WeakClassCache


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22  package org.jboss.util.collection;
23  
24  import java.lang.ref.WeakReference  ;
25  import java.util.Map  ;
26  import java.util.WeakHashMap  ;
27  
28  /**
29   * A weak class cache that instantiates does not a hold a
30   * strong reference to either the classloader or class.<p>
31   * 
32   * It creates the class specific data in two stages
33   * to avoid recursion.<p>
34   * 
35   * instantiate - creates the data<br>
36   * generate - fills in the details
37   *
38   * @author <a HREF="mailto:adrian@jboss.org">Adrian Brock</a>
39   */
40  public abstract class WeakClassCache
41  {
42     /** The cache */
43     protected Map   cache = new WeakHashMap  (); 
44  
45     /**
46      * Get the information for a class
47      * 
48      * @param clazz the class
49      * @return the info
50      */
51     public Object   get(Class   clazz)
52     {
53        if (clazz == null)
54           throw new IllegalArgumentException  ("Null class");
55        
56        Map   classLoaderCache = getClassLoaderCache(clazz.getClassLoader());
57  
58        WeakReference   weak = (WeakReference  ) classLoaderCache.get(clazz.getName());
59        if (weak != null)
60        {
61           Object   result = weak.get();
62           if (result != null)
63              return result;
64        }
65  
66        Object   result = instantiate(clazz);
67  
68        weak = new WeakReference  (result);
69        classLoaderCache.put(clazz.getName(), weak);
70        
71        generate(clazz, result);
72        
73        return result;
74     }
75     
76     /**
77      * Get the information for a class
78      * 
79      * @param name the name
80      * @param cl the classloader
81      * @return the info
82      * @throws ClassNotFoundException when the class cannot be found
83      */
84     public Object   get(String   name, ClassLoader   cl) throws ClassNotFoundException  
85     {
86        if (name == null)
87           throw new IllegalArgumentException  ("Null name");
88        if (cl == null)
89           throw new IllegalArgumentException  ("Null classloader");
90        Class   clazz = cl.loadClass(name);
91        return get(clazz);
92     }
93     
94     /**
95      * Instantiate for a class
96      * 
97      * @param clazz the class
98      * @return the result
99      */
100    protected abstract Object   instantiate(Class   clazz);
101    
102    /**
103     * Fill in the result
104     * 
105     * @param clazz the class
106     * @param result the result
107     */
108    protected abstract void generate(Class   clazz, Object   result);
109    
110    /**
111     * Get the cache for the classloader
112     * 
113     * @param cl the classloader
114     * @return the map
115     */
116    protected Map   getClassLoaderCache(ClassLoader   cl)
117    {
118       synchronized (cache)
119       {
120          Map   result = (Map  ) cache.get(cl);
121          if (result == null)
122          {
123             result = CollectionsFactory.createConcurrentReaderMap();
124             cache.put(cl, result);
125          }
126          return result;
127       }
128    }
129 }
130
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