+ /**
+ * Used to enqueue the PhantomReferences for read-only TransactionProxy instances. The
+ * FinalizableReferenceQueue is safe to use statically in an OSGi environment as it uses some
+ * trickery to clean up its internal thread when the bundle is unloaded.
+ */
+ private static final FinalizableReferenceQueue phantomReferenceQueue =
+ new FinalizableReferenceQueue();
+
+ /**
+ * This stores the TransactionProxyCleanupPhantomReference instances statically, This is
+ * necessary because PhantomReferences need a hard reference so they're not garbage collected.
+ * Once finalized, the TransactionProxyCleanupPhantomReference removes itself from this map
+ * and thus becomes eligible for garbage collection.
+ */
+ private static final Map<TransactionProxyCleanupPhantomReference,
+ TransactionProxyCleanupPhantomReference> phantomReferenceCache =
+ new ConcurrentHashMap<>();
+
+ /**
+ * A PhantomReference that closes remote transactions for a TransactionProxy when it's
+ * garbage collected. This is used for read-only transactions as they're not explicitly closed
+ * by clients. So the only way to detect that a transaction is no longer in use and it's safe
+ * to clean up is when it's garbage collected. It's inexact as to when an instance will be GC'ed
+ * but TransactionProxy instances should generally be short-lived enough to avoid being moved
+ * to the old generation space and thus should be cleaned up in a timely manner as the GC
+ * runs on the young generation (eden, swap1...) space much more frequently.
+ */
+ private static class TransactionProxyCleanupPhantomReference
+ extends FinalizablePhantomReference<TransactionProxy> {
+
+ private final List<ActorSelection> remoteTransactionActors;
+ private final AtomicBoolean remoteTransactionActorsMB;
+ private final ActorContext actorContext;
+ private final TransactionIdentifier identifier;
+
+ protected TransactionProxyCleanupPhantomReference(TransactionProxy referent) {
+ super(referent, phantomReferenceQueue);
+
+ // Note we need to cache the relevant fields from the TransactionProxy as we can't
+ // have a hard reference to the TransactionProxy instance itself.
+
+ remoteTransactionActors = referent.remoteTransactionActors;
+ remoteTransactionActorsMB = referent.remoteTransactionActorsMB;
+ actorContext = referent.actorContext;
+ identifier = referent.identifier;
+ }
+
+ @Override
+ public void finalizeReferent() {
+ LOG.trace("Cleaning up {} Tx actors for TransactionProxy {}",
+ remoteTransactionActors.size(), identifier);
+
+ phantomReferenceCache.remove(this);
+
+ // Access the memory barrier volatile to ensure all previous updates to the
+ // remoteTransactionActors list are visible to this thread.
+
+ if(remoteTransactionActorsMB.get()) {
+ for(ActorSelection actor : remoteTransactionActors) {
+ LOG.trace("Sending CloseTransaction to {}", actor);
+ actorContext.sendOperationAsync(actor,
+ new CloseTransaction().toSerializable());
+ }
+ }
+ }
+ }
+
+ /**
+ * Stores the remote Tx actors for each requested data store path to be used by the
+ * PhantomReference to close the remote Tx's. This is only used for read-only Tx's. The
+ * remoteTransactionActorsMB volatile serves as a memory barrier to publish updates to the
+ * remoteTransactionActors list so they will be visible to the thread accessing the
+ * PhantomReference.
+ */
+ private List<ActorSelection> remoteTransactionActors;
+ private AtomicBoolean remoteTransactionActorsMB;
+
+ private final Map<String, TransactionContext> remoteTransactionPaths = new HashMap<>();
+