package org.infinispan.util.concurrent;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.locks.LockSupport;
import org.infinispan.commons.api.Lifecycle;
import org.infinispan.commons.util.concurrent.ConcurrentHashSet;
/**
* A class that handles restarts of components via multiple threads. Specifically, if a component needs to be restarted
* and several threads may demand a restart but only one thread should be allowed to restart the component, then use
* this class.
* <p/>
* What this class guarantees is that several threads may come in while a component is being restarted, but they will
* block until the restart is complete.
* <p/>
* This is different from other techniques in that: <ul> <li>A simple compare-and-swap to check whether another thread
* is already performing a restart will result in the requesting thread returning immediately and potentially attempting
* to use the resource being restarted.</li> <li>A synchronized method or use of a lock would result in the thread
* waiting for the restart to complete, but on completion will attempt to restart the component again.</li> </ul> This
* implementation combines a compare-and-swap to detect a concurrent restart, as well as registering for notification
* for when the restart completes and then parking the thread if the CAS variable still indicates a restart in progress,
* and finally deregistering itself in the end.
*
* @author Manik Surtani
* @since 4.0
*/
public class SynchronizedRestarter {
private AtomicBoolean restartInProgress = new AtomicBoolean(false);
private ConcurrentHashSet<Thread> restartWaiters = new ConcurrentHashSet<Thread>();
public void restartComponent(Lifecycle component) throws Exception {
// will only enter this block if no one else is restarting the socket
// and will atomically set the flag so others won't enter
if (restartInProgress.compareAndSet(false, true)) {
try {
component.stop();
component.start();
}
finally {
restartInProgress.set(false);
for (Thread waiter : restartWaiters) {
try {
LockSupport.unpark(waiter);
}
catch (Throwable t) {
// do nothing; continue notifying the rest
}
}
}
} else {
// register interest in being notified after the restart
restartWaiters.add(Thread.currentThread());
// check again to ensure the restarting thread hasn't finished, then wait for that thread to finish
if (restartInProgress.get()) LockSupport.park();
// de-register interest in notification
restartWaiters.remove(Thread.currentThread());
}
}
}