src/com/google/common/base/FinalizableReferenceQueue.java - platform/external/guava - Git at Google

 /*
  * Copyright (C) 2007 Google Inc.
  *
  * 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 com.google.common.base;

 import java.io.FileNotFoundException;
 import java.io.IOException;
 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.
  *
  * <p>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
  */
 public class FinalizableReferenceQueue {

   /*
    * The Finalizer thread keeps a phantom reference to this object. When the
    * client (ReferenceMap, for example) 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
       = "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<Object> queue;

   /**
    * Whether or not the background thread started successfully.
    */
   final boolean threadStarted;

   /**
    * Constructs a new queue.
    */
   @SuppressWarnings("unchecked")
   public FinalizableReferenceQueue() {
     // We could start the finalizer lazily, but I'd rather it blow up early.
     ReferenceQueue<Object> queue;
     boolean threadStarted = false;
     try {
       queue = (ReferenceQueue<Object>) startFinalizer.invoke(null,
           FinalizableReference.class, this);
       threadStarted = true;
     } catch (IllegalAccessException e) {
       // Finalizer.startFinalizer() is public.
       throw new AssertionError(e);
     } catch (Throwable t) {
       logger.log(Level.INFO, "Failed to start reference finalizer thread."
           + " Reference cleanup will only occur when new references are"
           + " created.", t);
       queue = new ReferenceQueue<Object>();
     }

     this.queue = queue;
     this.threadStarted = threadStarted;
   }

   /**
    * Repeatedly dequeues references from the queue and invokes
    * {@link FinalizableReference#finalizeReferent()} on them until the queue
    * is empty. This method is a no-op if the background thread was created
    * successfully.
    */
   void cleanUp() {
     if (threadStarted) {
       return;
     }

     Reference<?> reference;
     while ((reference = queue.poll()) != null) {
       /*
        * This is for the benefit of phantom references. Weak and soft
        * references will have already been cleared by this point.
        */
       reference.clear();
       try {
         ((FinalizableReference) reference).finalizeReferent();
       } catch (Throwable t) {
         logger.log(Level.SEVERE, "Error cleaning up after reference.", t);
       }
     }
   }

   /**
    * Iterates through the given loaders until it finds one that can load
    * Finalizer.
    *
    * @return Finalizer.class
    */
   private static Class<?> loadFinalizer(FinalizerLoader... loaders) {
     for (FinalizerLoader loader : loaders) {
       Class<?> finalizer = loader.loadFinalizer();
       if (finalizer != null) {
         return finalizer;
       }
     }

     throw new AssertionError();
   }

   /**
    * Loads Finalizer.class.
    */
   interface FinalizerLoader {

     /**
      * Returns Finalizer.class or null if this loader shouldn't or can't load
      * it.
      *
      * @throws SecurityException if we don't have the appropriate privileges
      */
     Class<?> loadFinalizer();
   }

   /**
    * Tries to load Finalizer from the system class loader. If Finalizer is
    * in the system class path, we needn't create a separate loader.
    */
   static class SystemLoader implements FinalizerLoader {
     public Class<?> loadFinalizer() {
       ClassLoader systemLoader;
       try {
         systemLoader = ClassLoader.getSystemClassLoader();
       } catch (SecurityException e) {
         logger.info("Not allowed to access system class loader.");
         return null;
       }
       if (systemLoader != null) {
         try {
           return systemLoader.loadClass(FINALIZER_CLASS_NAME);
         } catch (ClassNotFoundException e) {
           // Ignore. Finalizer is simply in a child class loader.
           return null;
         }
       } else {
         return null;
       }
     }
   }

   /**
    * Try to load Finalizer in its own class loader. If Finalizer's thread
    * had a direct reference to our class loader (which could be that of
    * a dynamically loaded web application or OSGi bundle), it would prevent
    * our class loader from getting garbage collected.
    */
   static class DecoupledLoader implements FinalizerLoader {

     private static final String LOADING_ERROR = "Could not load Finalizer in"
         + " its own class loader. Loading Finalizer in the current class loader"
         + " instead. As a result, you will not be able to garbage collect this"
         + " class loader. To support reclaiming this class loader, either"
         + " resolve the underlying issue, or move Google Collections to your"
         + " system class path.";

     public Class<?> loadFinalizer() {
       try {
         /*
          * We use URLClassLoader because it's the only concrete class loader
          * implementation in the JDK. If we used our own ClassLoader subclass,
          * Finalizer would indirectly reference this class loader:
          *
          * Finalizer.class ->
          *   CustomClassLoader ->
          *     CustomClassLoader.class ->
          *       This class loader
          *
          * System class loader will (and must) be the parent.
          */
         ClassLoader finalizerLoader = newLoader(getBaseUrl());
         return finalizerLoader.loadClass(FINALIZER_CLASS_NAME);
       } catch (Exception e) {
         logger.log(Level.WARNING, LOADING_ERROR, e);
         return null;
       }
     }

