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/*
* Copyright (C) 2007 The Guava Authors
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package jersey.repackaged.com.google.common.base;
import jersey.repackaged.com.google.common.annotations.VisibleForTesting;
import java.io.Closeable;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.logging.Level;
import java.util.logging.Logger;
/**
* A reference queue with an associated background thread that dequeues references and invokes
* {@link 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 {@code
* finalizeReferent()} on the remaining references.
*
* @author Bob Lee
* @since 2.0 (imported from Google Collections Library)
*/
public class FinalizableReferenceQueue implements Closeable {
/*
* The Finalizer thread keeps a phantom reference to this object. When the client (for example, a
* map built by MapMaker) no longer has a strong reference to this object, the garbage collector
* will reclaim it and enqueue the phantom reference. The enqueued reference will trigger the
* Finalizer to stop.
*
* If this library is loaded in the system class loader, FinalizableReferenceQueue can load
* Finalizer directly with no problems.
*
* If this library is loaded in an application class loader, it's important that Finalizer not
* have a strong reference back to the class loader. Otherwise, you could have a graph like this:
*
* Finalizer Thread runs instance of -> Finalizer.class loaded by -> Application class loader
* which loaded -> ReferenceMap.class which has a static -> FinalizableReferenceQueue instance
*
* Even if no other references to classes from the application class loader remain, the Finalizer
* thread keeps an indirect strong reference to the queue in ReferenceMap, which keeps the
* Finalizer running, and as a result, the application class loader can never be reclaimed.
*
* This means that dynamically loaded web applications and OSGi bundles can't be unloaded.
*
* If the library is loaded in an application class loader, we try to break the cycle by loading
* Finalizer in its own independent class loader:
*
* System class loader -> Application class loader -> ReferenceMap -> FinalizableReferenceQueue
* -> etc. -> Decoupled class loader -> Finalizer
*
* Now, Finalizer no longer keeps an indirect strong reference to the static
* FinalizableReferenceQueue field in ReferenceMap. The application class loader can be reclaimed
* at which point the Finalizer thread will stop and its decoupled class loader can also be
* reclaimed.
*
* If any of this fails along the way, we fall back to loading Finalizer directly in the
* application class loader.
*/
private static final Logger logger = Logger.getLogger(FinalizableReferenceQueue.class.getName());
private static final String FINALIZER_CLASS_NAME = "jersey.repackaged.com.google.common.base.internal.Finalizer";
/** Reference to Finalizer.startFinalizer(). */
private static final Method startFinalizer;
static {
Class finalizer = loadFinalizer(
new SystemLoader(), new DecoupledLoader(), new DirectLoader());
startFinalizer = getStartFinalizer(finalizer);
}
/**
* The actual reference queue that our background thread will poll.
*/
final ReferenceQueue