Cache


1   package dynaop.util;
2   
3   import java.lang.ref.Reference  ;
4   import java.lang.ref.ReferenceQueue  ;
5   import java.lang.ref.SoftReference  ;
6   import java.lang.ref.WeakReference  ;
7   import java.util.Collections  ;
8   import java.util.HashMap  ;
9   import java.util.Map  ;
10  import java.util.WeakHashMap  ;
11  
12  /**
13   * Cache with weak keys and soft values.
14   * 
15   * @author Bob Lee (crazybob@crazybob.org)
16   */
17  public abstract class Cache {
18  
19      static Object   NULL_VALUE = new Object  ();
20      
21      Map   map;
22      ReferenceQueue   queue = new ReferenceQueue  ();
23      
24      /**
25       * Creates cache.
26       * 
27       * @param weakKeys Use weak references for keys.
28       */
29      public Cache(boolean weakKeys) {
30          this.map = (weakKeys) ? 
31              (Map  ) new WeakHashMap  () : new HashMap  ();
32          this.map = Collections.synchronizedMap(this.map);
33      }
34      
35      /**
36       * Defaults to weak keys.
37       */
38      public Cache() {
39          this(true);
40      }
41      
42      /**
43       * Creates value for key. Called by getter if 
44       * value isn't cached.
45       */
46      protected abstract Object   create(Object   key);
47      
48      /**
49       * Gets value for key. Creates if necessary.
50       */
51      public Object   get(Object   key) {
52          Object   value = internalGet(key);
53          if (value == null) {
54              value = create(key);
55              if (value == null)
56                  value = NULL_VALUE;
57              this.map.put(key, 
58                  new ValueReference(key, value));
59          }
60          return (value == NULL_VALUE) ? null : value;
61      }
62      
63      void cleanUp() {
64          Reference   reference;
65          while ((reference = this.queue.poll()) != null)
66              map.remove(
67                  ((ValueReference) reference).getKey());
68      }
69      
70      Object   internalGet(Object   key) {
71          cleanUp();
72          Reference   reference = (Reference  ) map.get(key);
73          return (reference == null) ? 
74              null : reference.get();
75      }
76      
77      class ValueReference extends SoftReference   {
78          
79          WeakReference   keyReference;
80          
81          ValueReference(Object   key, Object   value) {
82              super(value, queue);
83              this.keyReference = new WeakReference  (key); 
84          }
85          
86          Object   getKey() {
87              return this.keyReference.get();
88          }
89      }
90  }
91
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