gov/nasa/worldwind/AbsentResourceList.java

   1 /*
   2 Copyright (C) 2001, 2006 United States Government
   3 as represented by the Administrator of the
   4 National Aeronautics and Space Administration.
   5 All Rights Reserved.
   6 */
   7 package gov.nasa.worldwind;
   8
   9 /**
  10  * @author tag
  11  * @version $Id$
  12  */
  13 public class AbsentResourceList
  14 {
  15     // Absent resources: A resource is deemed absent if a specified maximum number of attempts have been made to retrieve it.
  16     // Retrieval attempts are governed by a minimum time interval between successive attempts. If an attempt is made
  17     // within this interval, the resource is still deemed to be absent until the interval expires.
  18     private static final int DEFAULT_MAX_ABSENT_RESOURCE_TRIES = 2;
  19     private static final int DEFAULT_MIN_ABSENT_RESOURCE_CHECK_INTERVAL = 10000;
  20
  21     private int maxTries = DEFAULT_MAX_ABSENT_RESOURCE_TRIES;
  22     private int minCheckInterval = DEFAULT_MIN_ABSENT_RESOURCE_CHECK_INTERVAL;
  23
  24     private static class AbsentResoureEntry
  25     {
  26         long timeOfLastMark; // meant to be the time of the most recent attempt to find the resource
  27         int numTries;
  28     }
  29
  30     private final java.util.concurrent.ConcurrentHashMap<Long, AbsentResoureEntry> possiblyAbsent =
  31         new java.util.concurrent.ConcurrentHashMap<Long, AbsentResoureEntry>();
  32
  33     private java.util.SortedSet<Long> definitelyAbsent = java.util.Collections.synchronizedSortedSet(
  34         new java.util.TreeSet<Long>());
  35
  36     public AbsentResourceList()
  37     {
  38     }
  39
  40     public AbsentResourceList(int maxTries, int minCheckInterval)
  41     {
  42         this.maxTries = Math.max(maxTries, 1);
  43         this.minCheckInterval = Math.max(minCheckInterval, 500);
  44     }
  45
  46     public final void markResourceAbsent(long resourceID)
  47     {
  48         if (this.definitelyAbsent.contains(resourceID))
  49             return;
  50
  51         AbsentResoureEntry entry = this.possiblyAbsent.get(resourceID);
  52         if (entry == null)
  53             this.possiblyAbsent.put(resourceID, entry = new AbsentResoureEntry());
  54
  55         ++entry.numTries;
  56         entry.timeOfLastMark = System.currentTimeMillis();
  57
  58         if (entry.numTries >= this.maxTries)
  59         {
  60             this.definitelyAbsent.add(resourceID);
  61             this.possiblyAbsent.remove(resourceID);
  62             // entry can now be garbage collected
  63         }
  64     }
  65
  66     public final boolean isResourceAbsent(long resourceID)
  67     {
  68         if (this.definitelyAbsent.contains(resourceID))
  69             return true;
  70
  71         AbsentResoureEntry entry = this.possiblyAbsent.get(resourceID);
  72         //noinspection SimplifiableIfStatement
  73         if (entry == null)
  74             return false;
  75
  76         return (System.currentTimeMillis() - entry.timeOfLastMark) < this.minCheckInterval;
  77     }
  78
  79     public final void unmarkResourceAbsent(long resourceID)
  80     {
  81         this.definitelyAbsent.remove(resourceID);
  82         this.possiblyAbsent.remove(resourceID);
  83     }
  84 }