     /**
      * Gets URL for base of path containing Finalizer.class.
      */
     URL getBaseUrl() throws IOException {
       // Find URL pointing to Finalizer.class file.
       String finalizerPath = FINALIZER_CLASS_NAME.replace('.', '/') + ".class";
       URL finalizerUrl = getClass().getClassLoader().getResource(finalizerPath);
       if (finalizerUrl == null) {
         throw new FileNotFoundException(finalizerPath);
       }

       // Find URL pointing to base of class path.
       String urlString = finalizerUrl.toString();
       if (!urlString.endsWith(finalizerPath)) {
         throw new IOException("Unsupported path style: " + urlString);
       }
       urlString = urlString.substring(0,
           urlString.length() - finalizerPath.length());
       return new URL(finalizerUrl, urlString);
     }

     /** Creates a class loader with the given base URL as its classpath. */
     URLClassLoader newLoader(URL base) {
       return new URLClassLoader(new URL[] { base });
     }
   }

   /**
    * Loads Finalizer directly using the current class loader. We won't be
    * able to garbage collect this class loader, but at least the world
    * doesn't end.
    */
   static class DirectLoader implements FinalizerLoader {
     public Class<?> loadFinalizer() {
       try {
         return Class.forName(FINALIZER_CLASS_NAME);
       } catch (ClassNotFoundException e) {
         throw new AssertionError(e);
       }
     }
   }

   /**
    * Looks up Finalizer.startFinalizer().
    */
   static Method getStartFinalizer(Class<?> finalizer) {
     try {
       return finalizer.getMethod("startFinalizer", Class.class, Object.class);
     } catch (NoSuchMethodException e) {
       throw new AssertionError(e);
     }
   }
 }
	/*
	* Copyright (C) 2007 Google Inc.
	*
	* 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 com.google.common.base;

	import java.io.FileNotFoundException;
	import java.io.IOException;
	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.
	*
	* <p>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
	*/
	public class FinalizableReferenceQueue {

	/*
	* The Finalizer thread keeps a phantom reference to this object. When the
	* client (ReferenceMap, for example) 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
	= "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<Object> queue;

	/**
	* Whether or not the background thread started successfully.
	*/
	final boolean threadStarted;

	/**
	* Constructs a new queue.
	*/
	@SuppressWarnings("unchecked")
	public FinalizableReferenceQueue() {
	// We could start the finalizer lazily, but I'd rather it blow up early.
	ReferenceQueue<Object> queue;
	boolean threadStarted = false;
	try {
	queue = (ReferenceQueue<Object>) startFinalizer.invoke(null,
	FinalizableReference.class, this);
	threadStarted = true;
	} catch (IllegalAccessException e) {
	// Finalizer.startFinalizer() is public.
	throw new AssertionError(e);
	} catch (Throwable t) {
	logger.log(Level.INFO, "Failed to start reference finalizer thread."
	+ " Reference cleanup will only occur when new references are"
	+ " created.", t);
	queue = new ReferenceQueue<Object>();
	}

	this.queue = queue;
	this.threadStarted = threadStarted;
	}

	/**
	* Repeatedly dequeues references from the queue and invokes
	* {@link FinalizableReference#finalizeReferent()} on them until the queue
	* is empty. This method is a no-op if the background thread was created
	* successfully.
	*/
	void cleanUp() {
	if (threadStarted) {
	return;
	}

	Reference<?> reference;
	while ((reference = queue.poll()) != null) {
	/*
	* This is for the benefit of phantom references. Weak and soft
	* references will have already been cleared by this point.
	*/
	reference.clear();
	try {
	((FinalizableReference) reference).finalizeReferent();
	} catch (Throwable t) {
	logger.log(Level.SEVERE, "Error cleaning up after reference.", t);
	}
	}
	}

