LRU原理
LRU（Least recently used，最近最少使用）算法根据数据的历史访问记录来进行淘汰数据，其核心思想是“如果数据最近被访问过，那么将来被访问的几率也更高”。
实现1
命中时需要遍历链表，找到命中的数据块索引，然后需要将数据移到头部。
import java.util.ArrayList; import java.util.Collection; import java.util.LinkedHashMap; import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock; import java.util.Map; /** * 类说明：利用LinkedHashMap实现简单的缓存， 必须实现removeEldestEntry方法，具体参见JDK文档 * * @author dennis * * @param <K> * @param <V> */ public class LRULinkedHashMap<K, V> extends LinkedHashMap<K, V> { private final int maxCapacity; private static final float DEFAULT_LOAD_FACTOR = 0.75f; private final Lock lock = new ReentrantLock(); public LRULinkedHashMap(int maxCapacity) { super(maxCapacity, DEFAULT_LOAD_FACTOR, true); this.maxCapacity = maxCapacity; } @Override protected boolean removeEldestEntry(java.util.Map.Entry<K, V> eldest) { return size() > maxCapacity; } @Override public boolean containsKey(Object key) { try { lock.lock(); return super.containsKey(key); } finally { lock.unlock(); } } @Override public V get(Object key) { try { lock.lock(); return super.get(key); } finally { lock.unlock(); } } @Override public V put(K key, V value) { try { lock.lock(); return super.put(key, value); } finally { lock.unlock(); } } public int size() { try { lock.lock(); return super.size(); } finally { lock.unlock(); } } public void clear() { try { lock.lock(); super.clear(); } finally { lock.unlock(); } } public Collection<Map.Entry<K, V>> getAll() { try { lock.lock(); return new ArrayList<Map.Entry<K, V>>(super.entrySet()); } finally { lock.unlock(); } } }
实现2
传统意义的LRU算法是为每一个Cache对象设置一个计数器，每次Cache命中则给计数器+1，而Cache用完，需要淘汰旧内容，放置新内容时，就查看所有的计数器，并将最少使用的内容替换掉。
非线程安全，若实现安全，则在响应的方法加锁。
import java.util.HashMap; import java.util.Map.Entry; import java.util.Set; public class LRUCache<K, V> { private int currentCacheSize; private int CacheCapcity; private HashMap<K,CacheNode> caches; private CacheNode first; private CacheNode last; public LRUCache(int size){ currentCacheSize = 0; this.CacheCapcity = size; caches = new HashMap<K,CacheNode>(size); } public void put(K k,V v){ CacheNode node = caches.get(k); if(node == null){ if(caches.size() >= CacheCapcity){ caches.remove(last.key); removeLast(); } node = new CacheNode(); node.key = k; } node.value = v; moveToFirst(node); caches.put(k, node); } public Object get(K k){ CacheNode node = caches.get(k); if(node == null){ return null; } moveToFirst(node); return node.value; } public Object remove(K k){ CacheNode node = caches.get(k); if(node != null){ if(node.pre != null){ node.pre.next=node.next; } if(node.next != null){ node.next.pre=node.pre; } if(node == first){ first = node.next; } if(node == last){ last = node.pre; } } return caches.remove(k); } public void clear(){ first = null; last = null; caches.clear(); } private void moveToFirst(CacheNode node){ if(first == node){ return; } if(node.next != null){ node.next.pre = node.pre; } if(node.pre != null){ node.pre.next = node.next; } if(node == last){ last= last.pre; } if(first == null || last == null){ first = last = node; return; } node.next=first; first.pre = node; first = node; first.pre=null; } private void removeLast(){ if(last != null){ last = last.pre; if(last == null){ first = null; }else{ last.next = null; } } } @Override public String toString(){ StringBuilder sb = new StringBuilder(); CacheNode node = first; while(node != null){ sb.append(String.format("%s:%s ", node.key,node.value)); node = node.next; } return sb.toString(); } class CacheNode{ CacheNode pre; CacheNode next; Object key; Object value; public CacheNode(){ } } public static void main(String[] args) { LRUCache<Integer,String> lru = new LRUCache<Integer,String>(3); lru.put(1, "a"); // 1:a System.out.println(lru.toString()); lru.put(2, "b"); // 2:b 1:a System.out.println(lru.toString()); lru.put(3, "c"); // 3:c 2:b 1:a System.out.println(lru.toString()); lru.put(4, "d"); // 4:d 3:c 2:b System.out.println(lru.toString()); lru.put(1, "aa"); // 1:aa 4:d 3:c System.out.println(lru.toString()); lru.put(2, "bb"); // 2:bb 1:aa 4:d System.out.println(lru.toString()); lru.put(5, "e"); // 5:e 2:bb 1:aa System.out.println(lru.toString()); lru.get(1); // 1:aa 5:e 2:bb System.out.println(lru.toString()); lru.remove(11); // 1:aa 5:e 2:bb System.out.println(lru.toString()); lru.remove(1); //5:e 2:bb System.out.println(lru.toString()); lru.put(1, "aaa"); //1:aaa 5:e 2:bb System.out.println(lru.toString()); } }
原文：https://www.cnblogs.com/bopo/p/9255654.html