public class FinalizableReferenceQueue
extends Object
implements Closeable

A reference queue with an associated background thread that dequeues references and invokes FinalizableReference.finalizeReferent() on them.

Keep a strong reference to this object until all of the associated referents have been finalized. If this object is garbage collected earlier, the backing thread will not invoke finalizeReferent() on the remaining references.

As an example of how this is used, imagine you have a class MyServer that creates a a ServerSocket, and you would like to ensure that the ServerSocket is closed even if the MyServer object is garbage-collected without calling its close method. You could use a finalizer to accomplish this, but that has a number of well-known problems. Here is how you might use this class instead:

 public class MyServer implements Closeable {
   private static final FinalizableReferenceQueue frq = new FinalizableReferenceQueue();
   // You might also share this between several objects.

   private static final Set<Reference<?>> references = Sets.newConcurrentHashSet();
   // This ensures that the FinalizablePhantomReference itself is not garbage-collected.

   private final ServerSocket serverSocket;

   private MyServer(...) {
     ...
     this.serverSocket = new ServerSocket(...);
     ...
   }

   public static MyServer create(...) {
     MyServer myServer = new MyServer(...);
     final ServerSocket serverSocket = myServer.serverSocket;
     Reference<?> reference = new FinalizablePhantomReference<MyServer>(myServer, frq) {
       @Override public void finalizeReferent() {
         references.remove(this):
         if (!serverSocket.isClosed()) {
           ...log a message about how nobody called close()...
           try {
             serverSocket.close();
           } catch (IOException e) {
             ...
           }
         }
       }
     };
     references.add(reference);
     return myServer;
   }

   @Override public void close() {
     serverSocket.close();
   }
 }