	/**
	* Iterates through the given loaders until it finds one that can load
	* Finalizer.
	*
	* @return Finalizer.class
	*/
	private static Class<?> loadFinalizer(FinalizerLoader... loaders) {
	for (FinalizerLoader loader : loaders) {
	Class<?> finalizer = loader.loadFinalizer();
	if (finalizer != null) {
	return finalizer;
	}
	}

	throw new AssertionError();
	}

	/**
	* Loads Finalizer.class.
	*/
	interface FinalizerLoader {

	/**
	* Returns Finalizer.class or null if this loader shouldn't or can't load
	* it.
	*
	* @throws SecurityException if we don't have the appropriate privileges
	*/
	Class<?> loadFinalizer();
	}

	/**
	* Tries to load Finalizer from the system class loader. If Finalizer is
	* in the system class path, we needn't create a separate loader.
	*/
	static class SystemLoader implements FinalizerLoader {
	public Class<?> loadFinalizer() {
	ClassLoader systemLoader;
	try {
	systemLoader = ClassLoader.getSystemClassLoader();
	} catch (SecurityException e) {
	logger.info("Not allowed to access system class loader.");
	return null;
	}
	if (systemLoader != null) {
	try {
	return systemLoader.loadClass(FINALIZER_CLASS_NAME);
	} catch (ClassNotFoundException e) {
	// Ignore. Finalizer is simply in a child class loader.
	return null;
	}
	} else {
	return null;
	}
	}
	}

	/**
	* Try to load Finalizer in its own class loader. If Finalizer's thread
	* had a direct reference to our class loader (which could be that of
	* a dynamically loaded web application or OSGi bundle), it would prevent
	* our class loader from getting garbage collected.
	*/
	static class DecoupledLoader implements FinalizerLoader {

	private static final String LOADING_ERROR = "Could not load Finalizer in"
	+ " its own class loader. Loading Finalizer in the current class loader"
	+ " instead. As a result, you will not be able to garbage collect this"
	+ " class loader. To support reclaiming this class loader, either"
	+ " resolve the underlying issue, or move Google Collections to your"
	+ " system class path.";

	public Class<?> loadFinalizer() {
	try {
	/*
	* We use URLClassLoader because it's the only concrete class loader
	* implementation in the JDK. If we used our own ClassLoader subclass,
	* Finalizer would indirectly reference this class loader:
	*
	* Finalizer.class ->
	* CustomClassLoader ->
	* CustomClassLoader.class ->
	* This class loader
	*
	* System class loader will (and must) be the parent.
	*/
	ClassLoader finalizerLoader = newLoader(getBaseUrl());
	return finalizerLoader.loadClass(FINALIZER_CLASS_NAME);
	} catch (Exception e) {
	logger.log(Level.WARNING, LOADING_ERROR, e);
	return null;
	}
	}

	/**
	* Gets URL for base of path containing Finalizer.class.
	*/
	URL getBaseUrl() throws IOException {
	// Find URL pointing to Finalizer.class file.
	String finalizerPath = FINALIZER_CLASS_NAME.replace('.', '/') + ".class";
	URL finalizerUrl = getClass().getClassLoader().getResource(finalizerPath);
	if (finalizerUrl == null) {
	throw new FileNotFoundException(finalizerPath);
	}

	// Find URL pointing to base of class path.
	String urlString = finalizerUrl.toString();
	if (!urlString.endsWith(finalizerPath)) {
	throw new IOException("Unsupported path style: " + urlString);
	}
	urlString = urlString.substring(0,
	urlString.length() - finalizerPath.length());
	return new URL(finalizerUrl, urlString);
	}

	/** Creates a class loader with the given base URL as its classpath. */
	URLClassLoader newLoader(URL base) {
	return new URLClassLoader(new URL[] { base });
	}
	}

	/**
	* Loads Finalizer directly using the current class loader. We won't be
	* able to garbage collect this class loader, but at least the world
	* doesn't end.
	*/
	static class DirectLoader implements FinalizerLoader {
	public Class<?> loadFinalizer() {
	try {
	return Class.forName(FINALIZER_CLASS_NAME);
	} catch (ClassNotFoundException e) {
	throw new AssertionError(e);
	}
	}
	}

	/**
	* Looks up Finalizer.startFinalizer().
	*/
	static Method getStartFinalizer(Class<?> finalizer) {
	try {
	return finalizer.getMethod("startFinalizer", Class.class, Object.class);
	} catch (NoSuchMethodException e) {
	throw new AssertionError(e);
	}
	}
	}