weakhashtable.java

comming logging 的源码 可以封装log4j等日志系统。 源码中使用了很多的设计模式

        Referenced keyRef = new Referenced(key, queue);
        return super.put(keyRef, value);
    }
    
    /**
     *@see Hashtable
     */    
    public void putAll(Map t) {
        if (t != null) {
            Set entrySet = t.entrySet();
            for (Iterator it=entrySet.iterator(); it.hasNext();) {
                Map.Entry entry = (Map.Entry) it.next();
                put(entry.getKey(), entry.getValue());
            }
        }
    }
    
    /**
     *@see Hashtable
     */      
    public Collection values() {
        purge();
        return super.values();
    }
    
    /**
     *@see Hashtable
     */     
    public Object remove(Object key) {
        // for performance reasons, only purge every 
        // MAX_CHANGES_BEFORE_PURGE times
        if (changeCount++ > MAX_CHANGES_BEFORE_PURGE) {
            purge();
            changeCount = 0;
        }
        // do a partial purge more often
        else if ((changeCount % PARTIAL_PURGE_COUNT) == 0) {
            purgeOne();
        }
        return super.remove(new Referenced(key));
    }
    
    /**
     *@see Hashtable
     */    
    public boolean isEmpty() {
        purge();
        return super.isEmpty();
    }
    
    /**
     *@see Hashtable
     */    
    public int size() {
        purge();
        return super.size();
    }
    
    /**
     *@see Hashtable
     */        
    public String toString() {
        purge();
        return super.toString();
    }
    
    /**
     * @see Hashtable
     */
    protected void rehash() {
        // purge here to save the effort of rehashing dead entries
        purge();
        super.rehash();
    }
    
    /**
     * Purges all entries whose wrapped keys
     * have been garbage collected.
     */
    private void purge() {
        synchronized (queue) {
            WeakKey key;
            while ((key = (WeakKey) queue.poll()) != null) {
                super.remove(key.getReferenced());
            }
        }
    }
    
    /**
     * Purges one entry whose wrapped key 
     * has been garbage collected.
     */
    private void purgeOne() {
        
        synchronized (queue) {
            WeakKey key = (WeakKey) queue.poll();
            if (key != null) {
                super.remove(key.getReferenced());
            }
        }
    }
    
    /** Entry implementation */
    private final static class Entry implements Map.Entry {
    
        private final Object key;
        private final Object value;
        
        private Entry(Object key, Object value) {
            this.key = key;
            this.value = value;
        }
    
        public boolean equals(Object o) {
            boolean result = false;
            if (o != null && o instanceof Map.Entry) {
                Map.Entry entry = (Map.Entry) o;
                result =    (getKey()==null ?
                                            entry.getKey() == null : 
                                            getKey().equals(entry.getKey()))
                            &&
                            (getValue()==null ?
                                            entry.getValue() == null : 
                                            getValue().equals(entry.getValue()));
            }
            return result;
        } 
        
        public int hashCode() {

            return (getKey()==null ? 0 : getKey().hashCode()) ^
                (getValue()==null ? 0 : getValue().hashCode());
        }

        public Object setValue(Object value) {
            throw new UnsupportedOperationException("Entry.setValue is not supported.");
        }
        
        public Object getValue() {
            return value;
        }
        
        public Object getKey() {
            return key;
        }
    }
    
    
    /** Wrapper giving correct symantics for equals and hashcode */
    private final static class Referenced {
        
        private final WeakReference reference;
        private final int           hashCode;

        /**
         * 
         * @throws NullPointerException if referant is <code>null</code>
         */        
        private Referenced(Object referant) {
            reference = new WeakReference(referant);
            // Calc a permanent hashCode so calls to Hashtable.remove()
            // work if the WeakReference has been cleared
            hashCode  = referant.hashCode();
        }
        
        /**
         * 
         * @throws NullPointerException if key is <code>null</code>
         */
        private Referenced(Object key, ReferenceQueue queue) {
            reference = new WeakKey(key, queue, this);
            // Calc a permanent hashCode so calls to Hashtable.remove()
            // work if the WeakReference has been cleared
            hashCode  = key.hashCode();

        }
        
        public int hashCode() {
            return hashCode;
        }
        
        private Object getValue() {
            return reference.get();
        }
        
        public boolean equals(Object o) {
            boolean result = false;
            if (o instanceof Referenced) {
                Referenced otherKey = (Referenced) o;
                Object thisKeyValue = getValue();
                Object otherKeyValue = otherKey.getValue();
                if (thisKeyValue == null) {                     
                    result = (otherKeyValue == null);
                    
                    // Since our hashcode was calculated from the original
                    // non-null referant, the above check breaks the 
                    // hashcode/equals contract, as two cleared Referenced
                    // objects could test equal but have different hashcodes.
                    // We can reduce (not eliminate) the chance of this
                    // happening by comparing hashcodes.
                    if (result == true) {
                        result = (this.hashCode() == otherKey.hashCode());
                    }
                    // In any case, as our c'tor does not allow null referants
                    // and Hashtable does not do equality checks between 
                    // existing keys, normal hashtable operations should never 
                    // result in an equals comparison between null referants
                }
                else
                {
                    result = thisKeyValue.equals(otherKeyValue);
                }
            }
            return result;
        }
    }
    
    /**
     * WeakReference subclass that holds a hard reference to an
     * associated <code>value</code> and also makes accessible
     * the Referenced object holding it.
     */
    private final static class WeakKey extends WeakReference {

        private final Referenced referenced;
        
        private WeakKey(Object key, 
                        ReferenceQueue queue,
                        Referenced referenced) {
            super(key, queue);
            this.referenced = referenced;
        }
        
        private Referenced getReferenced() {
            return referenced;
        }
     }
}