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package com.sun.enterprise.ee.web.sessmgmt;
import java.lang.ref.WeakReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.util.concurrent.ConcurrentHashMap;
import java.util.logging.Level;
import java.util.logging.Logger;
import com.sun.logging.LogDomains;
/**
* Enables fine grain session level synchronization by maintaining a cache of synchronization monitors for sessions.
* Monitors are looked up by session id, type being String.
* Key constraint maintained by this class is that at any point in time,
* there is only one monitor per session id.
*
* The monitors in the cache are aggressively recovered whenever a monitor is no longer being used.
* All threads operating atomically on session related to session id must use same monitor.
* When monitor is no longer referenced by any thread, it is okay for it to be reclaimed.
*
* @author ha team
*/
public class ReplicationSessionMonitors {
/**
* Our cache of monitor object keyed by session id(string), value is KeyedReference that references
* a monitor to be used in synchronization blocks to ensure atomic operations relating to a session.
* Map is ConcurrentHashMap<String, KeyedReference>
*/
private ConcurrentHashMap replicatedSessionMonitors = null;
private ReferenceQueue replicatedSessionMonitorsRefQueue = new ReferenceQueue();
private static final int CONCURRENCY_LEVEL = 100;
final private Logger _logger;
private static class KeyedReference extends WeakReference {
private String key;
private KeyedReference(Object objRef, ReferenceQueue q, String key) {
super(objRef, q);
this.key = key;
}
public String toString() {
return "key=" + key + " resolvesTo=" + get();
}
}
/**
*
* @param logger logger to use for log events.
* @param size initial size for Map of session ids to monitor objects
* @param loadFactor loadfactor for map
*/
public ReplicationSessionMonitors(Logger logger, int size, float loadFactor) {
_logger = logger;
replicatedSessionMonitors = new ConcurrentHashMap(size, loadFactor, CONCURRENCY_LEVEL);
replicatedSessionMonitorsRefQueue = new ReferenceQueue();
}
/**
* Return an existing or new ReplicationSession monitor for id.
*
* @param id - a session id
* @return Monitor for session <code>id</code>
*/
public Object get(String id) {
Object result = null;
//first look in map - most of the time this will succeed
Reference resultRef = (Reference)replicatedSessionMonitors.get(id);
if(resultRef != null) {
result = resultRef.get();
if(result != null) {
return result;
} else {
// cover case that GC released weak reference already.
// remove map entry to released monitor and use this method
//again to insert a new monitor for id.
//only remove if value is still resultRef to null - thread safe
remove(id, resultRef);
return get(id);
}
}
Object keepReachable
= new Object(); // not likely but keep strong ref to new Monitor so not gc'ed
Reference ref
= new KeyedReference(keepReachable, replicatedSessionMonitorsRefQueue, id);
Reference refToResult
= (Reference)replicatedSessionMonitors.putIfAbsent(id, ref);
//this means we just put ref into the map so we return keepReachable
if(refToResult == null) {
return keepReachable;
}
result = refToResult.get();
if(result != null) {
return result;
} else {
// cover case that reference was released already.
// remove map entry to released monitor and use this method
//again to insert a new monitor for id.
//only remove if value is still resultRef to null - thread safe
remove(id, refToResult);
return get(id);
}
}
private Object remove(String id, Object toRemove) {
boolean removed = replicatedSessionMonitors.remove(id, toRemove);
if(removed) {
((KeyedReference)toRemove).key = null;
}
if (_logger.isLoggable(Level.FINEST)) {
_logger.finest("replicatedSession monitors remove id=" + id + " succeeded=" + removed);
}
return removed;
}
private void assertOnReferenceQueue(KeyedReference ref) {
if (_logger.isLoggable(Level.FINE)) {
if (ref.get() != null) {
_logger.fine("assertion failed: monitor for session id: " + ref.key + " was on ReferenceQueue but it" +
" but its get() returned non-null");
}
}
}
/**
* Remove entries from cache with values of weak references that no longer reference an object.
*/
public void processExpired() {
KeyedReference toBeRemoved = null;
while ((toBeRemoved = (KeyedReference)this.replicatedSessionMonitorsRefQueue.poll()) != null) {
if(toBeRemoved.key == null) continue;
assertOnReferenceQueue(toBeRemoved);
Object removed = remove(toBeRemoved.key, toBeRemoved);
if (removed == null && _logger.isLoggable(Level.FINE)) {
_logger.fine("processExpiredReplicaMonitor: unable to remove " + toBeRemoved + " sessionId=" + toBeRemoved.key);
}
}
if (_logger.isLoggable(Level.FINER)) {
int exitSize = replicatedSessionMonitors.size();
if (exitSize != 0) {
_logger.finer("exit ReplicationSessionMonitors.processExpired numberOfEntries=" + exitSize);
if (_logger.isLoggable(Level.FINEST)) {
int i = 1;
for (Object keyRef : replicatedSessionMonitors.values()) {
_logger.finest("ReplicationSessionMonitors.processExpired: replicatedSessionMonitor[" + i + "]: " + ((KeyedReference) keyRef));
i++;
}
}
}
}
}
}