Synchronizing a "Selector"

Question

regarding thread safety of java.nio.channels.Selector:
"a selector's key and selected-key are not, *in general*,
safe for use by concurrent threads. if such a thread
might modify one of these sets directly then access
*can* be controlled by synchronizing on the set itself".
well... i want 2 threads to share one Selector instance.
and with some extra effort, the docs imply i can.
note:
the following use of a SynchronizedSet does not work.
Selector selector = Selector.open();
new Listener(selector);
new Server(selector);
public class Listener implements Runnable {
  public Listener(Selector selector) { ... }
  public void run() {
    while(true) {
      Set syncSet = Collections.synchronizedSet(selector.keys());
      Iterator it = syncSet.iterator();
      while(it.hasNext()) {
        ... // view the contents of the "keys" in Selector.
        ... // and add channels to Selector.
    } // _ while() _
  }  // __ run() __
} // ___ class ___ class ___
public class Server implements Runnable {
  public Server(Selector selector) { ... }
  public void run() {
    while(true) {
      selector.select();
      Set syncSet = Collections.synchronizedSet(selector.selectedKeys());
      Iterator it = syncSet.iterator();
      while(it.hasNext()) {
         SelectionKey key = (SelectionKey) it.next();
         it.remove();
         if(key.isAcceptable()) {
      SocketChannel sokChannel = (SocketChannel) setSokChannel.accept();
         sokChannel.register(selector, SelectionKey.OP_READ);
    } // _ while() _
  } // ___ run() ___
}depending on timing, i get different errors.
one popular error is when i try:
"SelectionKey key = (SelectionKey) iterator.next()"
i get "NullPointerException".
and, sometimes, i don't get an error, but what i
add in one thread("Server") does not show-up in the
other("Client") thread.
this is doable; i need some help and wisdom.
thanks.

Answer 1

>
Set syncSet = Collections.synchronizedSet(selector.keys());will create a synchronized set wrapping the selector.keys() set. It will not synchronize the selector.keys() itself. Look at the api-documentation of Collections:
Returns a synchronized (thread-safe) set backed by the specified set. In order to guarantee serial access, it is critical that all access to the backing set is accomplished through the returned set.So you should find a way to synchonize your threads by using either the same synchronized list in all of them or by sychronizing using a synchronized block - something like:
synchronized ( selector.keys() ){
....}or
synchronized( selector ){
}should do the job I think.