How can I use a Selector in a thread safe way?

Question

Hello,
I'm using a server socket with a java.nio.channels.Selector contemporarily by 3 different threads (this number may change in the future).
From the javadoc: Selectors are themselves safe for use by multiple concurrent threads; their key sets, however, are not.
Following this advise, I wrote code in this way:
         List keys = new LinkedList(); //private key list for each thread
         while (true) {
          keys.clear();
          synchronized(selector) {
              int num = selector.select();
              if (num == 0)
               continue;
              Set selectedKeys = selector.selectedKeys();
              //I expected this code to produce disjoint key sets on each thread...
              keys.addAll(selectedKeys);
              selectedKeys.clear();
          Iterator it = keys.iterator();
          while (it.hasNext()) {
              SelectionKey key = (SelectionKey) it.next();
              if ((key.readyOps() & SelectionKey.OP_ACCEPT) == SelectionKey.OP_ACCEPT) {
               Socket s = serverSocket.accept();
               SocketChannel sc = s.getChannel();
               sc.configureBlocking( false );
               sc.register( selector, SelectionKey.OP_READ );
              } else if ((key.readyOps() & SelectionKey.OP_READ) == SelectionKey.OP_READ) {
//.....Unfortunately synchronizing on the selector didn't have the effect I expected. When another thread select()s, it sees the same key list as the other thread that select()ed previously. When control arrives to serverSocket.accept(), one thread goes ahead and the other two catch an IllegalBlockingModeException.
I'm not willing to handle this exception, the right thing to do is giving disjoint key sets to each thread. How can I achieve this goal?
Thanks in advance

Answer 1

A single thread won't be enough cause after reading data from the socket I do some processing on it that may take long.So despatch that processing to a separate thread.
Most of this processing is I/O boundI/O bound on the socket? or something else? If it's I/O bound on the socket that's even more of a reason to use a single thread.
Anyway I think I'll use a single thread with the selector, put incoming data in a queue and let other 2 or 3 threads read from it.Precisely. Ditto outbound data.
Thanks for your replies. But I'm still curious: why is a selector thread safe if it can't be used with multiple threads because of it's semantics?It can, but there are synchronization issues to overcome (with Selector.wakeup()), and generally the cost of solving these is much higher than the cost of a single-threaded selector solution with worker threads for the application processing.