LRU缓存实现-LinkedHashMap

LRU是Least Recently Used 的缩写，翻译过来就是“最近最少使用”.

LRU缓存的思想

• 固定缓存大小，需要给缓存分配一个固定的大小。
• 每次读取缓存都会改变缓存的使用时间，将缓存的存在时间重新刷新。
• 需要在缓存满了后，将最近最久未使用的缓存删除，再添加最新的缓存。

按照LinkedHashMap自身已经实现了顺序存储，默认情况下是按照元素的添加顺序存储，也可以启用按照访问顺序存储，传入的第三个参数accessOrder为true的时候，就按访问顺序对LinkedHashMap排序，为false的时候就按插入顺序，默认是为false的。

当把accessOrder设置为true后，就可以将最近访问的元素置于最前面，这样就可以满足上述的第二点。即最近读取的数据放在最前面，最早读取的数据放在最后面，然后它还有一个判断是否删除最老数据的方法，默认是返回false，即不删除数据，我们使用LinkedHashMap实现LRU缓存的方法就是对LinkedHashMap实现简单的扩展，扩展方式有两种，一种是inheritance，采用inheritance方式实现比较简单，而且实现了Map接口，在多线程环境使用时可以使用 Collections.synchronizedMap()方法实现线程安全操作,一种是delegation,delegation方式实现更加优雅一些，但是由于没有实现Map接口，所以线程同步就需要自己搞定了.

//delegation

import java.util.ArrayList;
import java.util.Collection;
import java.util.LinkedHashMap;
import java.util.Map;

/**
 * LRU(最近最少使用),Java模拟实现，基于linkedHashMap
 * @param <K>
 * @param <V>
 */
public class LRUCache<K,V> {
    private static final float hashTableLoadFactor = 0.75f;
    private LinkedHashMap<K,V> map;
    private int cacheSize;

    /**
     * 创建LRUCache,true访问顺序
     * @param cacheSize
     */
    public LRUCache(int cacheSize){
        this.cacheSize = cacheSize;
        int hashTableCapacity = (int) Math.ceil(cacheSize / hashTableLoadFactor) + 1;
        map = new LinkedHashMap<K, V>(hashTableCapacity, hashTableLoadFactor,
                true) {
                private static final long serialVersionUID = 1;

            @Override
            protected boolean removeEldestEntry(Map.Entry<K, V> eldest) {
                return size() > LRUCache.this.cacheSize;
            }
        };
    }

    /**
     * Retrieves an entry from the cache.<br>
     * The retrieved entry becomes the MRU (most recently used) entry.
     *
     * @param key
     *            the key whose associated value is to be returned.
     * @return the value associated to this key, or null if no value with this
     *         key exists in the cache.
     */
    public synchronized V get(K key) {
        return map.get(key);
    }
    /**
     * Adds an entry to this cache. The new entry becomes the MRU (most recently
     * used) entry. If an entry with the specified key already exists in the
     * cache, it is replaced by the new entry. If the cache is full, the LRU
     * (least recently used) entry is removed from the cache.
     *
     * @param key
     *            the key with which the specified value is to be associated.
     * @param value
     *            a value to be associated with the specified key.
     */
    public synchronized void put(K key, V value) {
        map.put(key, value);
    }

    /**
     * Clears the cache.
     */
    public synchronized void clear() {
        map.clear();
    }

    /**
     * Returns the number of used entries in the cache.
     *
     * @return the number of entries currently in the cache.
     */
    public synchronized int usedEntries() {
        return map.size();
    }

    /**
     * Returns a <code>Collection</code> that contains a copy of all cache
     * entries.
     *
     * @return a <code>Collection</code> with a copy of the cache content.
     */
    public synchronized Collection<Map.Entry<K, V>> getAll() {
        return new ArrayList<Map.Entry<K, V>>(map.entrySet());
    }

    public static void main(String[] args) {
        LRUCache<String,String> c = new LRUCache<>(3);
        c.put("1", "one"); // 1
        c.put("2", "two"); // 2 1
        c.put("3", "three"); // 3 2 1
        c.put("4", "four"); // 4 3 2

        if (c.get("2") == null){
            throw new Error();//2 4 3
        }
        c.put("5", "five"); // 5 2 4
        c.put("4", "second four"); // 4 5 2
        if (c.usedEntries() != 3){
            throw new Error();
        }
        if (!c.get("4").equals("second four"))
            throw new Error();
        if (!c.get("5").equals("five"))
            throw new Error();
        if (!c.get("2").equals("two"))
            throw new Error();
        for (Map.Entry<String,String> entry : c.getAll()){
            System.out.println(entry.getKey() + ":" + entry.getValue());
        }
    }
}

//inheritance
import java.util.LinkedHashMap;
 import java.util.Map;

 public LRUCache<K, V> extends LinkedHashMap<K, V> {
 private int cacheSize;

 public LRUCache(int cacheSize) {
 super(16, 0.75, true);
 this.cacheSize = cacheSize;
 }

 protected boolean removeEldestEntry(Map.Entry<K, V> eldest) {
 return size() >= cacheSize;
 }
 }