multithreading - NetBeans / Java / New hint: Thread.sleep called in loop

Question

In NetBeans, there's a new hint that says: Thread.sleep called in loop.

Question 1: How/when can it be a problem to sleep in a loop?

Question 2: If it's a problem, what should I do instead?

UPDATE: Question 3: Here's some code. Tell me in this case if I should be using something else instead of Thread.Sleep in a loop. In short, this is used by a server which listens for client TCP connections. The sleep is used here in case the max number of sessions with clients is reached. In this situation, I want the application to wait until a free session becomes available.

public class SessionManager {
    private static final int DEFAULT_PORT = 7500;
    private static final int SLEEP_TIME = 200;
    private final DatabaseManager database = new DatabaseManager();
    private final ServerSocket serverSocket = new ServerSocket(DEFAULT_PORT);

public SessionManager() throws IOException, SQLException
{
}

public void listen()
{
while (true)
    if (Session.getSessionCount() < Session.getMaxSessionCount())
        try
        {
             new Thread(new Session(database, serverSocket.accept())).start();
        }
        catch (IOException ex) { ex.printStackTrace(); }
    else
        try
        {
            Thread.sleep(SLEEP_TIME);
        }
        catch (InterruptedException ex) { ex.printStackTrace(); }
}

public static void main(String[] args) throws IOException, SQLException
{
new SessionManager().listen();
}
}

Answer 1

Calling sleep in a loop typically leads to poor performance. For example:

while (true) {
    if (stream.available() > 0) {
       // read input
    }
    sleep(MILLISECONDS);
}

If MILLISECONDS is too large, then this code will take a long time to realize that input is available.

If MILLISECONDS is too small, then this code will waste a lot of system resources check for input that hasn't arrived yet.

Other uses of sleep in a loop are typically questionable as well. There's usually a better way.

If it's a problem, what should I do instead?

Post the code and maybe we can give you a sensible answer.

EDIT

IMO, a better way to solve the problem is to use a ThreadPoolExecutor.

Something like this:

public void listen() {
    BlockingQueue queue = new SynchronousQueue();
    ThreadPoolExecutor executor = new ThreadPoolExecutor(
            1, Session.getMaxSessionCount(), 100, TimeUnit.SECONDS, queue);
    while (true) {
        try {
            queue.submit(new Session(database, serverSocket.accept()));
        } catch (IOException ex) { 
            ex.printStackTrace();
        }
    }
}

This configures the executor to match the way your code currently works. There are a number of other ways you could do it; see the javadoc link above.