Java NIO - Key Cancellation and Registering with New Selector

Question

Hi
Sometimes when a channel is unregistered from a particular selector (key.cancel() ) and then registered with another selector, the registration does not happen. It just blocks in that registration method..It does not execute after that..
I am doing
key.cancel();
selector2.wakeup();
channel.register(selector2, SelectionKey.OP_READ,this);
System.out.println("Associated with another selector");The System.Out does not get executed, and blocks at earlier statement.
What could be the reason?
What is the best way to deregister with a selector and register to another selector
Thanks

Answer 1

Synchronization is the reason. register() and select() are synchronized against each other. wakeup() causes the select() to exit but if the selector got round its loop and back to select() before this code got to execute the register it will still block of course.
You could have selector2's select loop do a little sleep if it finds zero ready keys, or maybe reduce its selection timeout so that register will only block for a finite amount at worst.
I don't think selection timeouts should ever be infinite and in practice I rarely set them above a minute. If that's still too long to wait you could consider reducing it further.