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利用LinkedHashMap简单实现基于LRU策略的缓存

2012-11-18 15:35 1066 查看
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++;
}



                                            
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