利用LinkedHashMap简单实现基于LRU策略的缓存

为了提高访问效率，对资源的访问一般会考虑采取一定的缓存策略。基于访问时间的缓存策略：LRU (LeastRecentlyUsed)是一种应用广泛的缓存算法。该算法维护一个缓存项队列，队列中的缓存项按每项的最后被访问时间排序。当缓存空间已满时，将处于队尾即删除最后一次被访问时间距现在最久的项，将新的区段放入队列首部。

缓存一般也是会被多线程访问，需要考虑线程访问安全问题，对于为考虑多线程访问而实现的LinkedHashMap可以参考http://code.google.com/p/concurrentlinkedhashmap/

Java程序中利用LinkedHashMap可以非常方便的实现基于LRU策略的缓存

public class QuestionBankCache{
private LinkedHashMap<String, QuestionBank> qbanksMap;
private static final int MAX_ENTRIES = 100;
public QuestionBankCache() {
qbanksMap = new LinkedHashMap(MAX_ENTRIES, .75F,true) {
   protected boolean removeEldestEntry(Map.Entry eldest) {    
   return size() > MAX_ENTRIES;    
   } 
};
} 
}

LinkedHashMap相比HashMap类，HashMap中元素是无序的，而LinkedHashMap中是按照一定顺序排列元素的，其中有个重要的成员变量accessOrder，该属性默认为false，LinkedHashMap仅根据插入顺序对元素排序；如果在其构造函数中指定该属性为true，则LinkedHashMap即可根据访问顺序对元素排序。

    /**
     * The iteration ordering method for this linked hash map: <tt>true</tt>
     * for access-order, <tt>false</tt> for insertion-order.
     *
     * @serial
     */
    private final boolean accessOrder;

1，访问元素的实现

LinkedHashMap的get函数，可以看到实现方法是先根据key值找出元素，并调用了该元素的recordAccess方法

    public V get(Object key) {
        Entry<K,V> e = (Entry<K,V>)getEntry(key);
        if (e == null)
            return null;
        e.recordAccess(this);
        return e.value;
    }

recordAccess方法的实现是在LinkedHashMap Entry类中，该方法在accessOrder属性为true时 将该元素移到容器的header处，以实现按照访问顺序进行排序的目的

    /**
     * LinkedHashMap entry.
     */
    private static class Entry<K,V> extends HashMap.Entry<K,V> {
        // These fields comprise the doubly linked list used for iteration.
        Entry<K,V> before, after;

        Entry(int hash, K key, V value, HashMap.Entry<K,V> next) {
            super(hash, key, value, next);
        }

        /**
         * Removes this entry from the linked list.
         */
        private void remove() {
            before.after = after;
            after.before = before;
        }

        /**
         * Inserts this entry before the specified existing entry in the list.
         */
        private void addBefore(Entry<K,V> existingEntry) {
            after  = existingEntry;
            before = existingEntry.before;
            before.after = this;
            after.before = this;
        }

        /**
         * This method is invoked by the superclass whenever the value
         * of a pre-existing entry is read by Map.get or modified by Map.set.
         * If the enclosing Map is access-ordered, it moves the entry
         * to the end of the list; otherwise, it does nothing.
         */
        void recordAccess(HashMap<K,V> m) {
            LinkedHashMap<K,V> lm = (LinkedHashMap<K,V>)m;
            if (lm.accessOrder) {
                lm.modCount++;
                remove();
                addBefore(lm.header);
            }
        }

        void recordRemoval(HashMap<K,V> m) {
            remove();
        }
    }




2，插入元素的实现
插入元素是通过父类的put方法实现，可以看到该方法中会调用recordAccess方法（覆盖方式）和addEntry方法（新增方式），这两个方法在LinkedHashMap类中都进行了重构。

    /**
     * Associates the specified value with the specified key in this map.
     * If the map previously contained a mapping for the key, the old
     * value is replaced.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
     *         (A <tt>null</tt> return can also indicate that the map
     *         previously associated <tt>null</tt> with <tt>key</tt>.)
     */
    public V put(K key, V value) {
        if (key == null)
            return putForNullKey(value);
        int hash = hash(key.hashCode());
        int i = indexFor(hash, table.length);
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }

        modCount++;
        addEntry(hash, key, value, i);
        return null;
    }
LinkedHashMap中重构(Override）addEntry方法的实现，可以看到调用了removeEldesEntry方法，该方法我们在缓存类中进行了重构(Override)，目的是保持缓存中的最大元素个数固定，而LinkedHashMap默认实现是通过动态扩大容量来保证元素空间的


    /**
     * This override alters behavior of superclass put method. It causes newly
     * allocated entry to get inserted at the end of the linked list and
     * removes the eldest entry if appropriate.
     */
    void addEntry(int hash, K key, V value, int bucketIndex) {
        createEntry(hash, key, value, bucketIndex);

        // Remove eldest entry if instructed, else grow capacity if appropriate
        Entry<K,V> eldest = header.after;
        if (removeEldestEntry(eldest)) {
            removeEntryForKey(eldest.key);
        } else {
            if (size >= threshold)
                resize(2 * table.length);
        }
    }

    /**
     * This override differs from addEntry in that it doesn't resize the
     * table or remove the eldest entry.
     */
    void createEntry(int hash, K key, V value, int bucketIndex) {
        HashMap.Entry<K,V> old = table[bucketIndex];
        Entry<K,V> e = new Entry<K,V>(hash, key, value, old);
        table[bucketIndex] = e;
        e.addBefore(header);
        size++;
    }