Class FinalizableReferenceQueue

java.lang.Object
com.google.common.base.FinalizableReferenceQueue
All Implemented Interfaces:
Closeable, AutoCloseable

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 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) {
       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;
   }

   public void close() {
     serverSocket.close();
   }
 }
 
Since:
2.0
  • Field Details

    • logger

      private static final Logger logger
    • FINALIZER_CLASS_NAME

      private static final String FINALIZER_CLASS_NAME
      See Also:
    • startFinalizer

      private static final Method startFinalizer
      Reference to Finalizer.startFinalizer().
    • queue

      final ReferenceQueue<Object> queue
      The actual reference queue that our background thread will poll.
    • frqRef

      final PhantomReference<Object> frqRef
    • threadStarted

      final boolean threadStarted
      Whether or not the background thread started successfully.
  • Constructor Details

    • FinalizableReferenceQueue

      public FinalizableReferenceQueue()
      Constructs a new queue.
  • Method Details

    • close

      public void close()
      Specified by:
      close in interface AutoCloseable
      Specified by:
      close in interface Closeable
    • cleanUp

      void cleanUp()
      Repeatedly dequeues references from the queue and invokes FinalizableReference.finalizeReferent() on them until the queue is empty. This method is a no-op if the background thread was created successfully.
    • loadFinalizer

      private static Class<?> loadFinalizer(FinalizableReferenceQueue.FinalizerLoader... loaders)
      Iterates through the given loaders until it finds one that can load Finalizer.
      Returns:
      Finalizer.class
    • getStartFinalizer

      static Method getStartFinalizer(Class<?> finalizer)
      Looks up Finalizer.startFinalizer().