//
   //  ========================================================================
   //  Copyright (c) 1995-2014 Mort Bay Consulting Pty. Ltd.
   //  ------------------------------------------------------------------------
   //  All rights reserved. This program and the accompanying materials
   //  are made available under the terms of the Eclipse Public License v1.0
   //  and Apache License v2.0 which accompanies this distribution.
   //
   //      The Eclipse Public License is available at
  //      http://www.eclipse.org/legal/epl-v10.html
  //
  //      The Apache License v2.0 is available at
  //      http://www.opensource.org/licenses/apache2.0.php
  //
  //  You may elect to redistribute this code under either of these licenses.
  //  ========================================================================
  //
  
  package org.eclipse.jetty.util;
  
  import java.lang.ref.PhantomReference;
  import java.lang.ref.ReferenceQueue;
  import java.lang.ref.WeakReference;
  import java.util.concurrent.ConcurrentHashMap;
  import java.util.concurrent.ConcurrentMap;
  
  import org.eclipse.jetty.util.component.AbstractLifeCycle;
  import org.eclipse.jetty.util.log.Log;
  import org.eclipse.jetty.util.log.Logger;
  
  /**
   * A facility to detect improper usage of resource pools.
   * <p>
   * Resource pools usually have a method to acquire a pooled resource
   * and a method to released it back to the pool.
   * <p>
   * To detect if client code acquires a resource but never releases it,
   * the resource pool can be modified to use a {@link LeakDetector}.
   * The modified resource pool should call {@link #acquired(Object)} every time
   * the method to acquire a resource is called, and {@link #released(Object)}
   * every time the method to release the resource is called.
   * {@link LeakDetector} keeps track of these resources and invokes method
   * {@link #leaked(org.eclipse.jetty.util.LeakDetector.LeakInfo)} when it detects that a resource
   * has been leaked (that is, acquired but never released).
   * <p>
   * To detect whether client code releases a resource without having
   * acquired it, the resource pool can be modified to check the return value
   * of {@link #released(Object)}: if false, it means that the resource was
   * not acquired.
   * <p>
   * IMPLEMENTATION NOTES
   * <p>
   * This class relies on {@link System#identityHashCode(Object)} to create
   * a unique id for each resource passed to {@link #acquired(Object)} and
   * {@link #released(Object)}. {@link System#identityHashCode(Object)} does
   * not guarantee that it will not generate the same number for different
   * objects, but in practice the chance of collision is rare.
   * <p>
   * {@link LeakDetector} uses {@link PhantomReference}s to detect leaks.
   * {@link PhantomReference}s are enqueued in their {@link ReferenceQueue}
   * <em>after</em> they have been garbage collected (differently from
   * {@link WeakReference}s that are enqueued <em>before</em>).
   * Since the resource is now garbage collected, {@link LeakDetector} checks
   * whether it has been released and if not, it reports a leak.
   * Using {@link PhantomReference}s is better than overriding {@link #finalize()}
   * and works also in those cases where {@link #finalize()} is not
   * overridable.
   *
   * @param <T> the resource type.
   */
  public class LeakDetector<T> extends AbstractLifeCycle implements Runnable
  {
      private static final Logger LOG = Log.getLogger(LeakDetector.class);
  
      private final ReferenceQueue<T> queue = new ReferenceQueue<>();
      private final ConcurrentMap<String, LeakInfo> resources = new ConcurrentHashMap<>();
      private Thread thread;
  
      /**
       * Tracks the resource as been acquired.
       *
       * @param resource the resource that has been acquired
       * @return whether the resource has been tracked
       * @see #released(Object)
       */
      public boolean acquired(T resource)
      {
          String id = id(resource);
          return resources.putIfAbsent(id, new LeakInfo(resource, id)) == null;
      }
  
      /**
       * Tracks the resource as been released.
       *
       * @param resource the resource that has been released
       * @return whether the resource has been acquired
       * @see #acquired(Object)
       */
      public boolean released(T resource)
     {
         String id = id(resource);
         return resources.remove(id) != null;
     }
 
     /**
      * Generates a unique ID for the given resource.
      *
      * @param resource the resource to generate the unique ID for
      * @return the unique ID of the given resource
      */
     protected String id(T resource)
     {
         return String.valueOf(System.identityHashCode(resource));
     }
 
     @Override
     protected void doStart() throws Exception
     {
         super.doStart();
         thread = new Thread(this, getClass().getSimpleName());
         thread.setDaemon(true);
         thread.start();
     }
 
     @Override
     protected void doStop() throws Exception
     {
         super.doStop();
         thread.interrupt();
     }
 
     @Override
     public void run()
     {
         try
         {
             while (isRunning())
             {
                 @SuppressWarnings("unchecked")
                 LeakInfo leakInfo = (LeakInfo)queue.remove();
                 if (LOG.isDebugEnabled())
                     LOG.debug("Resource GC'ed: {}", leakInfo);
                 if (resources.remove(leakInfo.id) != null)
                     leaked(leakInfo);
             }
         }
         catch (InterruptedException x)
         {
             // Exit
         }
     }
 
     /**
      * Callback method invoked by {@link LeakDetector} when it detects that a resource has been leaked.
      *
      * @param leakInfo the information about the leak
      */
     protected void leaked(LeakInfo leakInfo)
     {
         LOG.warn("Resource leaked: " + leakInfo.description, leakInfo.stackFrames);
     }
 
     /**
      * Information about the leak of a resource.
      */
     public class LeakInfo extends PhantomReference<T>
     {
         private final String id;
         private final String description;
         private final Throwable stackFrames;
 
         private LeakInfo(T referent, String id)
         {
             super(referent, queue);
             this.id = id;
             this.description = referent.toString();
             this.stackFrames = new Throwable();
         }
 
         /**
          * @return the resource description as provided by the resource's {@link Object#toString()} method.
          */
         public String getResourceDescription()
         {
             return description;
         }
 
         /**
          * @return a Throwable instance that contains the stack frames at the time of resource acquisition.
          */
         public Throwable getStackFrames()
         {
             return stackFrames;
         }
 
         @Override
         public String toString()
         {
             return description;
         }
     }
 }