WeakIdentityHashMap


1   /*
2    * @(#)WeakIdentityHashMap.java 1.2 05/11/17
3    * 
4    * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
5    * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
6    */
7   
8   package com.sun.jmx.mbeanserver;
9   
10  import static com.sun.jmx.mbeanserver.Util.*;
11  
12  import java.lang.ref.Reference  ;
13  import java.lang.ref.ReferenceQueue  ;
14  import java.lang.ref.WeakReference  ;
15  
16  import java.util.Map  ;
17  
18  
19  /**
20   * <p>A map where keys are compared using identity comparison (like
21   * IdentityHashMap) but where the presence of an object as a key in
22   * the map does not prevent it being garbage collected (like
23   * WeakHashMap).  This class does not implement the Map interface
24   * because it is difficult to ensure correct semantics for iterators
25   * over the entrySet().</p>
26   *
27   * <p>Because we do not implement Map, we do not copy the questionable
28   * interface where you can call get(k) or remove(k) for any type of k,
29   * which of course can only have an effect if k is of type K.</p>
30   *
31   * <p>This map does not support null keys.</p>
32   */
33  /*
34   * The approach
35   * is to wrap each key in a WeakReference and use the wrapped value as
36   * a key in an ordinary HashMap.  The WeakReference has to be a
37   * subclass IdentityWeakReference (IWR) where two IWRs are equal if
38   * they refer to the same object.  This enables us to find the entry
39   * again.
40   */
41  class WeakIdentityHashMap<K, V> {
42      private WeakIdentityHashMap() {}
43      
44      static <K, V> WeakIdentityHashMap<K, V> make() {
45          return new WeakIdentityHashMap<K, V>();
46      }
47      
48      V get(K key) {
49      expunge();
50      WeakReference  <K> keyref = makeReference(key);
51      return map.get(keyref);
52      }
53  
54      public V put(K key, V value) {
55      expunge();
56      if (key == null)
57          throw new IllegalArgumentException  ("Null key");
58      WeakReference  <K> keyref = makeReference(key, refQueue);
59      return map.put(keyref, value);
60      }
61  
62      public V remove(K key) {
63      expunge();
64      WeakReference  <K> keyref = makeReference(key);
65      return map.remove(keyref);
66      }
67  
68      private void expunge() {
69      Reference  <? extends K> ref;
70      while ((ref = refQueue.poll()) != null)
71          map.remove(ref);
72      }
73  
74      private WeakReference  <K> makeReference(K referent) {
75      return new IdentityWeakReference<K>(referent);
76      }
77  
78      private WeakReference  <K> makeReference(K referent, ReferenceQueue  <K> q) {
79      return new IdentityWeakReference<K>(referent, q);
80      }
81  
82      /**
83       * WeakReference where equals and hashCode are based on the
84       * referent.  More precisely, two objects are equal if they are
85       * identical or if they both have the same non-null referent.  The
86       * hashCode is the value the original referent had.  Even if the
87       * referent is cleared, the hashCode remains.  Thus, objects of
88       * this class can be used as keys in hash-based maps and sets.
89       */
90      private static class IdentityWeakReference<T> extends WeakReference  <T> {
91      IdentityWeakReference(T o) {
92          this(o, null);
93      }
94  
95      IdentityWeakReference(T o, ReferenceQueue  <T> q) {
96          super(o, q);
97          this.hashCode = (o == null) ? 0 : System.identityHashCode(o);
98      }
99  
100     public boolean equals(Object   o) {
101         if (this == o)
102         return true;
103         if (!(o instanceof IdentityWeakReference))
104         return false;
105         IdentityWeakReference wr = (IdentityWeakReference) o;
106         Object   got = get();
107         return (got != null && got == wr.get());
108     }
109 
110     public int hashCode() {
111         return hashCode;
112     }
113 
114     private final int hashCode;
115     }
116 
117     private Map  <WeakReference  <K>, V> map = newMap();
118     private ReferenceQueue  <K> refQueue = new ReferenceQueue  <K>();
119 }
120
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