LinkedHashMap實現LRU緩存的底層原理及實現步驟

如何通過linkedhashmap實現高速緩存?

要求:存儲特定個數的熱點數據,不常訪問的數據自動刪除掉.

只需要:自定義LinkedHashMap的匿名子類,重寫removeEldestEntry方法.

 

LinkedHashMap<Object, Object> map = new LinkedHashMap<Object, Object>(5, 0.75F,
    true) {
  @Override//重寫的這個方法會在調用afterNodeInsertion方法時被使用到.
  protected boolean removeEldestEntry(Entry eldest) {
    return size() > 5;
  }
};
map.put(1, 1);
map.put(2, 2);
map.put(3, 3);
map.put(4, 4);

Object o = map.get(2);

map.put(5, 5);
map.put(6, 6);
Set<Object> objects = map.keySet();
objects.forEach(e -> System.out.print(e + " "));
System.out.println();
map.values().forEach(e -> System.out.print(e + " "));

//控制檯輸出

3 4 2 5 6
3 4 2 5 6

2放到了"末尾",1被"頂出".

常用的數據會一直在尾部

源碼解釋:

hashmap的put方法會調用afterNodeInsertion方法,afterNodeInsertion方法中會用到removeEldestEntry方法進行判斷.所以需要重寫這個方法.

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
               boolean evict) {
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    else {
        Node<K,V> e; K k;
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        else if (p instanceof TreeNode)
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        else {
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) {
                    p.next = newNode(hash, key, value, null);
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        treeifyBin(tab, hash);
                    break;
                }
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                p = e;
            }
        }
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    ++modCount;
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;
}

void afterNodeInsertion(boolean evict) { // possibly remove eldest
    LinkedHashMap.Entry<K,V> first;
    if (evict && (first = head) != null && removeEldestEntry(first)) {
        K key = first.key;
        removeNode(hash(key), key, null, false, true);
    }
}