket (although it’s not related to that, I’m just saying it has to be initialized again because I close it when I disable the share). However, after disabling, it never renters the listening loop, even when openConnection() is called.

Anyone know what’s wrong here?

Answer 1

There’s not enough code shown to show any errors. But here are some comments that might help.

• Both the shutdown and listening booleans need to be volatile. Any fields shared between threads need to be somehow synchronized otherwise changes to their values will not be seen by other threads.

• serverSocket will also need to be volatile since it seems to be created by the caller of openConnection() but consumed in the while loop. You might consider just setting active to the true in openConnection() and have serverSocket managed entirely by the accept thread.

• clientSockets looks to be a collection. That will need to be a synchronized connection since, again, it looks like it is accessed by multiple threads. Again, a better pattern would be for the closeAllClientConnections() call to just set a boolean and the thread itself would do the closing. That removes any race conditions around using the collections, etc..

• Looks to be that if you are ! terminating that your accept thread will spin. At the very least you should put some Thread.sleep(100) or something to slow it down. A wait/notify would even be better.

It is important to realize that it is not just about what happens “at the same time” in a threaded program. It is also about memory caching. The accept thread could have added something to the clientSockets ArrayList1 a minute ago and another thread may not see those changes if the list is not synchronized somehow. Even worse is the prospect that certain portions of the ArrayList may have been updated in memory but not others which might cause an exception. To get a synchronized collection, you should create your ArrayList like:

List<...> clientSockets = Collections.synchronizedList(new ArrayList<...>());

Sounds like you should read some of the documentation about why synchronization is necessary:

http://docs.oracle.com/javase/tutorial/essential/concurrency/sync.html